Sivers asymmetry in J/ψ production in COMPASS 2010 proton data

The Sivers asymmetry in J/ψ production in scattering of muons off transversely polarized protons μ(+) + p(↑)→μ(+) + J/ψ + X is measured in two z-bins (inclusive and exclusive production) in the COMPASS 2010 data. Only J/ψ’s decaying into muons are detected. About 2 230 and 4450 J/ψ events in the lower and the higher z-bins are found. The Sivers asymmetry is extracted using a simple double-ratio method and is found to be compatible with zero in the lower z bin and to have a slight preference for negative values in the higher z bin.