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

TL;DR

A significant target-spin asymmetry of the distribution in the azimuthal angle straight phi of the pion relative to the lepton scattering plane was formed for pi(+) electroproduction on a longitudinally polarized hydrogen target.

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.

Figures (3)

• Figure 1: Kinematic planes for pion production in semi-inclusive deep-inelastic scattering.
• Figure 2: Target-spin analyzing powers for $\pi^+$: ${\it A}^{\sin\phi}_{UL}$ (squares) and ${\it A}^{\sin 2\phi}_{UL}$ (circles) as a function of Bjorken $x$. Error bars show the statistical uncertainty and the band represents the systematic uncertainties for ${\it A}^{\sin\phi}_{UL}$. As shown in Table \ref{['Tabxdis']}, $\langle Q^2 \rangle$ varies with $x$.
• Figure 3: Target-spin analyzing powers in the $\sin \phi$ moment as a function of transverse momentum, for $\pi^+$ (squares) and $\pi^-$ (circles). Error bars show the statistical uncertainties and the band represents the systematic uncertainties.