Charm and Beauty Production at HERA–B

The HERA–B experiment at DESY has acquired a data-set of approximately 300,000 decays J/ψ→ l+l− during its 2002/2003 data-taking period. These data are used to analyze the production of heavy quarks in proton-nucleus interactions at a center-of-mass energy of 41.6 GeV. In this article, preliminary results of two measurements are discussed, a measurement of nuclear effects in the production of J/ψ mesons and a measurement of the bb production cross section. HERA–B: DETECTOR, TRIGGER AND DATA-SET The HERA–B detector, depicted in Fig. 1, is a fixed-target spectrometer with large angular acceptance. Protons from the halo of the HERA proton beam are collided with an internal wire target. The target consists of up to eight thin wires of different materials, distributed in two target stations. Each wire can be moved independently to adjust the interaction rate. The HERA–B detector consists of the following sub-detectors: A silicon micro-strip vertex detector is used to reconstruct and separate primary and secondary vertices. The tracking detectors comprise micro-strip gaseous chambers with gas electron multiplier foils in the inner acceptance region of high particle flux and honeycomb drift chambers in the outer region. Particle identification is performed by a ring-imaging Čerenkov counter, an electromagnetic calorimeter, and a four-layer muon detector. Lepton pairs from J/ψ decays are enriched by a multi-level trigger system. The first trigger level is implemented as a hardware track trigger in the outer tracking detector. Starting points for the track search are provided by pretriggers in the muon detector and the calorimeter. At the second trigger level, a software trigger running on a PC farm, track candidates are extrapolated to the vertex detector, and a two-prong vertex fit is performed. Accepted events are reconstructed online. The HERA–B experiment has taken data between October 2002 and February 2003. The data sample recorded with the lepton pair trigger contains approximately 150,000 decays into each of the channels J/ψ→ e+e− and J/ψ→ μ+μ−, 90,000 of which were taken with two target wires of different materials simultaneously. Based on this data set, the HERA–B collaboration has analyzed the production of charmonium states such as J/ψ, ψ(2S) and χc, as well as the production of open and hidden beauty, e.g. bb and Υ.