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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Hydrogen Deuterium PID detectors Tracking detectors g e' DVCS at HERMES QNP09

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 ~150-1400 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

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 ~150-1400 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

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

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