Polarization of Lambda and Anti-Lambda in 920 GeV Fixed-Target Proton-Nucleus Collisions

TL;DR

This study measures Lambda and Anti-Lambda baryon polarization in 920 GeV proton-nucleus collisions, finding small polarization effects consistent with previous data and zero polarization at mid-rapidity.

Contribution

First measurement of Lambda and Anti-Lambda polarization in fixed-target proton-nucleus collisions at this energy, extending understanding of polarization behavior across different x_F regions.

Findings

Lambda polarization agrees with previous negative x_F measurements.

Anti-Lambda polarization is consistent with zero.

Small deviation from zero polarization observed at negative x_F.

Abstract

A measurement of the polarization of Lambda and Anti-Lambda baryons produced in pC and pW collisions at sqrt(s)=41.6 GeV has been performed with the HERA-B spectrometer. The measurements cover the kinematic range of 0.6 GeV/c < p_T<1.2 GeV/c in transverse momentum and -0.15<x_F<0.01 in Feynman-x. The polarization results from the two different targets agree within the statistical error. In the combined data set, the largest deviation from zero, +0.054 +- 0.029, is measured for x_F < -0.07. Zero polarization is expected at x_F=0 in the absence of nuclear effects. The polarization results for the Lambda agree with a parametrization of previous measurements which were performed at positive x_F values, where the Lambda polarization is negative. Results of Anti-Lambda polarization measurements are consistent with zero.