Spin Asymmetry in Muon--Proton Deep Inelastic Scattering on a Transversely-Polarized Target

Abstract

We measured the spin asymmetry in the scattering of 100 GeV longitudinally-polarized muons on transversely polarized protons. The asymmetry was found to be compatible with zero in the kinematic range $0.006<x<0.6$, $1<Q^2<30\,~\mbox{GeV}^2$. {}From this result we derive the upper limits for the virtual photon--proton asymmetry $A_2$, and for the spin structure function $g_2$. For $x<0.15$, $A_2$ is significantly smaller than its positivity limit $\sqrt{R}$.

Figures (3)

• Figure 1: The event distribution as a function of the azimuthal angle $\phi$, for interactions in the upstream and downstream targets. The value $\phi=0$ corresponds to a scattered muon with transverse momentum parallel to the target polarization. The observed distribution mainly reflects the trigger acceptance.
• Figure 2: The transverse cross-section asymmetry $A_{\perp}$ as a function of $x$ for interactions in the upstream and downstream targets and for the combined data. The error bars represent the statistical errors.
• Figure 3: The asymmetry $A_2$ as a function of $x$. The solid line shows $\sqrt{R}$ from the SLAC parametrization [12], and the dashed line the results obtained with $\overline{g_2}=0$ in Eq. (1). The error bars represent statistical errors, only.