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6<br />
No.48 SEPTEMBER 27, 2018<br />
CLOSE UP<br />
WWW.DAY.KIEV.UA<br />
By Mariia PROKOPENKO,<br />
photos by Ruslan KANIUKA, The Day<br />
It is a coincidence that Dmytro and<br />
I are speaking on September 10, on<br />
the 10th anniversary of the<br />
launching of the Large Hadron<br />
Collider (LHC), the world’s largest<br />
accelerator of elementary particles,<br />
which is particularly supposed to shed<br />
light on the nature of dark matter. In<br />
essence, this problem is being tackled,<br />
albeit by different methods, at the Niels<br />
Bohr Institute in Copenhagen, Denmark,<br />
where PhD (Physics and Mathematics)<br />
Dmytro YAKUBOVSKYI has been<br />
working for two and a half years as<br />
postdoctoral fellow. At the same time, he<br />
remains a higher doctorate seeker at the<br />
Mykola Boholiubov Institute of<br />
Theoretical Physics in Kyiv and<br />
regularly comes to Kyiv to research<br />
dark matter and deliver lectures to both<br />
students and the public. We met during<br />
one of these visits. We discussed the<br />
current information about dark matter<br />
and approaches to scientific research in<br />
Denmark and Ukraine. We began with<br />
the “birthday boy,” i.e., the LHC.<br />
● ON THE LARGE HADRON<br />
COLLIDER<br />
“In essence, all detectors of the Large<br />
Hadron Collider probe different characteristics<br />
of various elementary particles, including<br />
the well-known ones and the Higgs<br />
boson which was first detected there.<br />
Small detectors ATLAS and CMS and the<br />
detector LHCb are used in this research,<br />
while ALICE is mostly used to explore the<br />
quark-gluon plasma [the so-called “liquid”<br />
form of matter which supposedly exists<br />
in a short period of time immediately<br />
after the Big Bang – Author].<br />
“Almost all the LHC detectors are also<br />
searching for the traces of still unknown<br />
particles that may form dark matter.<br />
They have detected no new particles so<br />
far, which affords ground for developing<br />
alternative models, including the one I<br />
work with. If the LHC fails to find even one<br />
dark matter particle, it will be very probable<br />
that the model we are developing and<br />
watching now, as well as certain experimental<br />
consequences of it, is correct.”<br />
● ON HIS OWN RESEARCH<br />
“Our group in Denmark explores the<br />
neutrino minimal extension of the Standard<br />
Model (often abbreviated as vMSM) of particle<br />
physics. The point is that the Standard<br />
Model has particles called neutrinos which<br />
differ from all the other by their properties.<br />
Moreover, these are in fact the only particles<br />
which the Standard Model does not explain<br />
fully. In particular, it does not explain<br />
a phenomenon discovered about 25 years<br />
ago, for the discovery of which the Nobel<br />
Prize in Physic was awarded in 2015 [to<br />
Takaaki Kajita and Arthur McDonald – Author].<br />
It is neutrino oscillations. Imagine<br />
that one type of neutrino turns into another<br />
and vice versa just during the motion.<br />
This process can be explained, for example,<br />
by means of quantum mechanics, but,<br />
from the viewpoint of oscillation observations<br />
in particle physics, it is a very rare and<br />
unexpected phenomenon. Moreover, these<br />
phenomena of oscillation cannot be explained<br />
by the Standard Model which became<br />
the cornerstone of elementary particle<br />
physics when the Higgs boson was discovered<br />
[in 2012].<br />
“Nobody doubts now that the Standard<br />
Model of particle physics excellently<br />
explains electromagnetic and strong interactions,<br />
i.e. how nucleons, individual<br />
particles in nuclei, interact with one another<br />
and why they keep together, as<br />
well as weak interaction which is responsible<br />
for beta-decay and thermonuclear fusion<br />
in the Sun – in fact the source of life.<br />
Even in outer space, we can see no serious<br />
deviations from the Standard Model, except<br />
for a few ones.<br />
“The aim of the model I work with is to<br />
describe the observed deviations, introducing<br />
as few additional entities as possible.<br />
It is like Occam’s razor – the principle<br />
that the simplest solution tends to be the<br />
right one. Some models introduce a lot of<br />
additional entities, such as supersymmetry.<br />
This is a good model. It was even glorified<br />
by Vakarchuk and others [the rock<br />
group Okean Elzy produced an album,<br />
Dark matter and where to look for it<br />
‘Supersymmetry,’ in 2003 – Author], but<br />
the problem is that this model needs twice<br />
as many particles as we know at the present<br />
moment. We know dozens of elementary<br />
particles now, and each of them needs<br />
a ‘superpartner.’ But we can see none of<br />
them. If the Large Hadron Collider fails to<br />
find traces of supersymmetric particles,<br />
this will mean that the phenomena that occur<br />
outside the Standard Model, such as<br />
neutrino oscillations, need a different solution.”<br />
● ON NON-STANDARDS<br />
IN THE STANDARD MODEL<br />
“The goal of elementary particle<br />
physics researchers is to construct a complete,<br />
closed, and self-consistent model.<br />
Ideally, it should describe all the phenomena<br />
we can see. All of its forecasts should<br />
be checked and all the new particles should<br />
be found. And this model should be free of<br />
internal contradictions.<br />
“It took the Standard Model of particle<br />
physics a long time to emerge – perhaps<br />
from the early 20th century, and this created<br />
a lot of problems, such as non-selfconsistency.<br />
As is known, the sum of probabilities<br />
should be equal to unity. But it<br />
turned out that the model, which described<br />
for the first time the interaction of neutrinos,<br />
beta-decay, and thermonuclear fusion<br />
in the Sun, stipulated that, oddly<br />
enough, the probability of high-energy<br />
processes should exceed unity. It’s nice that<br />
it describes processes very well at low energies,<br />
shows good coincidences with the<br />
experiment, and gives less-than-unity<br />
probabilities, so let it go on working. But<br />
it must be replaced where it gives absurd<br />
answers.<br />
“Actually, why were Weinberg, Salam,<br />
and Glashow awarded the Nobel Prize [in<br />
1979]? Because they were the first to formulate<br />
this model. It is a very complex construction.<br />
The researchers used Einstein’s<br />
idea that not only gravitation, but also other<br />
types of interaction can be connected<br />
with symmetries in a certain variety of<br />
space. This all was very well developed and<br />
resulted in the Standard Model. Now we<br />
can see that the Standard Models works almost<br />
flawlessly, but the question of neutrino<br />
oscillation, and not only this, still remains.”<br />
● ON THE PROBLEMS OF DARK<br />
MATTER AND ANTIMATTER<br />
“There are more problems with the<br />
Standard Model in outer space. Firstly, it<br />
is the problem of dark matter. From the<br />
viewpoint of gravitational interaction or<br />
dynamics, one fourth of our Universe consists<br />
of dark matter and only 5 percent of<br />
conventional substance. There is much<br />
more dark matter. We don’t know what it<br />
is, and there is a hypothesis that it consists<br />
of particles. If so, it’s not the particles we<br />
know – not electrons, protons, photons, or<br />
even common neutrinos. And the problem<br />
is how to find these particles.<br />
“There are dozens of hypotheses about<br />
what dark matter may consist of. Researchers<br />
are seeking for the manifestations<br />
of these particles in very diverse experiments<br />
and not only – for example, in<br />
radiation from space objects. There are<br />
even special detectors that can allegedly<br />
find dark matter particles colliding with<br />
conventional substance underground. This<br />
means we won’t see dark matter but will detect<br />
this ‘kick.’ Accordingly, there are<br />
many competing models because we don’t<br />
know the truth.<br />
“Another problem is antimatter. For<br />
instance, you and I consist of matter, of<br />
particles, but it has been known since the<br />
1930s that there are also antiparticles<br />
that very much resemble ‘ours.’ If you take<br />
a particle and an antiparticle and bring<br />
An interview with<br />
physicist Dmytro<br />
Yakubovskyi who<br />
conducts research<br />
in Denmark and Ukraine<br />
them closely together, they will annihilate<br />
each other and produce a lot of matter. In<br />
fact, according to the formula E = mc2, antiparticles<br />
are the most effective source of<br />
energy. There are very few antiparticles,<br />
for otherwise, in all likelihood, we would<br />
not exist – there would have only remained<br />
cosmic microwave background,<br />
i.e. photons that would have formed as a result<br />
of annihilation. But we see our world<br />
without any major traces of antimatter.<br />
Antimatter may exist in cosmic rays – one<br />
particle per hundred or thousand. This is<br />
good enough, but we can see neither clusters<br />
of antimatter, nor galaxies that consist<br />
of it. This poses a problem because the<br />
Standard Model envisions a very close<br />
quantity of matter and antimatter.<br />
“The model I explore allows explaining<br />
all these puzzles in theory within the<br />
framework of the Standard Model – in theory<br />
because we have seen so far only a probable<br />
signal of dark matter decay which can<br />
be explained by these very particles or,<br />
maybe, by many others. We still have a long<br />
way to go.”<br />
● ON THE CHARACTERISTICS<br />
OF DARK MATTER<br />
“It is known for sure that dark matter<br />
exists because there is a mass excess in the<br />
Universe. We observe an effect and know<br />
that it is impossible to explain all that we<br />
can see without it.<br />
“I go on speaking in the descending order<br />
of truth. Mass is measured in astronomy<br />
by the laws of gravitation. Therefore,<br />
you can always explain mass excess not by<br />
new particles but by your attempts to<br />
change the law of gravitation. This is possible<br />
hypothetically, and there are models<br />
that use it. For example, the Erik Verlinde<br />
model, or the model of modified Newtonian<br />
dynamics, describes very well the observation<br />
of certain objects or their classes,<br />
for example, galaxies. But if you look<br />
at the whole array of the gained data, you<br />
will see that these models describe, for example,<br />
clusters of galaxies not so well and<br />
need to be at least modified. Introducing a<br />
new substance – dark matter – will explain<br />
the observation better.<br />
“What is still less known, dark matter<br />
consists of particles outside the Standard<br />
ALMOST ALL DETECTORS OF THE LARGE HADRON COLLIDER ARE SEARCHING FOR THE TRACES OF STILL UNKNOWN<br />
PARTICLES THAT MAY FORM DARK MATTER. THEY HAVE DETECTED NO NEW PARTICLES SO FAR, WHICH AFFORDS<br />
GROUND FOR DEVELOPING ALTERNATIVE MODELS<br />
Model – 95 percent or more of them are the<br />
particles we don’t know. Common neutrinos<br />
may account for not more than 5 percent<br />
of dark matter. It is a challenge for<br />
particle physics to search for these new particles.<br />
“What do we know about new particles?<br />
One of the main characteristics of a<br />
particle is mass, and the difference between<br />
the lightest and the heaviest dark matter<br />
candidate particle is about 10 in the 40th<br />
power. These hypothetical particles may be<br />
very large – they are called wimpzillas (a<br />
blend of WIMP – weakly interacting massive<br />
particles – and Godzilla – Author].<br />
They may have a mass that corresponds to<br />
a rest energy of dozens of joules. It is very<br />
much from the viewpoint of elementary<br />
particles. The decay of these particles can<br />
form cosmic rays that have the energy of<br />
tens of joules. It is approximately like<br />
throwing half a brick with the speed of<br />
10 meters a second – this is the highest energy<br />
of a cosmic body detected on Earth,<br />
which is billions of times as high as the energy<br />
of the particles the LHC can produce.<br />
Or it may be so light particles that their de<br />
Broglie wave should be the size of the<br />
smallest galaxies.”<br />
● ON DARK ENERGY<br />
“There is also dark energy of which we<br />
know almost nothing [hypothetically, it accounts<br />
for three fourths of the Universe –<br />
Author]. We only know that it ‘behaves’<br />
like antigravitation. Roughly speaking,<br />
people saw that our Universe is not simply<br />
expanding but expanding with acceleration.<br />
If you toss up an apple or a stone, it<br />
will be moving away from Earth and slowing<br />
down. Conversely, the Universe expands<br />
as a whole, accelerating instead of<br />
slowing down. The interactions we know<br />
cannot explain this.<br />
“If the well-known equation of state of<br />
dark energy is anything to go by, the Universe<br />
will, unfortunately, be expanding<br />
eternally, and galaxies, their components,<br />
and maybe even atoms will fly away.”<br />
● THE WAY RESEARCH IS<br />
CONDUCTED<br />
“Everything depends on the model, but<br />
use is usually made of both theoretical calculation,<br />
including a computer-assisted<br />
analysis, and the analysis of a very large<br />
number of observation data. For example,<br />
the LHC generates hundreds of petabytes<br />
of data in a year (one petabyte = 2 in the<br />
50th power bytes – Author]. It is very<br />
much even at present, but it was unprecedentedly<br />
much 10 years ago. For this reason,<br />
a system of ramified calculations was<br />
specially invented because not a single,<br />
even the largest, computer cluster cannot<br />
process these data in real time.<br />
“The LHC is not important for our<br />
group, as far as searching for dark matter<br />
is concerned, but we need to examine X-ray<br />
spectrums of the galaxies observed by<br />
space telescopes – it is a couple of terabytes<br />
of data on the whole. Although these telescopes<br />
cost hundreds of millions of dollars,<br />
most of their observation data are available<br />
free of charge to anybody who wishes to<br />
process them. Of particular interest for us<br />
are, above all, observations of the Andromeda<br />
Galaxy or our galaxy. There is a<br />
lot of dark matter there because a galaxy<br />
needs it in order to form. We know the way<br />
dark matter is distributed across the sky,<br />
examine various areas, estimate how much<br />
dark matter should be there, compare observations,<br />
and check whether there are interesting<br />
signals.<br />
“One of the signals we are looking for<br />
is the so-called emission line. In 2014 our<br />
team, concurrently with a US group, detected<br />
for the first time a signal from this<br />
emission line with the energy of 3.5 kiloelectronvolts,<br />
which may be an explanation<br />
of dark matter decay. A corresponding particle<br />
of dark matter should be approximately<br />
70 times lighter than an electron<br />
and thousands of times heavier than a<br />
common neutrino – it is called sterile or<br />
right-handed neutrino, or heavy neutral<br />
lepton, depending on the field you work in.<br />
“As our research requires huge groups,<br />
we hope we will be the best-shooting battalion.<br />
It is a maxim that battles are won<br />
by best-shooting, not best-manned, battalions.”<br />
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