Beam-Spin Azimuthal Asymmetries in Pion Electroproduction at HERMES

TL;DR

This study measures the beam-spin azimuthal asymmetry $A_{LU}^{\sin\phi}$ in semi-inclusive pion electroproduction off hydrogen in SIDIS using the HERMES spectrometer and a polarized 27.6 GeV beam. The analysis relies on a factorized SIDIS framework where the cross section includes twist-2 and twist-3 TMD contributions, yielding a $\sin\phi$ modulation that is quantified through the $A_{LU}^{\sin\phi}$ observable across $x$, $z$, and $P_T$. The results show a positive $A_{LU}^{\sin\phi}$ of about $2\%$ for $\pi^+$, with a $z$-dependent increase, while $\pi^-$ and $\pi^0$ are consistent with zero within uncertainties; comparisons with quark-diquark and other TMD-based models provide qualitative agreement and highlight the role of vector-meson contamination. Overall, the work demonstrates sensitivity of beam-spin asymmetries to Collins-like and other TMD fragmentation functions, informing transversity-related measurements and global TMD analyses.

Abstract

We present the measurement of Single Beam-Spin Azimuthal Asymmetries in pion electroproduction off hydrogen in semi-inclusive deep inelastic scattering (SIDIS). The measurement was made using the HERMES spectrometer with an internal gas target and the polarized 27.6 GeV electron(positron) beam of HERA using the data taken during the years 1996-2000. The $\sinφ$ modulation of the azimuthal asymmetry was measured for semi-inclusive $π^+,π^-$ and $π^0$. The dependence of the asymmetry on the Bjorken $x$, pion relative ($z$) and transverse ($P_T$) momentum is presented. Results are compared to theoretical model calculation.

Figures (2)

• Figure 1: Definitions of kinematic planes and variables
• Figure 2: Dependence of beam SSA on $z$, $x_B$ and $P_T$ for pions. The measurement of the $x_B$ and $P_T$ dependences is made for $0.5<z<0.8$ range (indicated by full circles on the left column). Error bars represent the statistical uncertainty and the shaded bands represents an upper limit for a possible uncertainty from exclusive vector meson decay product contribution in the semi-inclusive pion sample. An additional 5.5% scale uncertainty is due to the beam polarization measurement.