Deeply Virtual Compton Scattering at HERMES

1. QNP09 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

2. QNP09 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 totarget

3. QNP09 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

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

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

6. QNP09 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!!!

7. QNP09 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 p0background corrected for the results

8. QNP09 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

9. QNP09 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.

10. QNP09 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.

11. QNP09 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:

12. QNP09 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.

13. QNP09 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]

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

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

16. QNP09 Hydrogen Deuterium The Recoil Detector A Recoil Detector installed to identify the recoiling protons Dedicated high luminosity run in 2006 - 2007 with unpolarized hydrogen and deuterium targets • PID detectors • Tracking detectors g e'

17. QNP09 HERA BEAM The Recoil Detector • Consists of silicon detector, scintillating fiber tracker and photon detector • Detection of recoiling proton • p-measurement ~150-1400 MeV/c • 76%  acceptance • p/PPID via dE/dx Photon Detector. Silicon Detector Improvement of momenta resolution Scintillating Fiber Tracker 1 tesla magnetic field

18. QNP09 HERA BEAM The Recoil Detector • Consists of silicon detector, scintillating fiber tracker and photon detector • Detection of recoiling proton • p-measurement ~150-1400 MeV/c • 76%  acceptance • p/PPID 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?

19. QNP09 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.

20. QNP09 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!!!