Study of χc Production Using HERA-B Data

In this thesis the production of the charmonium states χc1 and χc2 in protonnucleus collisions at a proton-nucleon center-of-mass energy √ s = 41.6 GeV was studied. The data used for the analysis have been taken by the fixed-target experiment HERA-B that uses the HERA proton beam to scatter protons off the nuclei of different wire targets. About 122 · 103 recorded muonic J/ψ decays, J/ψ → μ+μ−, resulted in almost 10 000 reconstructed χc → J/ψγ. The ratio Rχc = ∑ i=1,2 σ(χci)Br(χci → J/ψγ)/σ(J/ψ), which is the ratio of J/ψ from χc decays to all produced J/ψ, was measured in the kinematical range −0.35 < x F < 0.15 for carbon and tungsten wire targets. The average over both materials yields Rχc = 0.190+0.030 −0.029. Despite the small separation of the masses of the two χc states, comparable to the detector resolution, the ratio R12 = Rχc1/Rχc2 was measured yielding R12 = 1.30+0.59 −0.37 which corresponds to a production cross section ratio σ(χc1) σ(χc2) = 0.74 +0.34 −0.22. By using the known J/ψ production cross section, the χc1 and χc2 production cross sections are calculated to be σ(χc1) = (153 ± 27) nb/nucleon and σ(χc2) = (207 ± 39) nb/nucleon, respectively. All results were obtained under the assumption that both the J/ψ and χc states are produced without polarization. In addition a study of possible deviations of Rχc and R12 due to the polarization of J/ψ and χc was performed. By varying the polarization parameter, λobs, of all produced J/ψ by 2σ around the value measured by HERA-B, and assuming fully polarized χc states, the maximum variations of Rχc and R12 were evaluated. These studies show that Rχc could change up to 21% and R12 from −11% to +16% relative to the values calculated without polarization.