Angular distributions of leptons from J/psi's produced in 920 GeV fixed-target proton-nucleus collisions

TL;DR

This study analyzes the angular distributions of leptons from J/psi decays in proton-nucleus collisions at 41.6 GeV, revealing polarization effects and frame-dependent anisotropies in the decay patterns.

Contribution

It provides detailed measurements of J/psi polarization and angular anisotropies in fixed-target proton-nucleus collisions, highlighting frame-dependent effects.

Findings

J/psi's are produced with longitudinal polarization.

Polar anisotropy is maximal at low transverse momentum.

Significant azimuthal anisotropy appears in the helicity frame at higher p_T.

Abstract

A study of the angular distributions of leptons from decays of J/psi's produced in p-C and p-W collisions at sqrt{s}=41.6 GeV has been performed in the Feynman-x region -0.34 < x_F < 0.14 and for transverse momentum up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3 x 10^5 J/psi's reconstructed in both the e^+ e^- and mu^+ mu^- decay channels, indicate that J/psi's are produced with longitudinal polarization. The magnitude of the effect is maximal at low p_T. For p_T >1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises.