Measurement of Deeply Virtual Compton Scattering Beam-Spin Asymmetries

Abstract

1 tex file (6 pages), 4 (eps) figuresThe beam spin asymmetries in the hard exclusive electroproduction of photons on the proton (ep -> epg) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q^2 from 1 to 4.8 GeV^2, -t from 0.09 to 1.8 GeV^2), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A = a*sin(phi)/[1+c*cos(phi)]. This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector

Beam-helicity asymmetry arising from deeply virtual Compton scattering measured with kinematically complete event reconstruction

“…The measurement of the DVCS process on unpolarized protons is most sensitive to GPD H, which describes the transition that conserves the helicities of both the struck quark and the nucleon. Such measurements were performed by Hermes [6][7][8][9], H1 [10][11][12][13], and Zeus [14,15] at Hera, by the Hall A Collaboration [16] and by Clas [17][18][19] at Jefferson Lab.…”

Resumming soft and collinear contributions in deeply virtual Compton scattering

“…This lead to the first studies of non-perturbative non forward parton distributions, now called generalized parton distributions (GPDs), first in the fixed target experiment HERMES [1][2][3], and then at H1 and ZEUS, using the dominance of the DVCS contribution at small x Bj [4][5][6]. Almost simultaneously, the DVCS contribution was measured at JLAB, at CLAS [7,8] and at Hall A [9]. From the theory side, the interest for hard exclusive processes started with the Leipzig group [10].…”

Test of two new parametrizations of the generalized parton distributionH

“…As stated in the introduction, we will apply the 1CDD formalism to the computation of JLab Hall A and CLAS polarized beam observables, namely, beam helicitydependent and -independent cross sections [34] and beam-spin asymmetries [35]. All details about the evaluation of observables related to the ep !…”

“…[47] this amplitude is evaluated through an expansion at leading and subleading orders. Since a great wealth of data in the valence region [34,35] involve values of Q 2 ranging between 1 and 2 GeV 2 , the impact of this approximation is sizable when comparing theoretical expectations and measurements. In 2008 Guichon and Vanderhaeghen [51] elaborated on their previous numerical computations [27][28][29] to establish analytic expressions of the ep !…”

“…[32] for PDFs with an integrable singularity at the origin and confront it to DVCS experimental data. We will compare our model to Jefferson Lab Hall A [34] and CLAS measurements [35], which are very accurate and concern the valence sector. Therefore, we only implemented the one-component DD model for the singlet contribution of valence quarks.…”