Performance studies for the measurement of B0 mesons in proton-proton collisions at 13 TeV with the ALICE experiment

Abstract

The Quark-Gluon Plasma (QGP) is predicted by Quantum Chromodynamics (QCD) to exist at extremely high temperatures and/or energy densities. In this state of matter, quarks and gluons are no longer confined to hadrons and behave asymptotically free. Near Geneva, at the Large Hadron Collider (LHC), experiments to study the properties of the QGP are performed. This thesis focuses on the measurement of beauty production in proton-proton collisions in the ALICE experiment (A Large Ion Collider Experiment), which is one of the four experiments at the LHC. Performance studies with Pythia Monte-Carlo simulations are done to investigate two B0 meson decays. The new LHC centre-of-mass collision energy for proton-proton collisions (13 TeV) and the performance expected after the upgrade of the ALICE detector have been taken into account. This is done with realistic Monte-Carlo simulations, which describe the expected transverse momentum and pointing resolution for the upgraded Inner Tracking System (ITS) detector.

A hadronic and semi-leptonic decay of the B0 meson are studied in this thesis, respectively B0 -> D*+ + pi- and B0 -> D*+ + e- + ve. The D*+ meson decays into D0 + pi+ and the D0 meson into K- + pi+. The hadronic decay is relatively easy to reconstruct, however the branching ratio is quite low: (3.04 +- 0.4)*10^-3%. The semi-leptonic decay has a higher branching ratio (namely (5.52 +- 0.37)*10^-2%), but here a reconstruction procedure for the undetected neutrino is needed. This procedure is based on a recent analysis technique from the LHCb experiment. The yield extraction for both decays is performed by the invariant mass method for the expected number of events for Run-2. Unfortunately, due to limited statistics, this procedure worked only for the pT integrated invariant mass distribution. Therefore, a counting procedure have been used for the six investigated transverse momentum intervals (0 - 3, 3 - 6, 6 - 10, 10 - 15, 15 - 20 and 20 - 30 GeV/c).

A study with the selection cuts used in this thesis becomes profitable for the hadronic decay channel at 35 billion events. At this number of events, the amount of B0 meson decays will vary between 50 and 300 in all the pT intervals. This leads to a statistical significance higher than 2.5, which means that it is possible to perform the invariant mass technique. A study into the semi-leptonic decay will be more difficult because the reconstruction method for the momentum of the neutrino broadens the width of the signal and shifts the peak position. This is unfortunate, because already at 3.5 billion events there will be approximately 200 B0 meson decays, which pass the selection cuts in each pT interval. The statistical significance will lie between 3 and 10, but due to the width of the peak and the background shape it is still difficult to fit the signal.

So both decays studied in this thesis look promising to study beauty production in real proton-proton collisions at sqrt[s] = 13 TeV. However, a full AliRoot simulation is still needed to refine the results.