1. Deeply Virtual Compton Scattering at HERMES
Weilin Yu
II.Physikalisches Institut,
Justus-Liebig-Universität Gießen, Germany
for the HERMES Collaboration
The 5th International Conference on
Quarks and Nuclear Physics
Spin of nucleon, GPDs and DVCS
DVCS Results from HERMES
The HERMES Recoil Detector
DVCS at HERMES
QNP09

2. Nucleon Spin and GPDs HERMES : Phys. Rev. D75(2007)
Poor information
Accessible through:
Generalized Parton Distributions (GPDs)
four twist-2 GPDs when the partons do not flip helicity
Ji Relation:
t : momentum transfer squared to target
DVCS at HERMES
QNP09

3. Nucleon Spin and GPDs HERMES : Phys. Rev. D75(2007)
Poor information
Accessible through:
Generalized Parton Distributions (GPDs)
3D picture of nucleon
Parton Distribution
Function
Form Factors
GPDs
Longi. Momentum
Trans. Position
DVCS at HERMES
QNP09

4. Deeply Virtual Compton Scattering (DVCS)
DVCS : simplest/cleanest Hard Exclusive Process to access GPDs
DVCS
Interference term I
Same final state
Difficult to measure the DVCS
cross-section directly at HERMES!!!
Bethe-Heitler
DVCS at HERMES
QNP09

5. Deeply Virtual Compton Scattering (DVCS)
DVCS : simplest/cleanest Hard Exclusive Process to access GPDs
DVCS
Interference term I
Unpolarized Proton Target:
el beam charge, l beam helicity
Same final state
Bethe-Heitler
Azimuthal Asymmetries possible
DVCS at HERMES
QNP09

6. Azimuthal Asymmetries @ HERMES
Beam-charge asymmetry [BCA]
F1, F2 : Dirac and Pauli elastic nucleon form factors
Beam-Helicity asymmetry [BSA]
: Compton form factor
convolutions of GPDs
If the beams have both charges:
Asymmetries simultaneously extracted from data via the combined fitting :
BCA:
BSA(interference):
BSA(DVCS):
Interference and DVCS terms separated!!!
DVCS at HERMES
QNP09

7. DVCS @ HERMES Without Recoil Detector
Recoil protons identified by missing mass
[-1.52,1.72]
different for e- and e+
15% overall background
MC studies used to estimate uncertainties from alignment, acceptance, bin width
12% from associated BH production
Impossible to clean without the Recoil Detector!!!
Semi-inclusive p0 background corrected for the results
DVCS at HERMES
QNP09

8. Beam Charge Asymmetries
GPD model: Vanderhaeghen, Guichon, Guidal (VGG)
based on double distribution ansatz
Leading asymmetry
higher twist
fractions of associated BH production
The “VGG model with a D-term” is dis-favored by the BCA measurement
DVCS at HERMES
QNP09

9. Beam Helicity Asymmetries
BSA(DVCS term) from proton tagets data
const. term
higher twist
fractions of associated BH production
Pure DVCS squared asymmetries are compatible with zero, in agreement with model assumptions.
DVCS at HERMES
QNP09

10. Beam Helicity Asymmetries
BSA(interference term) from proton tagets data
const. term
Leading asymmetry
higher twist
fractions of associated BH production
VGG bands obtained by varying input parameters.
DVCS at HERMES
QNP09

11. Azimuthal Asymmetries @ HERMES
Transverse target-spin asymmetry [TTSA]
U: unpolarized beam, T: proton target with transversal polarization
AC and AUT can be simultaneously extracted if both beam charges are available:
DVCS at HERMES
QNP09

12. HERMES: First Measurement of TTSA
sensitive to Ju
Only asymmetry where GPD E is not suppressed
Model: VGG with variation of Ju while Jd = 0
JHEP 06 (2008) 066.
DVCS at HERMES
QNP09

13. Azimuthal Asymmetries @ HERMES
Transverse target-spin asymmetry [TTSA]
U: unpolarized beam, T: proton target with transversal polarization
AC and AUT can be simultaneously extracted if both beam charges are available:
Long. target-spin asymmetry [LTSA]
DVCS at HERMES
QNP09

14. DVCS from Deuterium Target
Low t : coherent
High t : incoherent
Proton and Deuterium targets results are compatible.
DVCS at HERMES
QNP09

15. DVCS from Nuclear Targets
No nuclear mass dependence observed within uncertainties
DVCS at HERMES
QNP09

16. The Recoil Detector
A Recoil Detector installed to identify the recoiling protons
Dedicated high luminosity run in with unpolarized hydrogen and deuterium targets
Hydrogen
Deuterium
PID detectors
Tracking detectors g e'
DVCS at HERMES
QNP09

17. HERA BEAM The Recoil Detector Detection of recoiling proton
Consists of silicon detector, scintillating fiber tracker and photon detector
Detection of recoiling proton
p-measurement ~ MeV/c
76%  acceptance
p/P PID via dE/dx
Photon Detector.
Silicon Detector
Improvement of momenta resolution
Scintillating Fiber Tracker
1 tesla magnetic field
DVCS at HERMES
QNP09

18. HERA BEAM The Recoil Detector Detection of recoiling proton
Consists of silicon detector, scintillating fiber tracker and photon detector
Detection of recoiling proton
p-measurement ~ MeV/c
76%  acceptance
p/P PID via dE/dx
Photon Detector.
Silicon Detector
Improvement of momenta resolution
Scintillating Fiber Tracker
1 tesla magnetic field
Backgroud suppression and study
semi-incl. : from 3% to <<1%
associated : from 12% to ~1%
asymmetries from the associated production?
DVCS at HERMES
QNP09

19. Conclusions
Hermes provides a very useful framework to study DVCS and GPDs.
Several DVCS azimuthal asymmetries were measured with respect to beam helicity and charge, and to target spin in the HERMES experiment.
The recoil detector took data in 2006 and 2007, with which DVCS can be directly measured and background can be rejected.
DVCS at HERMES
QNP09

20. HERMES Experiment HERA: longi. polar. e± beams, 27.5 GeV
Tracking detectors
PID detectors
Pure gas target: polarized H, D; unpolarized H, D, N, Ne, Kr, He,Xe
Forward spectrometer: 40 mrad < Q < 220 mrad
dp/p ~ 2%, dQ < 1 mrad
PID for e± : TRD, Preshower, Calorimeter
one lepton and one photon detected in the calorimeter for DVCS!!!
DVCS at HERMES
QNP09