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

A study of the angular distributions of leptons from decays of J/ψ's produced in p-C and p-W collisions at √(s) = 41.6 GeV has been performed in the J/ψ Feynman-x region.0.34 < xF < 0.14 and for J/ψ transverse momenta 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 . 105 J/ψ's reconstructed in both the e+e- and μ+μ- decay channels, indicate that J/ψ's are produced polarized. The magnitude of the effect is maximal at low pT. For pT > 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. (orig.)