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dimensions
of
particle
physics
spring 2013
A joint Fermilab/SLAC publication

A joint Fermilab/SLAC publication
On the cover
The Standard Model of particle physics has room for every known
type of particle—the electron and its heavier cousins, the quarks
that make up the atomic nucleus and the bosons that carry forces.
But while neutrinos are part of the Standard Model table, they
don’t really fit in; the fact that the three types of neutrinos can
change into one another violates Standard Model predictions.
What else will we learn from these maverick particles? See
“Neutrinos, the Standard Model Misfits,” page 10.

symmetry | spring 2013
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Editorial: The Discoveries Continue
The discoveries of 2012 point the way to more exciting physics in
2013 and the decades beyond.
Commentary: John Womersley
The “unreasonable demands” of pure research are an essential
driver for technology, enriching our bodies, minds and pocketbooks.
Commentary: Moishe Pripstein and George Trilling
Long-term funding and support for science pays huge dividends
from unexpected discoveries and applications—even when the
potential impact is unclear at the time of discovery.
Signal to Background
A brainy playground springs up at Fermilab; Picasso’s paint proves
common; a hipster detector searches for the crystalline “plink”
of dark matter; SLAC’s softball team applies science to sport;
a physicist fights for academic freedom.
Neutrinos, the Standard Model Misfits
For years, scientists thought that neutrinos fit perfectly into the
Standard Model. But they don’t. By better understanding these
strange, elusive particles, scientists seek to better understand the
workings of all the universe, one discovery at a time.
What’s Next at the Large Hadron Collider?
Experiments at the Large Hadron Collider made a major
discovery, but the world’s highest-energy particle accelerator
is just getting started.
Illuminating the Dark Universe
The pursuit of dark matter and dark energy is one of the most
exciting—and most challenging—areas of science. Now researchers
think they’re beginning to close in.
Deconstruction: Long-Baseline Neutrino Experiment
The Long-Baseline Neutrino Experiment aims to discover
whether neutrinos violate the fundamental matter–antimatter
symmetry of physics.
Essay: A Galaxy with a View
A physicist, a software developer and a writer step outside one night
to take in nature’s beauty at a mountaintop observatory in Chile.
Application: Cancer Detection
Gamma rays are great for revealing astrophysical phenomena
such as supermassive black holes and merging neutron stars.
They’re also proving excellent for detecting early stages of cancer.
Logbook: Higgs-like Particle
In June 2012, particle physicists on experiments at the Large
Hadron Collider had a secret to keep, just between themselves
and a few thousand colleagues.
Explain it in 60 Seconds: Spectroscopy
Spectroscopy is a technique that astronomers use to measure
and analyze the thousands of colors contained in the light emitted
by stars, galaxies and other celestial objects.

from the editor
Photo: Bradley Plummer, SLAC
The discoveries
continue
This is quite the time in particle physics. Some
of the most exciting discoveries in a decade have
been made over the past year, and the coming
years promise new
endeavors and new
findings.
2012 brought us
the discovery of a new
boson resembling the
Higgs, a triumph that
not only is a major
step toward understanding
how some
elementary particles
acquire mass but also
brings other longstanding
questions
about matter, energy,
space and time into
sharper focus.
Meanwhile, the Daya Bay Reactor Neutrino
Experiment, in combination with other experiments,
measured a key neutrino parameter, a significant
result that paves the way for future revelations
about neutrino properties and the imbalance
between matter and antimatter. And the Baryon
Oscillation Spectroscopic Survey looked further
back in time than ever before, spying on the very
early universe. Such observations are likely to
offer insight into dark energy.
Of course, the discoveries don’t end there.
2013 will be a year of new projects and new data.
The Dark Energy Survey will start mapping our
universe as it seeks to unravel the mystery of
dark energy. NOνA is powering up and beginning
its studies of the strange properties of neutrinos.
The Large Underground Xenon experiment is
starting its search for the quiet signals of dark
matter. The Large Hadron Collider is undergoing
upgrades, enabling it to climb to higher collision
energy and produce heavier particles, while analysis
continues on the existing LHC dataset.
Upgrades are also in the works for for the KEKB
accelerator and the Belle experiment. And we
expect additional results from a slew of other
projects and analyses, answering questions
about dark matter and dark energy, neutrino
properties, the asymmetry between matter and
antimatter, physics beyond the Standard Model,
the cosmic phenomena that emit the most
energetic form of light, and much, much more.
This year is also a time of long-range planning.
Through what’s called the Snowmass process,
the US particle physics community is now identifying
the most important and pressing scientific
questions and the experiments and techniques
needed to answer them. After the Snowmass
process is complete in the second half of 2013,
the Department of Energy will reestablish the
Particle Physics Project Prioritization Panel to
develop a clear strategy for the United States’
investment in the construction and operation of
particle physics projects over the coming
decades.
As thrilling as the past year has been, the future
promises to be even more so.
Kelen Tuttle, Editor-in-chief
Symmetry
PO Box 500
MS 206
Batavia Illinois 60510
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mail@symmetrymagazine.org
symmetry (ISSN 1931-8367)
is published by Fermi National
Accelerator Laboratory and
SLAC National Accelerator
Laboratory, funded by the
US Department of Energy
Office of Science. (c) 2013
symmetry All rights reserved
Editor-in-Chief
Kelen Tuttle
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Deputy Editor
Kathryn Jepsen
Managing Editor
Kurt Riesselmann
Senior Editor
Glennda Chui
Staff Writers
Glenn Roberts Jr.
Andre Salles
Ashley WennersHerron
Lori Ann White
Interns
Jessica Orwig
Joseph Piergrossi
Publishers
Farnaz Khadem, SLAC
Katie Yurkewicz, FNAL
Contributing Editors
Kandice Carter, JLab
Jennifer Gagné, TRIUMF
James Gillies, CERN
Sophie Kerhoas-Cavata, CEA
Karen McNulty Walsh, BNL
Vanessa Mexner, NIKHEF
Till Mundzeck, DESY
Vincenzo Napolano, INFN
Terry O’Connor, STFC
Paul Preuss, LBNL
Perrine Royole-Degieux, IN2P3
Yuri Ryabov, IHEP Protvino
Boris Starchenko, JINR
Rika Takahashi, KEK
Maury Tigner, LEPP
Tongzhou Xu, IHEP Beijing
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symmetry | spring 2013
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physics! Stay on top of
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subscribing to symmetry’s
electronic edition. We publish
new articles every week.
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commentary: john womersley
Being unreasonable:
the value of pure science
Photo: Cindy Arnold, Fermilab
Fermilab’s founding director, Robert R. Wilson,
famously responded to the question of how the
laboratory would help defend the United States
with the declaration that it would not. “It has
only to do with the respect with which we regard
one another, the dignity of men, our love of
culture,” he said. “It has to do with: Are we good
painters, good sculptors, great poets? I mean
all the things we really venerate in our country
and are patriotic about. It has nothing to do
directly with defending our country except to
make it worth defending.”
In the postwar era, those words had great
resonance. But science can—and has—delivered
so much more to benefit the nations that invest
in it. In today’s world of fiscal uncertainty, it is
imperative that scientists justify their work in
economic as well as scientific terms.
Last year, I spoke at a symposium marking the
end of Fermilab’s Tevatron particle collider. From
back-of-the-envelope calculations, I estimated that
the cost of construction and operation ($4 billion
in today’s terms) was returned approximately
tenfold in the value of training PhD students and
stimulating industry in superconducting magnets
and computing.
Fundamental physics places unreasonable
demands on technology and computing. When
organizations commit to a project on the scale of
the Tevatron, the Large Hadron Collider or any of
the proposed next-generation giant telescopes,
they are stimulating the future advances in these
areas that will make the projects possible. As
George Bernard Shaw said, “The reasonable man
adapts himself to the world; the unreasonable
one persists in trying to adapt the world to himself.
Therefore, all progress depends on the unreasonable
man.”
Around the world, scientists are gathering
the evidence that makes a compelling case: The
technological developments needed for pure
research are essential to the advances society
makes. Examples include magnetic resonance
imaging, synchrotrons, the Web and the people
inspired to engage in science or engineering
by the awe of pure science.
Today’s high-resolution MRI scanners would
not be possible without research done into
superconducting magnets for particle physics at
the UK Science and Technology Facilities
Council’s Rutherford Appleton Laboratory in the
1960s. The superconducting cables and joints
they invented now underpin a global MRI industry
worth $6.9 billion a year and growing at 7.7
percent a year.
The need for the international particle physics
community to collaborate across borders drove
the development of the Web at the European
laboratory CERN. The Web’s value to the economy
is now immense. In 2011, Americans spent $256
billion on retail and travel-related purchases online,
a 12 percent increase over 2010.
Data also show us that questions about the
fundamental nature of the universe grab public
attention. We know that competing in a global
knowledge economy requires a scientifically
trained workforce; and we have good evidence
that something like 90 percent of physics students
were originally motivated to study science
because of particle physics and astronomy.
One measure of our success is that in the United
Kingdom we’ve seen substantial increases in
physics enrollment in the past year, despite overall
decreases in student numbers.
These are just a few examples of how the
unreasonable requirements of science drive innovation
around the world. We owe it to ourselves,
to future generations of scientists, and most importantly
to the public who ultimately support us, to
make the best case we can for the importance and
impact of fundamental science. We have a great
story to tell, and we should not be shy in telling it!
John Womersley is the chief executive officer of the United
Kingdom’s Science and Technology Facilities Council and
a visiting professor at the University of Durham, University
College London and University of Oxford. He also previously
served as a scientific advisor to the US Department of
Energy and was a member of the DZero experiment at the
Tevatron from 1986 to 2005.
symmetry | spring 2013
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commentary: moishe pripstein and george trilling
The power
of basic science
Amid the worldwide excitement of the recent
discovery of what may be the long-sought Higgs
boson, some questions arose as to whether it
was worth the expense.
Particle physics is an important endeavor, one
that addresses profound human curiosity about
such fundamental questions as how the universe
evolved and why we have mass, and one that
trains generations of new scientists at the cutting
edge of research.
But, even more than that, the science is important
because new knowledge is power—even
when the potential impact is unclear at the time
of discovery. The challenge then is to ensure
that it is used for the benefit of society.
As one example among many, when the theory
of quantum mechanics was introduced in the
1920s, it seemed to many people to have no
relevance at all to the macroscopic world, and
therefore to our lives. As a physical theory of
the submicroscopic world, quantum mechanics
is applicable only to the world of very small
distances.
But within two decades, quantum mechanics
led to the invention of the transistor and the
development of solid-state electronics, which
dominate our lives today. No one could have
had any such inkling in the 1920s.
This is the power of pure research: hope for
the future.
Today, the largest particle-physics experiment,
the Large Hadron Collider in Geneva, Switzerland,
explores a whole range of transformational
research topics. In addition to the discovery of
the Higgs-like boson, the LHC is on the brink
of many unexpected discoveries. Is there, as
suggested by theory, a new hierarchy of particles,
called supersymmetric particles, beyond those
described by the Standard Model of particle physics?
What is dark matter? Do extra dimensions
exist, beyond the familiar three dimensions of
space and one of time that underlie our present
concept and experience? Will the unexplored
domains of high energy and large masses that the
LHC makes accessible lead to the discovery of
entirely new principles of nature? A few decades
from now, the answers to these questions—and
the others asked in particle physics and indeed
all science—may enable huge societal benefits.
So how do we realize this promise for the
future? First, by leveraging the special excitement
in so many fields of science today, such as in
particle physics with the discovery of a possible
Higgs and in cosmology with the discovery of
dark energy. Scientists must be more proactive in
transmitting this excitement to the public
at large, which could translate into increased
support for science education and research.
If the past is any guide, long-term funding
support for science pays huge dividends from
unexpected discoveries and applications. So,
despite fiscal uncertainties, if not now, when?
Moishe Pripstein is the former program director of the National
Science Foundation’s support of US LHC participation
at CERN. He is also a senior physicist (retired) at Lawrence
Berkeley National Laboratory.
George Trilling is a former chairperson of the Department of
Physics at the University of California, Berkeley, and a former
director of the Physics Division at Lawrence Berkeley
National Laboratory. He is also a former president of the
American Physical Society.
Photos: APS
symmetry | spring 2013
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signal to background
A brainy playground springs up at Fermilab; Picasso’s paint proves common;
a hipster detector searches for the crystalline “plink” of dark matter; SLAC’s
softball team applies science to sport; a physicist fights for academic freedom.
Precocious protons
Ask a bunch of 10-year-olds
this question: Would you rather
hear about the journey of
a proton through Fermilab’s
accelerators, or would you
rather be a proton and take
that journey yourself?
And now, go visit an ear
doctor, since the deafening
sound of kids shouting out the
second option has no doubt
caused some damage. It’s no
secret that hands-on education
experiences are more fun for
kids—it feels like recess, and yet
learning is happening.
That’s the guiding principle
behind the Physics Playground,
now under construction at
Fermilab’s Lederman Science
Center. The first attraction to
be built is a running track that
allows children to pretend to
be protons, antiprotons or
muons, as they run along trails
in the shape of the lab’s iconic
accelerator complex.
The track mirrors the route
a particle takes as it travels
through the linac, Fermilab’s
initial accelerator, then around
the booster and the main
injector, where the particle ramps
up to nearly the speed of light.
The path then leads to a replica
of the Tevatron ring, where
kids can pretend to be protons
or antiprotons, running around
the track in opposite directions
depending on their choice.
The Proton Run is the
brainchild of a veteran educator
who knows the value of mixing
fun with learning: Marge
Bardeen, head of Fermilab’s
Office of Education.
To construct the run, Bardeen
partnered with Susan Dahl,
who coordinates the Teacher
Resource Center at the
Lederman Center. Dahl
obtained funding for the Proton
Run in an unusual way: She
secured a gambling tax grant
from Kane County, Illinois, that
covered about half the cost.
She and other members of
the non-profit organization
Fermilab Friends for Science
Education will now help arrange
funds to build the rest of the
playground, which may also
include an energy-wave simulator
and a swing set that mimics
the behavior of neutrinos.
The Proton Run will open to
the public this spring.
Andre Salles
Illustration: Sandbox Studio, Chicago
symmetry | spring 2013
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The secret of
Picasso’s paint
Although he was one of the few
artists who attained wealth
from his trade, Pablo Picasso
used inexpensive, common
house paint for some of his
works. Perhaps more surprising
is that, decades after he painted
his greatest masterpieces, a
facility with fundamental physics
roots identified that paint by
using a powerful instrument to
peer, for the first time, at individual
pigment particles comprising
some of Picasso’s paintings.
Judging from letters
Picasso wrote to his dealer
and photographs of his studios,
art historians had previously
concluded that Picasso preferred
to use an ordinary house paint
manufactured by the famous
French company Ripolin. Yet
these letters and photographs
offer only visual identification,
which can leave room for error,
says Art Institute of Chicago
conservation scientist
Francesca Casadio, who was
part of a project the institute
led to study Picasso’s paints.
Volker Rose, a physicist at
Argonne National Laboratory,
happened to hear about
Casadio’s work and proposed
that Argonne’s synchrotron
light source, the Advanced
Photon Source, could offer
a closer look, thereby settling
any skepticism.
Synchrotrons were just getting
their start in the mid-1940s,
around the same time that
Picasso produced some of his
most famous works. With
their intense beams of X-rays,
synchrotrons are capable of
viewing the nanoscale structure
of just about any material—
from advanced electronics to
viruses.
Casadio leapt at the chance,
and soon four samples, each no
bigger than the point of
a pin, were carefully removed
from the edges of paintings
or where slight damage was
present. Rose and his colleagues
trained Argonne’s synchrotron
X-ray beam on these minuscule
samples and found that the
concentration and ratio of
impurities, including iron and
lead, within the paint chips
proved identical to mid-20th
century Ripolin paint samples.
The group had a match.
More surprising to Casadio
was the low level of impurities
within the pigments, which
meant that Ripolin was manufacturing
high-quality house
paint during that time. In fact,
the quality was akin to fine
artists’ paints.
Picasso was one of the first
artists to use common house
paint in his paintings. He likely
preferred it, Casadio says,
because it has a glossy finish
and takes days rather than
months to dry.
“The chemical characterization
of paints at the nanoscale
opens the path to a better
understanding of their fabrication,
possible provenance and
chemical reactivity,” she says.
To know this, she continues,
“is to know more about the artist,
the time and place he painted
and how best to preserve
the work.”
Jessica Orwig
Photo and painting courtesy of Conservation Department, The Art Institute of Chicago
symmetry | spring 2013
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signal to background
Photos: COUPP
symmetry | spring 2013
Tiny bubbles
In the hunt for dark matter, the
stealthy stuff that makes up
about a quarter of the universe
but neither emits nor absorbs
light, observational techniques
span from strange to stranger.
The Chicagoland Observatory
for Underground Particle Physics
may top the list.
The COUPP experiment is
the hipster of the dark-matter
crowd. Its detector is a bubble
chamber, a retro bit of 1952
technology originally used to
discover interactions through
the weak force. Its inner vessel
is a clear, quartz tank that
looks a bit like a champagne
flute without a stem.
The champagne in this case
is a clear, heavy liquid heated
and pressurized to the edge of
evaporation, “waiting for any
excuse to boil,” says University
of Chicago physicist Juan Collar,
head of the Fermilab-based
experiment. When a foreign
particle bumps into an atom
of the liquid, the champagne
bubbles.
But it’s stronger than a
gentle fizz. “When these bubbles
appear, it’s a rather violent
process,” Collar says. “You can
actually hear these things with
your ears. The crack is very
high-pitched. The inventor of
the bubble chamber described
it as a ‘plink,’ very crystalline.”
In old-school bubble chamber
experiments, the disturbance
would trigger a still camera to
take rapid photographs of the
bubbles as they formed. In the
upgraded, more-sensitive-thanever
COUPP experiment, motion
sensors connected to video
cameras set off the trigger.
The “plink” does not just alert
the scientists that a particle
has interacted in the detector;
it also gives them information
about what kind of particle it
was. An interaction with an alpha
particle, the most significant
source of distracting background
in the COUPP experiment, is
four or five times louder than an
interaction with a dark matter
particle would be.
Hear it for yourself on
symmetry’s website:
symmetrymagazine.org/COUPP.
Kathryn Jepsen
A different spin
Sporting both physics and physique,
SLAC National Accelerator
Laboratory employees field a
slow-pitch, co-ed softball team
each year, the Spinors, in a
Stanford University recreational
league. Although their competitors
are a mostly younger bunch
of graduate students and staff,
the SLAC team likes to think
they have physics in their favor.
In physics, spinors are used
to plot the spin properties of
elementary particles. First
described by French mathematician
Élie Joseph Cartan in
1913, spinors have a range of
applications in modern physics
and mathematics. They also
share a pronunciation with
“spinners”—baseball slang that
describes curveballs and sliders,
which are pitches thrown with
heavy spin on the ball.
The team’s logo even features
two softballs smashing
together, with smaller spheres
bursting out of the impact—
paying homage to the lab’s
particle collider experiments.
Softball traditions run deep
at SLAC; physics faculty and
students engaged in annual
softball championship games
on the Stanford campus as far
back as the 1950s, even before
the 1962 groundbreaking for
the lab’s two-mile-long linear
accelerator. Since then, an
annual softball game remains
an unbroken tradition.
Photo: Joe Faust, SLAC
8

Team manager Mike Woods,
a 21-year Spinors veteran, says
that since the Spinors first
formed in 1991, the team has
evolved to include a representative
slice of SLAC’s workforce:
men and women, ranging in age
from their 20s to their 80s, who
trade hits and runs with teams
that are often quite a bit
younger.
Woods describes the
Stanford recreational league
as “very laissez faire, very
social—we’ve never even had
hired umpires.” Based on work
schedules and availability, it’s
common for a different set
of players to show up to each
game.
And although the Spinors
haven’t yet won the league
championship, they’ll be back
again next year for another try.
Glenn Roberts Jr.
Fighting for academic
freedom, one scholar
at a time
A recent symposium honoring
Herman Winick’s illustrious
career in synchrotron development
boasted a stellar guest
list. It included friends and
colleagues from across SLAC
National Accelerator
Laboratory, where Winick has
spent the lion’s share of his
50-some-year career, and from
across the world, because
when Winick wasn’t building
experimental facilities at SLAC,
he was busy convincing other
scientists in other countries of
the worth of synchrotrons—
both as tools for discovery and
as teaching tools that could
help strengthen a local academic
community.
But Winick learned decades
ago that an academic community
is only as strong as the
freedom of its scholars. So in
addition to advocating for
synchrotrons, he advocates for
his colleagues, both in science
and beyond. A special guest
at the symposium was Natalia
Koulinka, a Belarusian journalist
who faced danger in her home
country due to her work. Hearing
of Koulinka’s plight through
colleagues, Winick, working
with the Scholars at Risk
Network hosted by New York
University, was instrumental
in bringing her to safety in the
United States.
Scholars at Risk is an international
network of higher
education institutions that promotes
academic freedom and
protects threatened scholars.
Its work includes helping
arrange temporary academic
positions in safe locations.
“Through this program hundreds
of careers, and undoubtedly
some lives, have been
saved,” Winick says. “These are
extraordinary people who have
taken great risks to promote
freedom and democracy in their
home countries. Working with
SAR enables universities such as
Stanford to give them a safe
place to continue their important
work, while at the same time
contributing to teaching and
research at the host university.”
Winick’s own work with
the network has resulted in five
scholars finding sanctuary. He
is now working with the Stanford
Development Office to fund
an endowment at the university
to ensure that additional scholars
can find sanctuary when
necessary.
Still, it’s Winick’s personal
touch that has had the biggest
impact. Koulinka says she’s
grateful for the institutional and
individual donations made on
her behalf. “But my gratitude to
Herman is very specific—he
took personal care of me,” she
says. “He didn’t have to, but
he did.” Now Koulinka is working
with Belarusian colleagues
to document the brief flowering
of journalism in her country
between its independence
in 1991 and when the current
regime took power in 1994.
“Someday I’d like to go back and
share all the knowledge I’ve
gained here,” she says.
Thanks to organizations like
SAR and people like Winick,
Koulinka may have that chance.
Lori Ann White
FROM LEFT: Belarusian journalist Natalia Koulinka with three of the people who helped bring her
to Stanford University and safety: Herman Winick, Nadejda Marques and Larry Diamond.
Photo: Fabricio Sousa, SLAC
symmetry | spring 2013
9

Illustrations: Sandbox Studio, Chicago
10

By Joseph Piergrossi
symmetry | spring 2013
11

eutrinos are as mysterious as they are
ubiquitous. One of the most abundant
particles in the universe, they pass through
most matter unnoticed; billions of them
are passing harmlessly through your body
right now. Their masses are so tiny that
so far no experiment has succeeded in
measuring them. They travel at nearly the speed of light—so
close, in fact, that a faulty cable connection at a neutrino
experiment at Italy’s Gran Sasso National Laboratory in 2011
briefly led to speculation they might be the only known
particle in the universe that travels faster than light.
Physicists have spent a lot of time exploring the properties
of these invisible particles. In 1962, they discovered
that neutrinos come in more than one type, or flavor. By the
end of the century, scientists had identified three flavors—
the electron neutrino, muon neutrino and tau neutrino—and
made the weird discovery that neutrinos could switch
flavor through a process called oscillation. This surprising
fact represents a revolution in physics—the first known
particle interactions that indicate physics beyond the
extremely successful Standard Model, the theoretical
framework that physicists have constructed over decades
to explain particles and their interactions.
Now scientists are gearing up for new neutrino studies
that could lead to answers to some big questions:
If you could put neutrinos on a scale, how much
would they weigh?
Are neutrinos their own antiparticles?
Are there more than three kinds of neutrinos?
Do neutrinos get their mass the same way other
elementary particles do?
Why is there more matter than antimatter in the
universe?
The answers to these questions not only offer a window
on physics beyond the Standard Model, but may also
open the door to answering questions about the universe
all the way back to its origins.
When it comes to finding neutrinos to study, scientists have
three choices.
They can catch naturally occurring neutrinos, such as
the ones produced by nuclear reactions in stars like our
sun, in collisions of cosmic particles with Earth’s atmosphere
or in stellar explosions known as supernovae. Stars like
our sun produce electron-flavor neutrinos, while cosmic
particles and supernovae produce a mixed bag of all three
neutrino flavors and their antineutrino counterparts.
Alternatively, scientists can investigate neutrinos made
in the nuclear reactors that generate power for homes
and businesses. Reactors produce electron-flavor antineutrinos.
Experiments to study neutrinos from this type
of source require the construction of a particle detector
near a nuclear power plant and yield valuable information
about neutrinos and their interactions with matter.
Finally, scientists can deliberately produce neutrinos
for experiments by firing protons from an accelerator
at pieces of graphite or similar targets, which then emit
specific types of neutrinos. Accelerator experiments
have the advantage of being able to examine either
neutrinos or antineutrinos. The intense beams of these
accelerator-made particles increase the chance for
a neutrino interaction to occur in detectors. In addition,
accelerators can produce neutrinos that have higher
energy than those emerging from reactors and the sun.
That makes accelerator experiments extremely valuable
in determining the exact nature of neutrinos.
The two types of manmade neutrino sources have
another advantage: Detectors can be placed at specific
distances from the source, depending on the science
to be done. The optimal distances can range from tens of
12

meters to a few hundred kilometers for reactor experiments
and hundreds to thousands of kilometers for long-baseline
oscillation experiments that use neutrinos from accelerators.
For example, the planned Long-Baseline Neutrino
Experiment, which will use an existing accelerator at Fermi
National Accelerator Laboratory, will have a detector
situated at what former LBNE Spokesperson Bob Svoboda
calls “the sweet spot”—a place just far enough away
that neutrinos should have close to maximum mixing of
their flavors by the time they hit the detector. “From this,
we can learn a great deal about how neutrinos change,”
says Svoboda, who is a professor at the University
of California, Davis. And since LBNE will produce both
neutrinos and antineutrinos, physicists can explore the
differences between matter and antimatter interactions
and what this might mean for the imbalance between
matter and antimatter in our universe.
Neutrino detectors also come in a variety of flavors. Since
neutrinos themselves are invisible to detectors, scientists
must take an indirect approach: They record the charged
particles and flashes of light created when a neutrino
hits an atom, and thus infer the neutrino’s presence.
Because the tiny neutrino interacts with matter so rarely,
the only way to detect it is to put lots of matter in its
way. Super-Kamiokande, a now-classic neutrino detector
in Japan, is filled with 50,000 tons of water. Neutrinos—
produced in Earth’s atmosphere, coming from the sun
and generated by an accelerator 295 kilometers away—
interact with water molecules and produce charged
particles. In turn, these particles produce blue flashes called
Cherenkov radiation. Light sensors within the water tank
capture and record the glow.
The new NOνA detector, under construction in Ash River,
Minnesota, advances SuperK’s technology. Instead of water,
NOνA will use liquid scintillator—a chemical that flashes as
particles pass through—to observe neutrinos fired at the
detector from Fermilab, about 800 kilometers away. At more
than 60 meters long and 15 meters tall, NOνA will be one of
the largest plastic structures in the world.
Instead of using one large tank filled with liquid, the
NOνA detector is highly segmented to glean more information
about each incoming neutrino’s identity and energy.
The 14,000 tons of liquid scintillator will be divided among
hundreds of thousands of tubes made of PVC plastic, says
Fermilab’s Pat Lukens, a project manager for the experiment.
When a neutrino hits a nucleus in the detector, producing
charged particles and flashes of light, researchers
will be able to tell precisely where the interaction occurred
and which way the particles went.
Another technology for getting more information about
neutrino interactions is a grid of wires submerged in a
detector liquid. Placed under high voltage, the wires attract
charged particles that appear when neutrinos interact with
the liquid. This technique, employed in the ICARUS neutrino
symmetry | spring 2013
13

detector in Italy, reveals the precise tracks of the charged
particles produced when neutrinos interact in liquid argon.
For the much larger LBNE detector, to be located at the
Sanford Lab in South Dakota, scientists are designing the
next generation of this type of detector.
The results of recent neutrino experiments have opened
the door to learning much more about neutrinos and
their habits. In 2011, researchers turned on the first set of
detectors at the Daya Bay Reactor Neutrino Experiment
in southern China, hoping to make a key measurement
that would help them understand how one type of neutrino
turns into another.
In March 2012, after only seven months of taking data,
the Daya Bay scientists announced success: They nailed
the measurement of θ 13 (pronounced theta-one-three), one
of three so-called “mixing angles” that describe the oscillation
of neutrinos between one flavor and another. Previous
experiments had shown that θ 13 had to be small, and
scientists had begun to wonder whether this mixing angle
might be zero. The Daya Bay result, in combination with
other neutrino measurements in Japan, South Korea, France
and the United States, showed that the angle is small,
but definitely not zero.
When the size of that angle was announced, neutrino
physicists from around the world cheered. The result
opened up the possibility that neutrinos behave differently
than antineutrinos, which in turn might help explain the
preponderance of matter over antimatter in the universe.
This leaves scientists in a good position to learn more
about one of the most abundant and ubiquitous particles
in the cosmos. New neutrino oscillation experiments “have
a good shot of reaching their goals,” says Boris Kayser,
a theorist at Fermilab. Using the θ 13 result, they could
determine the neutrino mass hierarchy and find out
whether neutrino interactions violate the matter-antimatter
symmetry. These are crucial steps toward understanding
whether neutrinos are the reason for the dominance of
matter over antimatter in our universe.
The most difficult question to answer, Kayser says, is
“What are the unknown unknowns?” While physicists
have some expectations about what they will see, neutrinos
again and again have proven themselves difficult to
predict. Given their bizarre nature, it’s entirely possible
that neutrinos may hold many more surprises for scientists
down the line.
Through experiments that
use a range of approaches
and technologies, physicists
are beginning to get a
fuller picture of neutrino
behavior. The results
could be key to answering
questions that have stymied
scientists for years.
Experiments have shown that neutrinos have a tiny, nonvanishing
mass. Although each neutrino must be a million
times lighter than an electron, their exact masses are not
known. Due to their abundance, neutrinos could account
for several percent of the mass of the universe and play
a significant role in the evolution of the universe.
The frequency of neutrino oscillations depends on the
mass difference among the three different neutrino types.
The NOνA experiment will soon begin to send neutrinos
from Fermilab to Ash River, Minnesota, a distance of 810
kilometers. Scientists hope that the observation of the
resulting oscillations will determine which type of neutrino
is the heaviest and which is the lightest.
Discovering this mass hierarchy is the first step. To
complete their understanding of neutrino masses, scientists
also need to determine the absolute neutrino mass scale
by measuring the mass of one of the neutrino types.
The KATRIN experiment in Germany will attempt to do just
that. The experiment will study the nuclear decay of
tritium, an unstable form of hydrogen. It will compare
the mass and kinetic energy of particles before and after
the decay, which produces an electron antineutrino.
Because the total energy of all particles involved in the
decay must be preserved, scientists can determine the
mass of the antineutrino if they can measure the kinetic
energy of particles with sufficient precision.
14

Scientists have observed the interactions of both neutrinos
and antineutrinos with matter. But it is not clear whether a
neutrino and its antiparticle are two separate particles. In
the case of charged particles, scientists easily can distinguish
particles and their antiparticles by their electric
charge. An electron has negative charge, and a positron has
positive charge. Neutrinos, however, have no electric
charge. So it’s possible that a neutrino could be its own
antiparticle. Theorists refer to this case as the Majorana
neutrino, in honor of Italian physicist Ettore Majorana,
who recognized this possibility. Alternatively, neutrinos
and antineutrinos could be separate particles and
behave according to the equations developed by theorist
Paul Dirac.
Several nuclear experiments, including the Enriched
Xenon Observatory in New Mexico and the Majorana
experiment in South Dakota, aim to settle the Majoranavs.-Dirac
neutrino question. They are examining radioactive
nuclei that exhibit the simultaneous decay of two neutrons—
a process known as double beta decay and first observed
in 1986. This nuclear reaction normally ejects two antineutrinos,
which carry away energy from this decay process.
If the Majorana theory is correct, the two antineutrinos
would also be neutrinos, and they could “cancel each
other out.” The result would be the occasional neutrinoless
double beta decay, in which neither neutrinos nor antineutrinos
are emitted. If experiments observed this rare process,
it would confirm the Majorana theory and pave the way for
many elegant theories that explain how neutrinos acquire
mass and why their mass is so much smaller than that of
any other particle of matter we know.
The Standard Model describes only three neutrino flavors,
each linked to the electron or one of its heavier cousins via
the weak nuclear force—the fundamental force responsible
for radioactive decay and the production of neutrinos.
But a variety of evidence suggests that additional neutrino
flavors may exist, with properties quite different from the
three known types of neutrinos. Experiments will continue to
look for these “sterile” neutrinos, which get their name
from the fact that they do not interact with other matter
through the weak force, as other neutrinos do.
According to the Standard Model, the field associated with
the Higgs boson provides quarks and charged leptons—a
group of elementary particles that includes the electron—
with mass. However, many scientists think the masses of
the ultra-light neutrinos arise, at least in part, in some
other, yet-unknown way. Experiments at the Large Hadron
Collider, which discovered a Higgs-like particle, won’t
be able to measure neutrino properties. Instead, future
nuclear experiments and neutrino oscillation experiments
such as NOνA and the Long-Baseline Neutrino Experiment
could weigh in on the origin of the neutrino masses.
“LBNE and NOνA could help us to interpret the results of
those nuclear experiments,” says Boris Kayser, a theorist
at Fermilab.
According to physicists’ current understanding of the
big bang, matter and antimatter formed in equal amounts
when the universe began. But if that were the case,
every last smidgen of matter should have collided with
every last smidgen of antimatter by now. This would have
released lots of energy and filled the universe with light
and radiation, but left it without any matter at all. “Why isn’t
the universe entirely energy?” asks Kayser. “Why didn’t
the matter and the antimatter annihilate each other as soon
as they were made?”
The answer to that question lies in something called
charge-parity symmetry violation. Finding the right kind of
CP violation to explain the preponderance of matter is a
top priority, and neutrinos are prime candidates. “It’s often
called the Holy Grail of neutrino physics,” says Mark
Messier, co-spokesperson of the NOνA experiment and
a professor at Indiana University.
Previous studies found CP violation—a difference in the
behavior of particles and their antiparticles—among
elementary particles known as quarks. But this CP violation
does not explain the overall matter-antimatter imbalance.
Neutrinos come into play because their incredible
lightness suggests, through a theory called the “see-saw
picture,” that they are the ultra-light relatives of very
heavy particles that lived briefly in the early universe. The
disintegration of these heavy particles may have violated
CP symmetry in a way that led to the present-day imbalance
between matter and antimatter. If that is indeed how
the imbalance arose, then scientists should also find CP
violation in the oscillation of today’s neutrinos.
symmetry | spring 2013
15

Photo: CERN
16

Experiments at the Large Hadron Collider
made a major discovery, but the world’s
highest-energy particle accelerator is just
getting started.
By Ashley WennersHerron
and Kathryn Jepsen
symmetry | spring 2013
17

The Large Hadron Collider, the largest particle accelerator in the
world, started colliding particles more than three years ago. Since
then, scientists have published more than 700 papers detailing the
knowledge they have gained at the cutting edge of particle physics.
Undisputedly, the most famous insight so far has been the discovery
of what could be the long-sought Higgs boson. This particle is thought
to arise from the fluctuation of the invisible “Higgs field” that pervades the
universe, imparting mass to particles that interact with it. Without the
Higgs field, our world would be a much different place.
Even during the excitement of that discovery, thousands of scientists—
more than 1800 of whom are based in the United States—continued the
important work of analyzing the continuing flood of new data pouring out
of their detectors.
There is still much to learn about the new, Higgs-like particle. And there
is still much more territory to cover in the search for new physics. The
LHC will expand its reach dramatically when scientists crank its energy
from 4 trillion to 6.5 trillion electronvolts in 2015.
Beyond discovery
In the LHC, superconducting magnets steer two beams of protons in opposite
directions along a 17-mile ring more than 300 feet beneath the border of
Switzerland and France. The beams cross paths in four locations along
the ring. When a proton from one beam collides with a proton from the
other, the energy of the collision can convert into mass, creating for
a moment new particles.
Massive particles created in collisions are unstable and quickly decay
into less massive particles, leaving a whole zoo of particles for scientists
to study.
Since the LHC turned on, the ATLAS, CMS, LHCb and ALICE experiments—
along with the smaller experiments TOTEM and LHCf—have discovered
a total of three particles.
“At the LHC, the streetlamps are just beginning to turn on, and we can
see under some of the lampposts now,” says John Ellis, a theorist and
professor at King’s College London. “Eventually, the pools of light will join
up and we’ll be able to see everything.”
In December 2011, one of the lamps revealed something new. The ATLAS
collaboration announced the first particle discovery at the LHC—a quark
and antiquark bound together named X b (3P) (pronounced kye-bee-threepee).
Although it had been predicted for years, it took the high rate of
collisions in the LHC to finally expose the particle. Scientists are still studying
it to understand how the quark and antiquark tie together through the
strong nuclear force, which makes the nucleus of an atom stick together, too.
The CMS collaboration found the limelight just a few months later, in May
2012, when they announced the discovery of the excited baryon Ξ b (pronounced
sai-bee), a particle composed of three quarks, including a bottom
quark. Scientists are now analyzing the particle; their work may reveal
insight into how quarks bind together.
And then, in July 2012, both the CMS and ATLAS collaborations
announced the discovery of a new particle that could be the Higgs boson.
Searching for new particles is just one continuing function of the LHC
experiments. Now that scientists have uncovered new particles, they have
another focus—finding out more about them.
The new Higgs-like particle, for example, seems to fulfill at least the
minimum role of the Higgs boson, as it interacts with particles in more or
less the expected way. But observations of the new particle’s properties—
its spin, parity and detailed interactions—could show it to be a different kind
of Higgs than the one predicted by the Standard Model, the theory used to
explain the makeup and interaction of particles and forces in our universe.
If it turns out that the particle is not the Standard Model Higgs boson,
scientists will learn that there are new phenomena whose descriptions may
require new underlying principles. One popular alternative model under
18

The Large Hadron Collider drives
two beams of particles on a
collision course around a 17-mile
ring located more than 300 feet
underground at the border of
France and Switzerland.
Photo: CERN
symmetry | spring 2013
19

Physicist Despina
Hatzifotiadou navigates a
maze of color-coded wires
at the ALICE detector, one
of six detectors at the Large
Hadron Collider.
Photo: antoniosaba/CERN
20

investigation is called supersymmetry. It posits that each particle of the
Standard Model has a related, more massive partner particle. In this model
and others, there would be more than one Higgs boson. Alternatively, it
could be that the Higgs boson is made of other, even smaller particles.
Or it could be that the Higgs exists in more than our three dimensions
of space.
“We could be looking at a new framework,” says Joao Varela, a physicist
with the Portuguese institute LIP and CMS deputy spokesperson. “It may
not be the Standard Model or even supersymmetry. It might be something
else entirely.”
Conversely, if the Higgs turns out to be the particle scientists expected
to find, physicists will have finally discovered every piece predicted in the
Standard Model.
More than new particles
Yet even with a Standard Model Higgs, questions will remain in particle
physics theory.
Particle physics research encompasses three intertwining frontiers:
the energy frontier, the intensity frontier and the cosmic frontier. Energy
frontier experiments involve converting energy into mass at particle colliders
such as the LHC; intensity frontier experiments use intense beams of
particles to study rare processes and make high-precision measurements;
cosmic frontier experiments use the cosmos as a laboratory and also
study particles that reach Earth from distant sources.
Work at all three frontiers aims in part to resolve a major contradiction
in particle physics theory. The masses of force-carrying particles such
as the Higgs boson, the W boson and the Z boson are all relatively similar,
between 80 and 125 times the mass of the proton. Within the Standard
Model, there is no explanation for why the masses of these particles—each
associated with a force that governs interactions between particles—
should have these values, nor why the Higgs mass should be so similar to
the other two. In fact, theorists have argued that these values are “unnatural”
in the Standard Model, and that the findings beg for an explanation.
Theorists have proposed many new models that can account for these
strangely low masses. All of these new models require the addition of new
fundamental particles to the Standard Model. Conveniently, some of the
extra particles predicted are good candidates to fill the role of dark matter,
the matter that scientists have found indirect evidence for in cosmic
frontier experiments but have never observed directly.
So far, LHC experiments have not found these extra fundamental particles.
(The two new particles found thus far, other than the Higgs-like boson,
are composite, not fundamental.) But even if they did, there would be
another hitch: Adding particles to fix the problems of the inexplicably light
Higgs and invisible dark matter causes a different kind of trouble. The
contradiction appears in something called flavor physics.
Some particles come in multiple copies with different masses. These
iterations are called flavors. Neutrinos, rarely interacting particles that are
a favorite subject of intensity frontier experiments, come in three flavors.
Likewise, there are three types of electrically charged leptons: the electron,
muon and tau. Quarks, the particles that make up atomic nuclei, come in
different types as well: up, down, charm, strange, bottom and top.
Sometimes a particle will transform from one flavor to the next. Based
on the Standard Model, scientists can predict how often this should happen,
if at all.
Scientists’ current predictions are calculated based on the Standard
Model. But if there’s something beyond that model, one or more undiscovered
particles, scientists expect to find that their Standard Model predictions for
flavor mixing do not precisely match their experimental results.
So far, that has not been the case. Measurements of flavor mixing at
intensity frontier experiments and energy frontier experiments—including
LHCb, CMS and ATLAS—have conformed nicely with standard predictions.
symmetry | spring 2013
21

“It’s the tension between the frontiers that’s really exciting,” says Andrew
Cohen, a theorist at Boston University. “We’re investigating this fundamental
mystery of the energy scale of the W, Z and Higgs bosons on all three fronts.”
Results from the LHC experiments will continue to provide essential
contributions toward resolving this conflict. Studying the properties of the
new, Higgs-like particle and conducting direct searches for new particles
will be the tasks of the ATLAS and CMS experiments. Flavor physics and
indirect searches for new particles are more the specialty of LHCb. In their
own ways, CMS, ATLAS and LHCb are all working to make more and more
precise measurements to more rigorously test the Standard Model.
The ALICE experiment has a slightly different specialty: delving into
understanding the behavior of the early universe. ALICE was designed to
study collisions of heavy ions, which produce a very hot state of matter
called the quark-gluon plasma. Scientists think the universe began in this
state, a primordial soup from which everything around us grew. ALICE
results, like those from the other LHC experiments, may have an important
impact on all three frontiers of particle physics.
More to come
Starting in March 2013, the LHC’s long shutdown will give scientists, engineers
and technicians the opportunity to upgrade the machine to run close to its
design energy. Each beam will operate at 6.5 trillion electronvolts.
Scientists expect to collect data from more than 200 quadrillion particle
collisions after the machine switches back on in 2015. At higher energies,
they will be able to see even more interesting events.
“The same amount of data at a higher energy is worth more,” says Ian
Hinchliffe, a physicist with Lawrence Berkeley National Laboratory
and member of the ATLAS collaboration. “With the planned upgrades, we’ll
increase the LHC’s sensitivity by a factor of 10.”
Albert Einstein’s famous theory of relativity states that the energy of
a particle is related to its mass; the two are different sides of the same
coin. The LHC puts the theory to work, pumping up particles to high energies
and smashing them into one another in order to transform that energy
into mass in the form of new particles.
Collisions at higher energies can create particles with more mass. At
13 trillion electronvolts, the LHC will be able to access a new realm of
masses and states of matter never before seen in manmade accelerators.
“The increase of energy gives a much greater reach, particularly for
heavy objects with a higher mass,” says Andy Lankford, a physicist with
the University of California, Irvine, and deputy spokesperson for ATLAS.
“It gives us the ability to explore the unknown.”
A high-energy future
Exploration at the LHC has only just begun.
“There are many reasons to be excited for the next five to 10 years and
beyond,” says Joe Incandela, a physicist with the University of California,
Santa Barbara, and CMS spokesperson.
Through careful studies, scientists will determine the properties of the
new, Higgs-like particle. They will find out whether the Standard Model
is a done deal or whether it has steered them astray. And they’ll have the
opportunity to find the unexpected.
Nature certainly has more mysteries for scientists to explore, and once
the accelerator begins running near full capacity in 2015, researchers will
have even better tools to search for new physics.
“We are at the beginning,” says Aleandro Nisati, who leads the ATLAS
collaboration’s studies of how the LHC upgrades will expand the potential
of physics analyses. “This is a new, big chapter in high-energy physics.”
22

University of California,
Santa Barbara, physicist
and CMS Spokesperson
Joe Incandela watches a
continually updating display
of recent collision
events on a screen in the
CMS control room.
Photo: CERN
symmetry | spring 2013
23

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24

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symmetry | spring 2013
U N I V E R S E
B y G l e n n d a C h u i
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The pursuit of dark matter and dark energy
is one of the most exciting—and most
challenging—areas of science. Now researchers
think they’re beginning to close in.
25

If you could use only 5 percent of the alphabet, you’d be stuck with
the letter A. Five percent of a complete daily diet is a slice of dry toast.
Yet that’s all we have, or at least all we can perceive, of the place we call
home.
Less than 5 percent of the universe is ordinary matter made of quarks,
electrons and neutrinos.
The rest is dark matter (23 percent) and dark energy (72 percent). They
have nothing in common, it seems, except our inability to see them directly
with the tools we have at hand and their profound influence on the visible
universe.
Like explorers at the brink of any new frontier, we want to know more.
We want to know why galaxies hold together rather than fly apart, why
the universe is expanding faster and faster, and how a shadow world of
unseen, unexpected particles and/or forces and/or things we can’t even
imagine yet could intertwine with our own.
We want to know what this dark stuff is.
In hot pursuit
The pursuit of dark matter and dark energy is one
of the most exciting challenges in physics, and
it’s being carried out on nearly every conceivable
front: deep underground and far out in space, on
the vast scale of the cosmos and the infinitesimally
tiny scale of subatomic particles, with sound
waves from the big bang and the brilliant remains
of exploding stars.
Although years of research have yet to reveal
the nature of dark matter and dark energy, few
scientists doubt they exist; their distinctive fingerprints
are all over the cosmos, shaping the growth
and evolution of the large-scale structures, such
as galactic filaments and clusters, we see today.
Now scientists think they’re beginning to
close in.
Theory, observations and sophisticated computer
simulations are converging on candidate
particles and promising techniques that seem likely,
at last, to tell us what dark matter is made of.
“You should be able to see actual dark matter
particles just beyond where the current generation
of experiments is,” says Glen Crawford, director
of the Research and Technology Division of the
Office of High Energy Physics at the US Department
of Energy, which is a major funder of dark
universe projects. “People have been coming
up with clever new ways of doing these dark
matter experiments.”
As for dark energy, scientists know almost
nothing about it at all. But over the next 10 to 15
years, projects now planned or under way will
map out the growth and evolution of the universe
ever more precisely and further back in time,
refining key measurements that offer strong clues
to what dark energy is.
Dark matter
The first hint that the dark universe existed came
in the 1930s with the realization that there must
be much more mass in clusters of galaxies than
we can see; otherwise they’d fly apart. This
invisible mass is the dark matter.
Although dark matter gives off no light, it does
interact with the rest of the universe through
gravity, and this gives us a way to find it. The
gravitational tug of unseen dark matter bends
light coming from faraway objects. By analyzing
these distortions, scientists have found clumpy
halos of dark matter surrounding many galaxies,
including our own. They’ve also found a huge
blob of dark matter that separated from normal
matter when galaxies in the Bullet Cluster collided.
Not all scientists are convinced that dark
matter can account for all this. There is, for
instance, a rival theory that contends these observations
can be explained by modifying gravity.
But the consensus, for now, is that dark matter
is real. And after considering various forms it
could take, most of the attention has focused on
two candidates: WIMPs and axions.
Enter the WIMP
The WIMP, or Weakly Interacting Massive Particle,
is thought to be abundant—by one estimate,
a billion of them pass through your body every
second—but also shy. The chance that a WIMP
will interact with the nucleus of an atom is
incredibly small. (Chances are that one per year,
26

at most, will interact with an atomic nucleus
inside you.)
But this very weak interaction with ordinary
stuff gives scientists a second way, other than
gravity, to detect dark matter. They set up detectors
made of very dense, ultra-pure material,
buried well underground to keep other particles
out, and wait for a WIMP to fly through. If and
when a WIMP hits the nucleus of one of the
atoms in the detector, it will nudge that nucleus
just a smidge out of place, producing a flash of
heat or light that can be recorded and analyzed.
(The WIMP itself goes happily on its way.)
These “direct detection” experiments may
have already seen WIMPs.
According to theory, our solar system’s movement
through the dark matter halo of our galaxy
should produce, from our perspective, “a wind of
WIMPs,” says Lauren Hsu, a particle astrophysicist
at Fermi National Accelerator Laboratory. The
Earth’s yearly orbit around the sun would cause
that wind to fluctuate, so that more WIMPs hit
detectors in June than in December. That’s the
pattern that DAMA/LIBRA, an experiment deep
under Italy’s Apennine Mountains, began seeing in
the mid-1990s.
However, while two other experiments reported
seeing something similar, it was not exactly
the same, and others saw no effect. So the jury
is still out, and researchers continue to look for
ways to understand the DAMA/LIBRA findings.
Today, about a dozen direct-detection experiments
are snugged into deep caverns in North
America, Europe and Asia. They include the
Cryogenic Dark Matter Search in Minnesota’s
Soudan Underground Laboratory; XENON100,
beneath 5000 feet of rock in Italy’s Gran Sasso
National Laboratory; the Large Underground
Xenon experiment, beneath the Black Hills of
South Dakota; and XMASS, taking data in Japan’s
Mozumi Mine.
As scientists continue to ratchet up the detectors’
size, purity and sensitivity, the chance of
unequivocally discovering a WIMP is getting better.
“I think this is a special time,” says Laura
Baudis, an experimental astroparticle physicist
at the University of Zurich, “because we needed
more than 10 years, even 15, to develop the
technology that was needed in order to test this
very low interaction of dark matter with normal
matter. Now we have reached, with at least some
of the technologies, the level where we can go
and build large detectors.”
Meanwhile, scientists have been scanning the
skies for indirect signs of WIMPish activity.
According to the prevailing theory, all the WIMPs
that ever existed were made in the nanosecond
after the big bang. Since then their number has
slowly declined, as they decay (quite rarely) into
other particles or meet up with their antimatter
counterparts and annihilate. Indirect WIMP
searches, including the orbiting Fermi Gamma-ray
Space Telescope and the PAMELA satellite, look
for the results of those decays and annihilations
in space, while IceCube and ANTARES look for
neutrinos produced by annihilations in the center
of the Earth and sun. So far, these searches
haven’t found any conclusive confirmation either.
It’s also hoped that WIMPs will show up in
particle collisions at CERN’s Large Hadron
Collider—not as particles per se, but as a certain
amount of energy and momentum that goes
missing in particular particle decays.
Aiming for axions
Still other experiments are pursuing axions, another
type of dark-matter particle that comes out of
a separate set of theories. It is even more weakly
interacting than the WIMP and even harder
to snag.
“If you had an axion on the table in front of you,
it would take 10 to the 50th years to decay. That’s
an extraordinary lifetime,” roughly a billion billion
billion billion times the age of the universe, says
Leslie Rosenberg, principal investigator for the
axion-detecting ADMX experiment at the
Illustration: Sandbox Studio, Chicago
symmetry | spring 2013
27

University of Washington. So for a long time
people thought it would be impossible to detect
the axion by looking for its decay products.
However, in the 1990s, Rosenberg and his
colleagues came up with the idea of scaling up
a technique that encourages axions to decay by
confining them in a very strong magnetic field.
Every blue moon, as he puts it, an axion would
interact with this magnetic field and produce
a detectable photon of microwave light.
It’s a difficult search, but Rosenberg says he
thinks an answer is near: “We’re on the cusp of
finally being able to have a definitive statement
about whether axions make the dark matter.”
A many-pronged approach
Actually, though, there’s no reason why the universe
should contain just one kind of dark matter.
After all, a lot of different particles make up our
measly 5 percent.
“Most studies that have been done have made
the simplest assumptions,” says physicist Aaron
Roodman of SLAC National Accelerator
Laboratory. “But there’s no reason to think dark
matter will be simple.”
That’s why scientists are taking so many
different approaches to the dark matter search.
Each technique has different strengths and
weaknesses, and different sources of systematic
error. And each one is sensitive to particles of
certain masses or characteristics, but not to others.
“No one’s ever going to believe the first signal,
even if it’s rock-solid, because it’s such a huge
problem,” says Jonathan Feng, a theoretical physicist
at the University of California, Irvine. “You
need to double-check it, confirm it in as many
ways as possible. And exactly how that’s going
to go probably depends on what the first signal
looks like.”
Dark energy
If dark matter is mysterious, dark energy is even
more so. No one had a clue it was out there
until the late 1990s, when observations of supernovae—exploding
stars that shine 5 billion
times brighter than the sun—showed that the
expansion of the universe is accelerating.
Something is pushing the cosmos outward,
counteracting the force of gravity—an observation
that earned three American scientists the 2011
Nobel Prize in physics.
What is this something? No one knows. It could
be Einstein’s cosmological constant, another way
of saying that “there is a mass, a density, to
completely empty space—that space weighs
something,” as cosmologist Rocky Kolb of the
University of Chicago puts it.
Another possibility is that dark energy is a
field—perhaps a fluctuating field known as
quintessence, named in allusion to a classic “fifth
element” proposed by the ancient Greeks, or
something like the Higgs field that imparts mass
to other particles.
While scientists haven’t thought of a way to
detect dark energy directly, they do have a
rough consensus about where to look for its
influence on the shape of the universe and on the
methods most likely to expose its character.
“At the moment our only measurement of dark
energy is what it has done to the expansion of
the universe,” says cosmologist David Schlegel of
Lawrence Berkeley National Laboratory. “We’re
measuring what there is to measure, basically.”
To make these measurements, scientists use
a combination of techniques to make detailed
images of the cosmos, translate those images
into 3D maps and figure out how fast big, bright
objects are moving away from us—and thus
how fast the universe was expanding at specific
points in its history.
They have three handy yardsticks for measuring
these faraway distances and speeds.
One is exploding stars—specifically Type Ia
supernovae, which all give off about the same
amount of light when they explode. The farther
away they are, the fainter they appear, so they
serve as unique markers on the cosmic distance
scale. Tracking their movements led to the
discovery of dark energy, and, as scientists find
and track more of them, they are refining the
story these stellar explosions tell.
A second is to study the clumping of matter
in the universe, the so-called “growth of structure.”
Scientists can infer how the universe has
expanded by measuring the number of galaxy
clusters with different masses and at different
times. And the distribution of all mass in the
universe can be studied by detecting the bending
of the light from distant galaxies by all matter,
dark or otherwise, between them and us.
28

29
symmetry | spring 2013

The third is “baryon acoustic oscillation,”
patterns of frozen sound waves left over from
the big bang. These patterns help determine
the average separation between galaxies, which
increases as the universe expands and serves
as another cosmic ruler.
All of these methods use spectroscopy—breaking
the light from distant objects into a rainbow of
colors for analysis. When an object is moving away
from us, its light is shifted down toward the red
end of the spectrum, just as a train’s whistle shifts
to a lower tone as it chugs away. This redshift tells
us how fast the object is moving as the cosmic
expansion carries it away.
From what they’ve learned so far, scientists
think this is what happened: For the first few billion
years after the big bang, emerging galaxies and
other clumps of matter were so close together
that their combined gravitational pull slowed the
expansion of the universe. But by about 5 billion
years ago, the galaxies had dispersed enough that
dark energy—which had been a constant, repulsive
force all along—won out over this gravitational
drag, and the expansion began to accelerate.
Seeking the invisible
In November 2012, the Baryon Oscillation
Spectroscopic Survey, part of the larger Sloan
Digital Sky Survey, released the first measurement
of how fast the cosmos was expanding
before this transition occurred, 3 billion years after
the big bang.
Another major project, the Dark Energy Survey,
will start mapping the universe in 2013 from a
telescope perched on a Chilean mountaintop. It
aims to chart the expansion of the universe and
the growth of large-scale cosmic structures back
14 billion years.
And in Texas, scientists are upgrading the
McDonald Observatory’s Hobby-Eberly Telescope
to carry out a dark-energy experiment called
HETDEX, which has a goal of measuring the
positions and movements of 1 million galaxies
starting in 2014.
Meanwhile, scientists are preparing a new
generation of dark-energy projects to attack the
problem from the ground and in space.
In the relatively near term, the US Department
of Energy has declared a need for an advanced
spectroscopic survey, and scientists are drawing
up a proposal for one.
Longer term, the Large Synoptic Survey
Telescope, which should begin construction in
Chile next year, will use the world’s largest digital
camera to create the deepest, widest and
fastest portrait of the night sky ever made from
the ground. The resulting data—an unprecedented
6 million gigabytes per year—will shed
light on both dark matter and dark energy.
And in Europe, scientists recently received
approval to build a billion-dollar space telescope,
Euclid, to study the expansion of the universe,
with an eye to understanding dark energy and
dark matter.
The ever-more-precise measurement of
cosmic structure and expansion should eventually
allow scientists to determine what’s called the
“equation of state”—the ratio of pressure to density—of
the dark energy itself, Roodman says.
If the dark energy is Einstein’s cosmological
constant and it has remained steady over time,
that ratio should be close to minus 1; weirdly, this
would mean it had negative pressure. If it has a
different value, scientists will know that the dark
energy is something else.
The path ahead
Scientists are confident that the experiments
under way and those now under construction will
bring us significantly closer to understanding
the dark universe around us. But in particle physics,
planning the next experiments—and developing
the technologies that make those experiments
possible—takes decades.
In August, hundreds of scientists will meet in
Minnesota for what is known as the Snowmass
Meeting—a once-a-decade gathering to identify
the most important and pressing questions in
particle physics and the experiments the United
States should pursue to answer them. The
meeting will cover all aspects of the field, from
experiments that smash particles together at
very high energies and intensities to detectors
and other instruments, theory, computing and
public outreach.
Dark matter and dark energy will be major
items on the agenda. And while the meeting is not
designed to set formal priorities, researchers
hope to reach consensus on the next generation
of approaches and projects to probe the dark
universe from a number of complementary
angles and with the funding realistically available.
“Are there clever little things that can be
done to push the envelope without breaking the
bank?” asks Feng, who is co-organizing the
part of the meeting that covers dark energy and
dark matter. “There are a lot of questions about
how to design a national program that covers as
many bases as possible within the budget.”
One thing seems certain: The dark universe
offers some of the most compelling science
imaginable.
“It’s the area where we know the least, so it’s a
natural place to look for discoveries,” Roodman
says. “I think in most cases the science should be
quite rich, in the sense that when you answer
some questions, there will be other questions to
ask. This is, I think, where a lot of the next discoveries
will come from.”
symmetry | spring 2013
31

deconstruction: long-baseline neutrino experiment
The US Department of Energy has approved the conceptual design of a new experiment
that will be a major test of our current understanding of neutrinos and their
mysterious role in the universe. Scientists can now proceed with the engineering
design of the Long-Baseline Neutrino Experiment, which aims to discover whether
neutrinos violate the fundamental matter–antimatter symmetry of physics. If they do,
physicists will be a step closer to answering the puzzling question of why the
universe is filled with matter while antimatter all but disappeared after the big bang.
So far, quarks are the only known particles that violate this fundamental symmetry.
But the observed effect in quark interactions is not of the right kind to explain the
abundance of matter over antimatter in our universe.
Scientists know that neutrino interactions also could violate matter–antimatter
symmetry. If so, how strong is the effect? Scientists designed the LBNE experiment
to discover the answer. They plan to break ground in 2015.
Text: Kurt Riesselmann
Illustration: Sandbox Studio, Chicago
Wilson Hall
NEUTRINO PRODUCTION
Fermilab,
ILLINOIS
EXISTING PROTON
ACCELERATOR
PARTICLE DETECTOR (upgrade)
Start on the prairie
Surrounded by 1000 acres of tallgrass prairie, the
accelerators at the Fermi National Accelerator
Laboratory in Batavia, Illinois, will produce beams
of muon neutrinos and antineutrinos for the
Long-Baseline Neutrino Experiment. Every 1.3
seconds, an accelerator will smash a batch of
protons into a graphite target to make short-lived
pions. Strong magnetic fields will guide and
focus the pions to form a beam that points toward
the LBNE detector in South Dakota. The pions
will travel a few hundred feet, decay and produce
muon neutrinos and antineutrinos.
Through the earth
Neutrinos can travel long distances through rock
and other matter without a scratch. The LBNE
neutrino beam will travel 800 miles straight through
the earth from Batavia, Illinois, to the Sanford
Lab in Lead, South Dakota—no tunnel necessary.
The trip will take less than one-hundredth
of a second, enough time for some of the muon
neutrinos to transform into electron neutrinos
and tau neutrinos. Scientists call this process
neutrino oscillation.
32

From around the world
The LBNE experiment will send beams of neutrinos and antineutrinos from the Department of Energy’s
Fermilab, 40 miles west of Chicago, to the Sanford Lab in the Black Hills of South Dakota. More
than 350 scientists and engineers from more than 60 institutions have joined the LBNE collaboration so
far. They come from universities and national laboratories in the United States, India, Italy, Japan
and the United Kingdom. The collaboration continues to grow, and project leaders seek and anticipate
further international participation.
Better than best
LBNE scientists plan to upgrade their ambitious experiment when additional resources become available,
possibly from international collaborators. At Fermilab, they would add an underground hall with
a particle detector that would measure the exact number of muon neutrinos produced. At the Sanford
Lab, they would construct a large particle detector thousands of feet underground, still within the
cone of the neutrino beam from Fermilab. Better shielded from cosmic rays than the planned detector
at the surface, this detector would be more sensitive to symmetry-violating neutrino interactions.
It also could look for neutrinos from supernovae and for the predicted but never observed decay of
protons. With support from their collaborators, the LBNE project team hopes to implement these
upgrades as soon as possible.
PARTICLE DETECTOR
Headframe
800 miles
Sanford Lab,
South Dakota
UNDERGROUND
PARTICLE
DETECTOR (upgrade)
Start on the prairie
Surrounded by 1000 acres of tallgrass prairie, the
accelerators at the Fermi National Accelerator
Laboratory in Batavia, Illinois, will produce beams
of muon neutrinos and antineutrinos for the
Long-Baseline Neutrino Experiment. Every 1.3
seconds, an accelerator will smash a batch of
protons into a graphite target to make short-lived
pions. Strong magnetic fields will guide and
focus the pions to form a beam that points toward
the LBNE detector in South Dakota. The pions
will travel a few hundred feet, decay and produce
muon neutrinos and antineutrinos.
EXISTING
LABS
Through the earth
Neutrinos can travel long distances through rock
and other matter without a scratch. The LBNE
neutrino beam will travel 800 miles straight through
the earth from Batavia, Illinois, to the Sanford
Lab in Lead, South Dakota—no tunnel necessary.
The trip will take less than one-hundredth
of a second, enough time for some of the muon
neutrinos to transform into electron neutrinos
and tau neutrinos. Scientists call this process
neutrino oscillation.
symmetry | spring 2013
33

essay: a galaxy with a view
A physicist, a software developer and a writer step outside one night to take in nature’s
beauty at a mountaintop observatory in Chile.
takes my eyes a few moments to adjust
It when I walk out the door of the Victor M.
Blanco telescope in Chile around 2 a.m.
I take a few careful steps into the moonless
October night with a physicist and a software
developer following close behind. We are working
the overnight shift at the Cerro Tololo Inter-
American Observatory, monitoring the newly
installed Dark Energy Camera. The most powerful
digital survey camera in the world, DECam will
spend the next several years studying the sky to
determine how dark energy has shaped the
evolution of the universe.
We have decided to do a little stargazing of
our own.
The stars seem to blink into being, one at
a time, as our eyes adjust. First come the brightest
ones, the touchstones of ancient constellations.
Orion raises his club and shield against the
charging bull, Taurus—both turned absurdly on
their heads in the southern sky. The Magellanic
Clouds, a pair of dwarf galaxies in orbit around
the Milky Way, glow in matching violet hues.
Layer by layer, dimmer stars appear, slowly
revealing the depth of the heavens.
This is the sky the first explorers saw, we say
to one another. We wonder if site surveyors
thought the same thing when they scaled the
mountain on horses and mules in the 1960s,
looking for the right place south of the equator
to build an international astronomical observatory.
At the time, there were fewer than a dozen
observatories in the Southern Hemisphere,
compared to closer to 100 in the north.
They chose this spot, 2200 meters above sea
level, far from the distracting lights of the towns
below. The winds from the Pacific Ocean, a
stripe of blue visible in the distance during the
daytime, are said to cleanse the air of dust.
Today clusters of telescopes under white and
silver domes dot the rocky landscape like enormous
eggs, each tucked into its own square nest.
Their glass eyes track the stars each night as
they sweep across the sky.
The stars’ motion is an illusion caused by the
rotation of the Earth, but we’re not the only
ones who are moving. Dark energy pushes the
universe to expand at an ever-increasing speed.
Depending on how dark energy works—something
DECam seeks to understand—the universe might
eventually be a much larger, colder, dimmer place.
If we could come back here in tens of billions
of years, these telescopes might see nothing but
blackness beyond the immediate neighborhood
of the Milky Way.
Tonight, though, we stand in the still, cold air,
amazed by a sky full of light.
Kathryn Jepsen
34

Photography by
Reidar Hahn, Fermilab
symmetry | spring 2013
35

application: cancer detection
Gamma cameras
see through
dense tissue
As the most energetic form of light, gamma rays
are great for revealing astrophysical phenomena
such as supermassive black holes and merging
neutron stars.
They’re also proving excellent for detecting
early stages of cancer.
“The search for the most violent events in the
universe has led to the development of the
most sensitive gamma-ray detectors,” says Gunnar
Maehlum, a former
particle physics
researcher who is now
general manager at
Gamma Medica-Ideas,
a company that
designs integrated
circuits for radiation
detection. “Due to
their superior performance,
they are now
being introduced into
medical diagnostic
A tumor hidden behind dense tissue
in a traditional mammogram (left) equipment.”
appears as a bright spot in a gamma-ray For tumors hidden
mammogram (right).
within dense tissue,
traditional screening
sometimes isn’t detailed enough to reveal the
cancer. This is especially true in the case of
breast cancer. For the 30 percent of women
with dense breast tissue, traditional X-ray mammography
doesn’t work well because, in an
X-ray image, dense tissue appears opaque and
white, just like a tumor.
Using detectors and integrated circuits
designed for particle-physics experiments, a group
of researchers in particle physics, nuclear medicine,
medical physics and astronomy developed
a compact semiconductor-based imager with high
spatial resolution that reveals tumors even within
dense tissue.
These gamma-ray mammography cameras use
cadmium-zinc-telluride detectors and are highly
accurate, says Michael K. O’Connor, a professor of
radiologic physics at Mayo Clinic in Rochester,
Minnesota.
The cameras offer improved resolution of tissue
with their 1.6-millimeter pixel size, about two
times better than conventional gamma cameras.
They can also image all the way to the edge of
the detector, unlike traditional gamma cameras,
which have a large ring of dead space around
the center.
Scans: Michael O’Connor, Mayo Clinic
“With these cameras you can detect tumors in
the 5- to 10-millimeter range,” O’Connor says.
“Ten millimeters is an important number for tumor
size: Once you can detect a tumor this size or
smaller, the prognosis gets much better. With a
tumor of this size, the chances are that it’s
localized and surgical removal of the tumor can
cure the patient.”
To detect potentially dangerous cell growth
using such a gamma camera, a physician injects
a radioactive tracer into the patient’s arm. Due
to cancer cells’ high metabolic activity, these cells
accumulate more of the tracer than normal
cells and so emit more gamma rays as it decays.
The camera can detect this and records a highresolution
image of the tumor.
There are approximately 15 to 20 of these
advanced gamma cameras currently in use at
hospitals around the United States, and they are
proving very successful. While traditional mammography
reveals about three tumors for each 1000
women with dense tissue screened, large clinical
trials at Mayo Clinic have shown that the gamma
cameras reveal about 10 cancers for each 1000
women with dense tissue screened.
“Because dense tissue diminishes the ability of
mammography to detect the cancer, but also
increases a woman’s chances of getting breast
cancer, it’s important to find alternative techniques,”
O’Connor says. “This is one of the most
promising. We’ve really been astounded by how
well it works.”
Kelen Tuttle
Photo: Gamma Medica, Inc.
symmetry | spring 2013 symmetry | spring 2013
36

logbook: higgs-like particle
CMS
PowerPoint slide from
Joe Incandela’s June
2012 presentation to the
CMS collaboration.
This week is a very important one for our experiment
Our Higgs results are being consolidated and finalised. There are preliminary
indications that we are close to 5σ using 2011+2012 data
more in Thursday/Friday approvals
This would clearly be an outstanding achievement, for which the Collaboration has
been working extremely hard for more than 20 years.
What struck me mostly over the last days:
Latest data useful for ICHEP recorded on Monday 18 June
Sign-off of the data quality made very efficiently on a very compressed schedule
Data processed very fast and successfully
On Sunday 24 June, first Higgs plots containing the full 2012 statistics were
circulating
Note: 2012 (“All good”) data included in our Higgs results:
5.9 fb -1 ~ 90% of the DELIVERED luminosity
IMPRESSIVE achievement at all levels, detector operation, Trigger and Data
Acquisition, Data Preparation, Computing, Performance and Physics analyses, and
the demonstration of a smoothly-working experiment and of people’s high
competence, extreme dedication, and collaborative spirit !
F.Gianotti, ATLAS Weekly, 26/6/2012
ATLAS
PowerPoint slide from
Fabiola Gianotti’s June
2012 presentation to the
ATLAS collaboration.
In June 2012,
particle physicists on
experiments at the Large
Hadron Collider had a secret to keep, just between
themselves and a few thousand colleagues.
Scientists on the ATLAS and CMS experiments at
the European laboratory CERN had both seen signs of a
possible Higgs boson rise above the significance level
customarily required to declare a new particle discovery.
The Higgs boson, an excitation of the field thought to
give mass to other subatomic particles, was the last missing
piece of the Standard Model of particle physics.
Outcomes from different searches had begun to line up,
and even the most obstinate skeptics had begun to agree:
This was looking more and more like the particle scientists
had been pursuing since theorists first predicted it in 1964.
It had been a difficult search, one that became a focal
point of programs at the Large Electron Positron collider
at CERN and later at the Tevatron collider at Fermilab. The
two largest experiments at the LHC tentatively planned
to release their results the first week of July—in two
individual presentations. Scientists designed the CMS and
ATLAS detectors in part to verify one another’s conclusions.
To keep the experiments independent, the scientists
needed to keep their data to themselves.
In late June, each of the two collaborations held an
historic, internal meeting in which it announced—for its
members’ ears only—Higgs search results that came close
to the traditional threshold for a discovery.
As ATLAS Spokesperson Fabiola Gianotti for the first
time presented the slide shown above, with its exhilarating
news, to the entire ATLAS collaboration, she joked that
they should be sure not to show happy faces to the CMS
collaboration.
CMS Spokesperson Joe Incandela presented the slide
at the top of this page during the week that the CMS
Higgs search results were shared with the full collaboration.
In the days that followed, Incandela sent a purposefully
vague email to a private CMS listserv saying little more
than “It has been tentatively agreed that we will show
approved results in a seminar at CERN. More details will
follow soon.”
Of course, ATLAS and CMS physicists interact too often
for the results to have stayed strictly confidential in the
final days before the big reveal. But it was still an electric
moment when, after both experiments presented similar
conclusions in their search for the Higgs, CERN Director-
General Rolf Heuer declared: “I think we have it.”
Kathryn Jepsen

symmetry
A joint Fermilab/SLAC publication
PO Box 500
MS 206
Batavia Illinois 60510
symmetry
USA
explain it in 60 seconds
is a technique that astronomers
use to measure
Spectroscopy
and analyze the hundreds of colors contained in the light
emitted by stars, galaxies and other celestial objects.
Ordinary telescopes show the directions in which
objects are located but offer no information on how far
away these objects are.
Spectroscopic surveys make use of the fact that, as
light travels to us from distant galaxies, it gets stretched
out by the expanding universe and appears redder. By
measuring the light spectrum of a galaxy, scientists can
determine its redshift and thus its distance.
The largest spectroscopic survey to date is the Baryon
Oscillation Spectroscopic Survey, which is being carried
out at the Sloan telescope and will record the spectra of
1.5 million galaxies by the time it’s completed in 2014. BOSS
will offer insight into one of the biggest mysteries of the
universe: dark energy, the enigmatic force that has accelerated
the universe’s expansion over the last 5 billion years.
An even more ambitious spectroscopic survey to
measure the redshifts of 20 million galaxies is now being
developed. In a few years, when this new spectroscopic
survey experiment goes online, we will finally realize the
massive scale of cosmic cartography necessary for truly
sensitive measurements of dark energy.
Klaus Honscheid and Eric Huff
Center for Cosmology and AstroParticle Physics,
The Ohio State University