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

Abstract

The beam-helicity asymmetry in exclusive electroproduction of real photons by the longitudinally polarized Hera positron beam scattering off an unpolarized hydrogen target is measured at Hermes. The asymmetry arises from deeply virtual Compton scattering and its interference with the Bethe-Heitler process. Azimuthal amplitudes of the beam-helicity asymmetry are extracted from a data sample consisting of ep → epγ events with detection of all particles in the final state including the recoiling proton. The installation of a recoil detector, while reducing the acceptance of the experiment, allows the elimination of background from ep → eN πγ events, which was estimated to contribute an average of about 12% to the signal in previous Hermes publications. The removal of this background from the present data sample is shown to increase the magnitude of the leading asymmetry amplitude by 0.054 ± 0.016 to −0.328 ± 0.027 (stat.) ± 0.045 (syst.).

Beam-helicity asymmetry in associated electroproduction of real photons ep → eγπN in the Δ-resonance region

“…This comparison provides evidence that the Monte Carlo description of the associated BH reaction used in previous analyses [12,[22][23][24]34] accounts for most of the observed yields.…”

“…The kinematic requirements on the identified positron and the photon cluster applied in ref. [12] are adjusted for this analysis as follows in order to optimize the selection of ep → eγπN events. The cluster is required to represent an energy deposition above 8 GeV in the calorimeter and above 1 MeV in the preshower detector.…”

“…In addition, adopting π + as proton candidates, the kinematic fit described in ref. [12] is performed in order to suppress ep → eγp background events. The following constraints on the χ 2 of kinematic event fitting and on the rdPID values are optimized and applied for the selection of events from the associated channels:…”

“…Following the approach of refs. [12,23,24], BH events are simulated using the Mo-Tsai formalism [25], by an event generator based on ref. [26] and described in detail in ref.…”

“…These asymmetry amplitudes are used to correct for the contribution from the SIDIS reaction in both the ep → eγπ 0 p and ep → eγπ + n channels. In order to correct for the small contribution from ep → eγp, its beam-helicity asymmetry amplitude A eγp measured with kinematically complete event reconstruction [12] is used. The slightly different kinematic constraints applied there are not expected to significantly affect this small correction.…”