Observation of a Single-Spin Azimuthal Asymmetry in Semi-Inclusive Pion Electro-Production

TL;DR

This paper reports the first measurement of single-spin asymmetries in semi-inclusive pion electro-production, revealing a significant azimuthal asymmetry linked to spin-dependent distribution and fragmentation functions.

Contribution

It provides the first experimental observation of target-spin asymmetry in semi-inclusive pion production, highlighting the role of chiral-odd distribution and T-odd fragmentation functions.

Findings

Observed a target-spin asymmetry of 0.022 in pion electro-production

Detected a significant azimuthal asymmetry in the distribution

Results suggest sensitivity to transverse polarization effects

Abstract

Single-spin asymmetries for semi-inclusive pion production in deep-inelastic scattering have been measured for the first time. A significant target-spin asymmetry of the distribution in the azimuthal angle phi of the pion relative to the lepton scattering plane was observed for pi+ electro-production on a longitudinally polarized hydrogen target. The corresponding analyzing power in the sin(phi) moment of the cross section is 0.022 +/- 0.005 +/- 0.003. This result can be interpreted as the effect of terms in the cross section involving chiral-odd spin distribution functions in combination with a time-reversal-odd fragmentation function that is sensitive to the transverse polarization of the fragmenting quark.