Exclusive Production of J/ψ Mesons in the ZEUS Detector

Abstract

Exclusive J/ψ production has been studied using the ZEUS detector at HERA. Two ranges of the photon virtuality, Q² < 1 GeV² and 0.15 < Q² < 0.8 GeV² are examined. The latter is defined by requiring the scattered electron to be detected in the Beam Pipe Calorimeter under low angles. The J/ψ mesons have been identified via the decay J/ψ -> e⁺ e^-. The cross sections are measured as a function of the photon-proton centre-of-mass energy, W. The data used were taken during the 1999 and 2000 running periods, which yielded an integrated luminosity of 68.7 pb^-1.<br /> The measured W dependences are in agreement with each other and the values obtained in high-Q² (2 < Q² < 100 GeV²) measurements. This leads to the conclusion that J/ψ production is a hard process for all Q² values and can be described with models based on perturbative QCD.<br /> Electron-pion separation of the ZEUS Transition Radiation Detector is measured. A method to combine TRD information and dE/dx measurements from the central drift chamber is developed and applied to the Q² < 1 GeV² J/ψ sample.