Abstract
The beam-helicity asymmetry in associated electroproduction of real photons, $ep\to e\gamma \pi N$, in the $\Delta$(1232)-resonance region is measured using the longitudinally polarized HERA positron beam and an unpolarized hydrogen target. Azimuthal Fourier amplitudes of this asymmetry are extracted separately for two channels, $ep\to e\gamma \pi^0 p$ and $ep\to e\gamma \pi^+ n$, from a data set collected with a recoil detector. All asymmetry amplitudes are found to be consistent with zero.
Highlights
Background corrections and systematic uncertaintiesThe Monte Carlo simulation shows that the selected samples of associated events contain contributions from two different sources of background
This approach is based on the assumption that the asymmetry for Semi-Inclusive DIS (SIDIS) π0 production is little affected by the requirement of the detection in the recoil detector of either a proton or a π+ satisfying the kinematic fit for the associated reaction
Monte Carlo studies showed [36] that the asymmetry extracted for SIDIS π0 production is insensitive to event selection using one or two photons
Summary
The data presented here were collected in 2006 and 2007 at Hermes (Desy) using the 27.6 GeV Hera positron beam and an unpolarized hydrogen gas target internal to the beam line For this measurement, the recoil detector [14] was used in conjunction with the forward spectrometer [15]. The scattered lepton and particles produced in the polar-angle range 0.04 rad < θ < 0.22 rad were detected by the forward spectrometer, for which the average leptonidentification efficiency was at least 98% with hadron contamination of less than 1%. For each detection layer i, a particle-identification discriminator rdPIDi, which depends on the reconstructed three-momentum |p| and on the energy deposition dE. Momentum reconstruction, and particle-identification procedures as well as detector performance studies are presented in ref. Momentum reconstruction, and particle-identification procedures as well as detector performance studies are presented in ref. [14]
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