J/ψ production study at the LHCb experiment

The large hadron collider (LHC) at Conseil European Pour Recherches Nucleaires (CERN) in Geneva is the largest particle physics accelerator. Proton-proton collisions up to a center-of-mass energy of 14 TeV to be produced at the LHC will offer great opportunity to address most fundamental questions in physics. The LHCb experiment is one of the four main experiments at the LHC. Dedicated to b physics studies, the primary goal of the LHCb experiment is to explore possible New Physics beyond the Standard Model through the studies of rare decays of charm and beauty-flavoured hadrons and precision measurements of CP-violating observables. Even at its first running phase, the enormous number of J/ψ particles to be collected by the LHCb detector will allow us to realize some very interesting physics studies on charmonium productions. The measurement may shed new light on the long standing puzzle in understanding the mechanism of quarkonia production in hadron-hadron collisions. In this thesis, studies of J/ψ production at LHCb are presented based on the fully-simulated Monte Carlo events. The procedure developed in the thesis will be used to analyze real data once enough statistics are accumulated. The study has been performed using a fully-simulated Monte Carlo sample generated at a center-of-mass energy of 14 TeV and a luminosity of 2 × 10^32 cm^−2 s^−1 to simulate multiple interactions. The sample size corresponds to an integrated luminosity of 0.79 pb^−1 . J/ψ events are reconstructed according to selection criteria optimized to achieve the best discriminating power against background processes. The study shows that 6.5 ×10^6 J/ψ events can be reconstructed in every pb^−1 of data, with a mass resolution of 11 MeV/c^2 and a S/B ∼ 18 in a ±3σ mass window. The production cross-sections of the prompt J/ψ and the J/ψ from b decays are measured in 28 bins of pT and η covering the region of 0 < pT < 7 GeV/c and 3 < η < 5. In each bin, a time-like variable is defined to distinguish the prompt J/ψ mesons from those of b decays. With the 0.79 pb−1 of data, we can achieve a 10% precision for ma jority of the bins. Possible systematic errors are estimated to be at the same level. The study also shows that the polarization of the J/ψ plays an important role in the cross-section determinations. It may contribute a systematic uncertainty up to 30% in some pT and η bins. Such an effect can be well reduced once an analysis on the J/ψ polarization is performed simultaneously. The measurement of the polarization parameters will also help in the understanding of the J/ψ production mechanism. In order to reduce the amount of simulated data needed for the efficiency estimations, a method is developed to describe the 2-D efficiency by three parameters and all the errors except statistic errors from data will depend on the three parameters. Based on the three parameters estimated from the same Monte-Carlo data, errors on the polarization parameters for the ma jority of the bins are determined to be around 0.1 for the prompt J/ψ and around 0.2 for the J/ψ from b decays. As the LHCb experiment has already collected 14 nb^−1 of data, part of the J/ψ analysis can be performed. Around 3,000 J/ψ candidates are reconstructed with a mass resolution of 16 MeV/c^2 and a S/B ∼ 1 in a ±3σ mass window. Based on the above sample, the cross sections as a function of pT are obtained. The preliminary cross section for J/ψ in the region pT ∈ [0, 9] GeV/c and y ∈ [2.5, 4] is 7.6 ± 0.3 µb where only the statistical error is quoted.