Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target

TL;DR

This paper reports the first measurement of single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target, revealing signals related to quark transversity and Sivers functions.

Contribution

It provides the first experimental evidence of quark transversity and Sivers functions through measured asymmetries in pion electroproduction with a transversely polarized target.

Findings

Evidence for quark transversity distribution via Collins asymmetry

Observation of Sivers asymmetry linked to quark transverse momentum

Potential influence of exclusive vector meson production on Sivers asymmetry

Abstract

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion ($\phi$) and the target spin axis ($\phi_S$) about the virtual photon direction and relative to the lepton scattering plane. The extracted Fourier component $\cmpi$ is a signal of the previously unmeasured quark transversity distribution, in conjunction with the so-called Collins fragmentation function, also unknown. The Fourier component $\smpi$ of the asymmetry arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production