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

TL;DR

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, and the Sivers asymmetry may be affected by exclusive vector meson production.

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 ($φ$) and the target spin axis ($φ_S$) 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

Figures (2)

• Figure 1: The definitions of the azimuthal angles of the hadron production plane and the axis of the relevant component $\vec{S}_\perp$ of the target spin, relative to the plane containing the momentum $\vec{k}$ ($\vec{k}'$) of the incident (scattered) lepton. Explicitly, $\phi = \frac{\vec{q}\times\vec{k}\cdot\vec{P}_h}{|\vec{q}\times\vec{k}\cdot\vec{P}_h|} \cos^{-1}{\frac{\vec{q}\times\vec{k}\cdot\vec{q}\times\vec{P}_h}{|\vec{q}\times\vec{k}||\vec{q}\times\vec{P}_h|}}$ and $\phi_S = \frac{\vec{q}\times\vec{k}\cdot\vec{S}_\perp}{|\vec{q}\times\vec{k}\cdot\vec{S}_\perp|} \cos^{-1} {\frac{\vec{q}\times\vec{k}\cdot\vec{q}\times\vec{S}_\perp}{|\vec{q}\times\vec{k}||\vec{q}\times\vec{S}_\perp|}}$, where $0<\cos^{-1}<\pi$.
• Figure 2: Virtual-photon Collins (Sivers) moments for charged pions as labelled in the upper (middle) panel, as a function of $x$ and $z$, multiplied by two to have the possible range $\pm1$. The error bars represent the statistical uncertainties. In addition, there is a common 8% scale uncertainty in the moments. The lower panel shows the relative contributions to the data from simulated exclusive vector meson production.