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

Understand

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.