Beauty at HERA-B : Measurement of the bb Production Cross Section in pN Collisions at sqrt(s) = 41.6 GeV

Abstract

This thesis reports on the measurement of the bb production cross section using the 2000 dataset of the HERA-B experiment. HERA-B is a fixed target experiment using the 920 GeV protons of the HERA accelerator of DESY, Hamburg. The production cross section of beauty quarks in pN collisions at fixed target energies is not well known. The two existing measurements are inconsistent and suffer from large errors. Theoretical calculations contain large uncertainties. A measurement by HERA-B could contribute to a better understanding of bb hadroprodution. The experiment is built as a forward spectrometer with multiple tracking and particle identification devices. Target wires of different materials can be moved into the halo of the proton beam to obtain the desired interaction rate. The trigger is designed to select the two leptons from J/psi->l+l- decays, both in the muon and in the electron channel. In this thesis only the electron channel was considered. The electron triggered events were used to search for evidence of the b->J/psiX->e+e-X decay. Most of the J/psi are produced directly at the interaction of the proton with the target. Only about 1 per 1000 J/psi is the decay product of a B meson. We measured the bb production cross section relative to the known cross section of the prompt J/psi, making use of the known branching ratio Br(bb->J/psiX). Many of the systematic uncertainties in trigger and detector efficiencies cancel when performing a relative measurement. Thus, for the measurement of the bb production cross section we determined not only the number of bb->J/psiX events, but also the number of prompt J/psi in the same sample. During the commissioning run of 2000, a sample of about 1 million di-electron triggered events was acquired. Most of the data was taken with a Carbon wire. For some runs a second wire of Titanium was inserted. The number of prompt J/psi in this data sample was determined by applying a fit to the spectrum of the invariant mass of the two tracks that resulted in a trigger decision. The non electron background under the J/psi peak in this spectrum was however high. Without strong electron identification criteria it was not possible to identify a clear J/psi signal. A clear signal could be obtained by requiring one or both electrons to have radiated a bremsstrahlung photon. The efficiency of this requirement was however low (34% per electron). The total number of prompt J/psi without bremsstrahlung requirement, 7080±550stat± 570sys, was determined by extrapolating using this measured efficiency. The B->J/psiX decays were selected by exploiting the long lifetime of the B meson. At HERA-B the mean decay length of B mesons is about 1.1 cm. We separated the B->J/psiX events from the J/psi produced at the target by requiring the J/psi and both electrons to be inconsistent with being produced at the target. The efficiency of the bremsstrahlung requirement was too low to be used for the low number of b->J/psi->e+e- events. However, the requirement of a two track vertex detached from the primary interaction point was enough to sufficiently reduce the background from combinatorics in the invariant mass spectrum. The number of b->J/psi->e+e- events was determined by applying a maximum likelihood fit to the invariant mass spectrum. We obtained 6.9+3.7-3.3 b->J/psi events. he obtained value for the bb cross section: s(bb) = 35+19-17(stat) ± 8(sys) nb / nucleon is compatible with existing measurements and QCD predictions. The good agreement with QCD predictions might seem remarkable, since for bb production measurements at LEP, HERA and the Tevatron, QCD predictions are off by a factor two or more. The statistical uncertainty of this measurement is however too high to significantly improve the empirical knowledge on beauty production. With the expected increase in statistics in the 2002/2003 run, HERA-B will be able to provide a precise measurement of s(bb).