RESEARCH STATEMENT: TANJA HORN My current research is ...

RESEARCH STATEMENT: TANJA HORN
My current research is aimed at unveiling the mysteries of hadron structure in the
transition region of QCD confinement to QCD asymptotic freedom. Building on the
results of my recent work, I have developed an extensive experimental program for
Jefferson Lab after the 12 GeV upgrade to study the transition to quark and gluon degrees
of freedom in exclusive meson production, e.g., the aspects of scaling related to QCD
factorization. My approved Jefferson Lab experiments for the studies of the pion (E12-
07-105) and the kaon (E12-09-011) systems will allow for quasi-model independent
comparison in the scaling regime. I have also initiated an experimental program in Hall B
aimed at studying nucleon structure through Timelike Compton scattering, and made a
major contribution to the development of the medium energy electron-ion collider
(mEIC) at Jefferson Lab. The latter would make it possible to study the spin, flavor, and
spatial distributions of quarks and gluons through GPDs in exclusive meson production. I
have also performed various studies related to detector development for 12 GeV magnetic
spectrometers and both medium- and high-energy e-p colliders.
A discussion of my current and future research is shown in the sections below.
MESON STRUCTURE and QCD FACTORIZATION
Understanding hadron structure in terms of quark and gluon degrees of freedom
remains an outstanding challenge in modern physics. In this quest, experiments involving
electroweak probes are an important source of information, and extensive studies have
mapped out the elastic form factors and quark-gluon distributions in the nucleon.
However, the structure of nature's simplest strongly interacting system, the pion, had thus
far proved more elusive. Pion beam experiments have only been able to provide data with
small photon virtuality (Q 2 ). My thesis experiment, where the electron was scattered on a
charged virtual pion in the cloud surrounding a nucleon, extended the Q 2 range by orders
of magnitude. This brought it into the regime where one could start comparing with
calculations using realistic QCD calculations, which asymptotically predicts a Q 2 scaling
of the pion form factor. While the data indeed showed such asymptotic behavior, a
discrepancy in the magnitude could indicate that there still was a significant contribution
from long-distance physics. Resolving this puzzle would require a better understanding of
the reaction mechanism, and data at even higher Q 2 . However, the two go hand-in-hand,
and the latter would be much easier to obtain if one could allow a higher four-momentum
transfer (-t) to the nucleon. As a follow-up to my PhD work, which has resulted in four
papers and numerous talks, I am planning to submit a proposal to the next Jefferson Lab
PAC for a 12 GeV experiment to investigate the higher order effects by measuring the
separated response functions in exclusive π° production, improving our knowledge of
non-pole contributions.
Such a measurement fits well into my general 12 GeV meson program, which is
focused on understanding hard exclusive meson production. My approved Jefferson Lab
experiments for studies of the pion (E12-07-105) and the kaon (E12-09-011) systems will
allow for quasi-model independent comparison in the scaling regime, mapping out the

transition from long-distance to short-distance physics, and provide important tests prerequisites
for nucleon structure studies by testing the applicability of QCD factorization.
For a long time, measurements of nucleon form factors provided the information
on the spatial density of partons, while their longitudinal momentum distributions were
obtained from deep inelastic scattering. Only recently was it realized that these
correspond to limiting cases of a more general way to characterize the structure of the
nucleon. The Wigner quantum phase space distributions of the quarks and gluons provide
a simultaneous description of both position and momentum of particles in a quantum
mechanical system, representing the closest analogue to a classical phase space density
allowed by the uncertainty principle. Mapping out the corresponding Generalized Parton
Distributions (GPDs) in the nucleon, and learning about the transverse momentum
distributions, has thus become an important goal. Experimentally, this relies on the
asymptotic freedom of QCD, which allows the hard, short-distance interaction of the
experimental probe (photon) with one parton (quark) to be unambiguously be separated
from the residual, soft interaction of the struck parton with the rest of the hadron, which
contains the long distance information about nucleon structure described by the GPDs.
Exclusive measurements of a photon or meson originating from the struck parton can
provide this information, but only if performed in a kinematic regime where hard-soft
factorization applies.
A central question of my research is to address the issue of where this occurs, by
studying the reaction mechanism with exclusive meson electroproduction in the transition
region between hadron and quark-gluon degrees of freedom. While there is no single
criterion for the applicability of factorization, it requires the cross section, separated into
components corresponding to longitudinal and transverse polarizations of the incoming
virtual photon, to exhibit the scaling properties predicted by QCD, and a confirmation
that σ L >> σ T . Currently, such L/T separated data are scarce. I recently completed a first
factorization study combining JLab 6 GeV data from the pion form factor and pionCT
experiments. My results are summarized in Phys. Rev. C78, 058201 (2008). More
conclusive results will be provided by my kaon and pion scaling experiments, and there
may be possibilities to extend the Q 2 reach even further at future facilities.
NUCLEON STRUCTURE
The mapping the nucleon structure through the measurement of GPDs is
undertaken by exclusive single-meson production and Compton scattering. So far the
latter has been restricted to Deeply Virtual Compton Scattering (DVCS), which is the
simplest exclusive process giving access to GPDs. It is, however, only a special case of
the reaction γ*p→γ*p, where the final state photon is real, and is not sufficient to
determine GPDs in a model independent way in both x and ξ. As a step towards the most
general measurement of GPDs, minimizing model dependent assumptions, I am
developing a program using Timelike Compton scattering (TCS), where the initial photon
is real and the final state one has a timelike virtuality. This provides an alternative and
potentially cleaner way to access GPDs compared to DVCS. And since higher order
corrections at finite Q 2 are different than in DVCS, a comparison can provide a better
understanding of the reaction mechanism. The TCS studies may also open the door to a
new class of Compton scattering experiments, where both the initial and final photons are
virtual, which would provide the most comprehensive set of information on GPDs. In

order to perform a first study of TCS at the current JLab energy, I submitted a request for
supplemental experimental equipment needed in order to run together with the g12
experiment in Hall B. The request was granted, and the experimental configuration could
be adapted to the TCS requirements. The experiment was completed in June 2008 and the
analysis is under way. Data taken in a different configuration are also available from the
e1-6 experiment, for which a proposal for a CLAS Approved Analysis is under way. If
the 6 GeV data analysis is successful, a CLAS12 proposal to study TCS will be
submitted, which may be followed by supplemental studies of heavy vector meson
production, and possibly a proposal for Hall D.
While opening up many opportunities, the 12 GeV upgrade of the Jefferson Lab
will not provide all the answers related to the structure of nucleon. New facilities will
emerge allowing an extension of my program to higher Q 2 (> 10 GeV 2 ) and smaller x (<
0.1), allowing a detailed measurements of the spin, flavor and spatial distribution of
quarks in the nucleon through non-diffractive meson production (e.g., π + , π°, K + ), and
giving access to gluon GPDs using non-diffractive processes. Among the latter, Compton
scattering as well as exclusive ρ 0 and J/Ψ production at high Q 2 also make it possible to
probe single quarks, which allows disentangling the singlet quark and gluon GPDs, and
test the QCD evolution. The J/Ψ provides a unique probe whose t-dependence contains
the information about the transverse spatial distribution of gluons. One option for
conducting such experiments would be an Electron-Ion Collider (EIC), for which I am
performing feasibility studies using a parametric Monte Carlo which I developed for that
purpose. I am also working on the design of a medium-energy electron-ion collider for
Jefferson Lab.