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

TL;DR

This paper reports a precise measurement of the beam-helicity asymmetry in deeply virtual Compton scattering using a kinematically complete event reconstruction, improving background suppression and providing detailed azimuthal amplitude data.

Contribution

It introduces a method with a recoil detector for background reduction in DVCS asymmetry measurements, enhancing the accuracy of azimuthal amplitude extraction.

Findings

Increased asymmetry amplitude after background removal

Quantified the asymmetry amplitude as -0.328 with uncertainties

Demonstrated the effectiveness of recoil detection in DVCS measurements

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 $ep\rightarrow ep\gamma$ 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 $ep\rightarrow eN\pi\gamma$ 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.).