GPD and TMD Studies at HERMES. Frank Ellinghaus University of Colorado October 2007 DNP 2007, Newport News, USA. scattered electron. real photon. electron. recoiling proton. proton. Generalized Parton Distributions (GPDs). Simplest/cleanest hard exclusive process:
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GPD and TMD Studies at HERMES
Frank Ellinghaus
University of Colorado
October 2007
DNP 2007, Newport News, USA
scattered
electron
real photon
electron
recoiling
proton
proton
Simplest/cleanest hard exclusive process:
e p -> e’ p’ g , Deeply-Virtual Compton Scattering (DVCS)
Access to the four leading quark GPDs
in real photon (DVCS) and Meson production !
DVCS
Bethe-Heitler (BH)
Indistinguishable processes
interfere with each other!
Small at
HERMES, JLab
kinematics
Calculable in
QED with
knowledge of FFs
Access to real part of
DVCS amplitude
Access to imaginary part of DVCS amplitude
Needs polarized (electron) beam
(other asymmetries with pol. Target!)
Needs both beam charges
(e+ and e-) beam
(simplest approx.)
7/1/07@ 1:09:56 am
27.6 GeV e+ and e-
<P> about 35-55%
pol. and unpol. Gas targets -> H, D, He, N, Ne, Kr, Xe
All data in the following taken before installation of Recoil Detector !
1 Tesla Superconducting Solenoid
Photon Detector
Scintillating Fiber Detector
HERA BEAM
Silicon Detector
Target Cell
Data taking with the recoil detector in 2006 and 2007
elastic BH: e p -> e p g
assoc. BH: e p -> e D+g (mainly)
semi-incl. : e p -> e p0 X (mainly)
About 15 % BG in exclusive bin (-1.5 < Mx < 1.7 GeV)
( simplest approx.)
HERMES,
(PRD 75, 2007)
Model by Vanderhaeghen, Guichon, Guidal (VGG),
based on double-distributions (Radyushkin)
Guzey/Teckentrup, PRD 74, 2006
Analysis based on tiny e- p sample (~ 700 events),
Now about 20 times more data on disk!
BCA has high sensitivity to GPD models!
Simplest approx. :
HERMES preliminary
Model by Vanderhaeghen, Guichon, Guidal (VGG),
based on double-distributions (Radyushkin)
Model (Guzey, Teckentrup) based on dual-parametrization (Polyakov, Shuvaev)
are in agreement with “all other” DVCS data so far:
-> Cross sections from H1/ZEUS (used for normalization)
-> BCA at HERMES
-> Published (PRL, 2001) AVERAGE BSA values from HERMES and CLAS
Size and kinematic dependence of the asymmetry is reproduced
More data with improved systematics to come, but BSA not very sensitive to
models.
Fourier expansion for unpolarized Hydrogen target:
<- Let’s not neglect it…
Zero Order approximation:
More realistic approximation:
“Usual” BSA is not only sensitive to interference term but gets contribution from
DVCS term.
“Usual” BSA depends on beam charge and size of the BCA.
Define:
New asymmetries can disentangle (both charges needed) the contributions
from the interference and the DVCS term
X. Ji, 1997
So far: Access to GPD H using unpolarized hydrogen targets
How to get to GPD E ?
GPD E (on a proton target) is always kinematically suppressed except in the transverse target-spin asymmetry TTSA:
Result from data taken on transversely polarized Hydrogen:
Largely independent on all model parameters but Ju
(F.E., Nowak, Vinnikov, Ye, EPJ C46 2006, hep-ph/0506264)
First model dependent extraction of Ju possible!
First model dependent constraint on total quark angular momentum!
Similar Method used by JLab Hall-A; “neutron” data has higher sensitivity to Jd
Value to be taken with care, since VGG does not seem to describe the available BSA data, but it is important to have a (first) method!
Second comparison to model calculations
(Guzey/Teckentrup, PRD 2006) suggest
small/negative value for Ju if Jd=0.
Model uncertainty bigger than uncertainties from measurements!
The way to go:
Constrain models for GPD H first by BSA/BCA.
(some model parameters might be the same for GPD E)
Compare remaining models to asymmetries sensitive to GPD E (Ju ,Jd).
The (only) other (promising) access
to the GPD E (J) on a p target :
AUT in excl. r0 production
Factorization proof for only
(Collins, Frankfurt, Strikman PRD 56, 1997)
(strongly simplified)
Factorization proof for only
(Collins, Frankfurt, Strikman PRD 56, 1997)
Frankfurt, Pobylitsa, Polyakov, Strikman,
PRD 60 (1999)
N(p+) – N(p-) versus
“tranverse momentum distribution of unpolarized quarks in a transversely polarized proton
Sivers
Collins
First measurement of DVCS on Neon (F.E. et al, hep-ex/0212019) triggered
first calculations for DVCS on Nuclei.
->Opens possibility to explore nuclear structure in terms of quarks and gluons,
EMC effect, (Anti-)Shadowing, CT, ….
Task: Find for each target upper (lower) -t
cut in order to compare the BSA for the
coherent (incoherent) production at similar
average kinematics:
Coherent BH
Incoherent BH
Semi-Inclusive BG
Resonances
82% coherent for heavier targets at –t =0.018 GeV2
and very similar average x and Q2
Guzey/Siddikov (J. Phys. G, 2006)
Promising, more data needed…
Guzey/Strikman (Phys.Rev. C, 2003)