Measurement of deeply virtual Compton scattering at HERA

Abstract

The cross section for deeply virtual Compton scattering in the reaction ep → eγp has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb −1 of e + p and 16.7 pb −1 of e − p collisions. Differential cross sections are presented as a function of the exchanged-photon virtuality, Q 2 , and the centre-of-mass energy, W , of the γ * p system in the region 5 < Q 2 < 100 GeV 2 and 40 < W < 140 GeV. The measured cross sections rise steeply with increasing W . The measurements are compared to QCD-based calculations.

A deeply virtual Compton scattering amplitude

“…At HERA the DVCS cross-section has been measured [3,4], in diffractive ep interactions, as a function of Q 2 , W and t that are respectively the photon virtuality, the invariant mass of the γ * p system and the squared 4-momentum transferred at the proton vertex; the diagram in Fig. 1a shows the production of a real photon at HERA.…”

Deeply virtual compton scattering in color dipole formalism

“…As usual, the kinematic variables are the c.m.s. energy squared s = W 2 γp = (p + q) 2 , where p and q are the proton and the photon momenta respectively, the photon virtuality squared Q 2 = −q 2 and the Bjorken scale x = Q 2 /(W 2 γp + Q 2 ). The DVCS imaginary part of the amplitude at zero momentum transfer reads as, where σ dip (x, r 2 ) is the dipole cross section, which depends on the scaling variablex…”

“…II (from H1 Coll. paper [3]) one compares their prediction with the existing measurement from H1 and ZEUS Collaborations [1][2][3]. The color dipole prediction from Donnachie-Dosh [11] is also presented for the sake of comparison.…”

“…(5), we use recent DVCS measurements at HERA [2,3] and the DIS cross cross section is obtained using the pQCD fits of the F p 2 structure function from Ref. [14].…”

“…In these cases, the difference with the inclusive case is the longitudinal components of the incoming and outgoing proton momentum, which depend on the photon virtuality Q 2 and the γ * p center of mass energy W . The recent data from DESY ep collider HERA on exclusive diffractive virtual Compton process [1][2][3] at large Q 2 becomes an important source to study the partons, in particular gluon, inside the proton for non-forward kinematics and its relation with the forward one (see Fig. I).…”