Production of the Charmonium States chic1 and chic2 in Proton Nucleus Interactions at sqrts = 41.6 GeV
Abstract
A measurement of the ratio Rchic = (chic -> Jpsi + gam)/ Jpsi in pC, pTi and pW interactions at 920 GeV/c (sqrts=41.6 GeV) in the Feynman-x range -0.35 < xF(Jpsi) < 0.15 is presented. Both mu+mu- and e+e- Jpsi decay channels are observed with an overall statistics of about 15000 chic events, which is by far the largest available sample in pA collisions. The result is Rchic = 0.188+-0.013(st)(+0.024)(-0.022)(sys) averaged over the different materials, when no Jpsi and chic polarisations are considered. The chic1 to chic2 production ratio R12 = Rchic1/Rchic2 is measured to be 1.02+-0.40, leading to a cross section ratio sigma(chic1)/sigma(chic2)=0.57+-0.23. The dependence of Rchic on the Feynman-x of the Jpsi, xF(Jpsi), and its transverse momentum, pT(Jpsi), 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 Rchic and R12 due to the dependence of acceptance on the polarization states of Jpsi and chic is performed. By varying the polarisation parameter, lambda(obs), of all produced Jpsi's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible chic1 and chic2 polarisations, it is shown that Rchic could change by a factor between 1.02 and 1.21 and R12 by a factor between 0.89 and 1.16.