Production of the charmonium states Χc1 and Χc2 in proton nucleus interactions at s = 41.6 GeV

A measurement of the ratio R xc = (Χ c → J/ψ + γ)/J/ψ in pC, pTi, and pW interactions at 920 GeV/c (s = 41.6 GeV) in the Feynman-x range -0.35 < x J/ψ F < 0.15 is presented. Both μ + μ - and e + e - J/ψ decay channels are observed with an overall statistics of about 15000 Χ c events, which is by far the largest available sample in pA collisions. The result is R Χc = 0.188 ± 0.013 st +0.024 -0.022sys averaged over the different materials, when no J/ ψ and Χ c polarizations are considered. The Χ c1 to Χ c2 production ratio R 12 = R Χc1 /R Χc2 is measured to be 1.02 ± 0.40, leading to a cross section ratio σ(Χc1) σ(Χ c2 ) = 0.57 ± 0.23. The dependence of R Χc on the Feynman-x of the J/ψ, x J/ψ F , and its transverse momentum, p J/ψ T , 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 R Χc and R 12 due to the dependence of acceptance on the polarization states of J/ψ and Χ c is performed. By varying the polarization parameter, λ obs , 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 Χ c1 and Χ c2 polarizations, it is shown that R Χc could change by a factor between 1.02 and 1.21 and R 12 by a factor between 0.89 and 1.16.