Beam-Spin Asymmetries in the Azimuthal Distribution of Pion Electroproduction

Abstract

A measurement of the beam-spin asymmetry in the azimuthal distribution of pions produced in semi-inclusive deep-inelastic scattering off protons is presented. The measurement was performed using the {HERMES} spectrometer with a hydrogen gas target and the longitudinally polarized 27.6 GeV positron beam of HERA. The sinusoidal amplitude of the dependence of the asymmetry on the angle $φ$ of the hadron production plane around the virtual photon direction relative to the lepton scattering plane was measured for $π^+,π^-$ and $π^0$ mesons. The dependence of this amplitude on the Bjorken scaling variable and on the pion fractional energy and transverse momentum is presented. The results are compared to theoretical model calculations.

Figures (7)

• Figure 1: Definition of kinematic planes for semi-inclusive deep-inelastic scattering.
• Figure 2: Beam SSA as a function of $\phi$ for $\pi^+$ electroproduction at mid-$z$ range. The solid curve represents a $\sin\phi$ fit, and the dashed one includes also the $\sin2\phi$ harmonic. Only statistical errors are shown.
• Figure 3: Dependence of the beam SSA on $z$, $x$, and $P_{h\perp}$. Results for the $x$ and $P_{h\perp}$ dependences are presented separately for the low-$z$ ($0.2<z<0.5$) and mid-$z$ ($0.5<z<0.8$) regions, indicated by open and full circles, respectively. For the high-$z$ ($0.8<z<1$, open squares) region only the $z$ dependence is pictured. The error bars represent the statistical uncertainty. An additional $5.5\%$ fractional scale uncertainty is due to the systematic uncertainty in the beam polarization measurement. Total systematic uncertainties do not exceed $0.005$.
• Figure 4: Top panel: amplitude $A_{LU}^{\sin\phi}$ for $\pi^+$ mesons originating from $\rho^0$ meson decays, obtained with Monte Carlo (band) and data (full circles). The open cross displays the asymmetry for the $\rho^0$ itself (Monte Carlo). Bottom panel: the fraction of pions in the SIDIS sample originating from VM decays.
• Figure 5: Dependence of $\widetilde{A}_{LU}^{\sin\phi}$ on $z$, $x$ and $P_{h\perp}$ for charged pions. The contribution from VM decays has been determined from a Monte-Carlo simulation and subsequently subtracted from the asymmetries. The measurement of the $x$ and $P_{h\perp}$ dependences is made separately for low ($0.2<z<0.5$) and middle ($0.5<z<0.8$) $z$-ranges (indicated by open and full circles, respectively). The error band indicates the uncertainties from Pythia and rhoMC.