References - Bogoliubov Laboratory of Theoretical Physics - JINR

LATEST RESULTS ON DEEPLY VIRTUAL COMPTON SCATTERING
AT HERMES
A.Borissov
DESY, on behalf of the HERMES collaboration
E-mail: borissov@mail.desy.de
Abstract
The HERMES experiment at DESY collected a rich data set for the analysis
of Deeply Virtual Compton Scattering (DVCS) utilizing the HERA longitudinally
polarized electron or positron beams with an energy of 27.6 GeV and longitudinally
and transversely polarized or unpolarized gas targets (H, D or heavier nuclei). The
azimuthal asymmetries measured in the exclusive DVCS production allow the access
to the real and imaginary parts of certain combinations of Generalized Parton
Distributions. Latest results on combined analyzes of beam-spin and beam-charge
asymmetries are presented.
1 Introduction
Hard exclusive electroproduction of real photons on nucleons, Deeply Virtual Compton
Scattering (DVCS) is one of the theoretically cleanest ways to access Generalized Parton
Distributions (GPDs). The theoretical framework of GPDs incorporates knowledge about
form factors and parton distribution functions. GPDs depend on four kinematic variables:
the squared four-momentum transfer t to the nucleon, the squared four-momentum transferred
by the virtual photon −Q 2 , x and ξ, which represent respectively the average and
half the difference of the longitudinal momentum fractions carried by the probed parton
in initial and final states. For the proton, there are four twist-2 GPDs per quark flavor:
Hq,Eq, � Hq, and � Eq.
Data from DVCS are indistinguishable from the electromagnetic Bethe-Heitler (BH)
process because of the same final state, see Fig. 1a. The real photon is radiated from the
struck quark in DVCS or from the initial or scattered lepton in BH. The cross section of
the exclusive photoproduction process can be written in the following form [1]:
dσ
dQ 2 dxBd|t|dφ =
xBe 6
32(2π) 4 Q 4√ 1+ɛ 2
�
|τDV CS| 2 + |τBH| 2 + I
�
, (1)
where τDV CS(τBH) is the DVCS (BH) production amplitude, I = τ ∗ DV CS τBH +τ ∗ BH τDV CS is
the interference term, xB is the Bjorken scaling variable. The amplitude of the BH process
dominates at HERMES kinematics. However, the kinematic dependence of the cross
section terms generate a set of azimuthal asymmetries which depend on the azimuthal
angle φ between the real-photon production plane and the lepton scattering plane.
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