Angular distributions of leptons from J/psi's produced in 920 GeV fixed-target proton-nucleus collisions
Abstract
A study of the angular distributions of leptons from decays of J/psi's produced in p-C and p-W collisions at sqrts=41.6 GeV has been performed in the Feynman-x region -0.34 < xF < 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 105 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 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.
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