Beauty photoproduction at HERA II with the ZEUS experiment
In this thesis the beauty photoproduction cross section has been measured with the ZEUS detector at HERA II using e+p data collected in 2004 corresponding to an integrated luminosity of 33 pb-1. Beauty production has been studied in the photoproduction regime, in which the boson exchanged between the proton and the positron is an almost real photon (virtuality, Q2 0 GeV2). Events were selected in which a bb pair is produced and at least one of the pair decays semi-leptonically into a muon. The beauty content of the sample was extracted using a combination of el (transverse momentum of the muon with respect to the closest jet) and impact parameter (distance of closest approach of the muon track to the primary vertex) methods. Measurements of the total and differential cross sections for beauty production were made. The measured cross sections are in good agreement with NLO QCD predictions and with the HERA I results that were obtained from a luminosity 3 times larger than that of the present measurement, using only the P 1 variable to separate the beauty signal from charm and light flavours and fixing the charm-to-light-flavour ratio from external measurements. An external constraint for the charm contribution is not needed in this analysis since its fraction is extracted from the fit to these data.