Production of the charmonium states χc1 and χc2 in proton nucleus interactions at s=41.6GeV

A measurement of the ratio Rχc=(χc→J/ψ+γ)/J/ψ in pC, pTi, and pW interactions at 920GeV/c (s=41.6GeV) in the Feynman-x range -0.35<xFJ/ψ<0.15 is presented. Both μ+μ- and e+e- J/ψ decay channels are observed with an overall statistics of about 15000 χc events, which is by far the largest available sample in pA collisions. The result is Rχc=0.188±0.013st+0.024-0.022sys averaged over the different materials, when no J/ψ and χc polarizations are considered. The χc1 to χc2 production ratio R12=Rχc1/Rχc2 is measured to be 1.02±0.40, leading to a cross section ratio σ(χc1) σ(χc2)=0.57±0.23. The dependence of Rχc on the Feynman-x of the J/ψ, xFJ/ψ, and its transverse momentum, pTJ/ψ, is studied, as well as its dependence on the atomic number, A, of the target. For the first time, an extensive study of possible biases on Rχc and R12 due to the dependence of acceptance on the polarization states of J/ψ and χc is performed. By varying the polarization parameter, λobs, of all produced J/ψ's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible χc1 and χc2 polarizations, it is shown that Rχc could change by a factor between 1.02 and 1.21 and R12 by a factor between 0.89 and 1.16. © 2009 The American Physical Society.