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/psi's produced in p-C and p-W collisions at root s = 41.6 GeV has been performed in the J/psi Feynman-x region -0.34 < x(F) < 0.14 and for J/psi 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 x 10(5) J/psi's reconstructed in both the e(+)e(-) and mu(+)mu(-) decay channels, indicate that J/psi's are produced polarized. 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.