Probing the Quark Gluon Plasma with B0s and B+ Mesons: Cross Sections in pp and Nuclear Modification Factors in PbPb Collisions

The quark gluon plasma (QGP) is one of the most exciting frontiers of particle physics. This exotic state of matter forms only under the uttermost conditions, of temperature and density, which are now accessible through relativistic heavy ion collisions. Studying it provides valuable information about its broad underlying theory, quantum chromodynamics. Due to b quarks being long lived particles that can record the QGP medium induced effects, hard probes such as B mesons have been identified as a phenomenological tool to learn about the QGP inner workings. In this thesis, the B0s and B+ production cross sections are first measured as functions of transverse momentum, pT , in proton-proton collisions at √s = 5.02 TeV. The data was collected with the CMS detector during the 2017 LHC run. The B mesons are exclusively reconstructed through the decay channels B0s → J/ψ(µ+µ−)φ(K+K−) and B+ → J/ψ(µ+µ−)K+. The production cross sections are compared against the FONLL theoretical prediction and found to be in overall good agreement. To quantify the QGP medium effects on the B meson spectra the nuclear modification factors, RAA, are computed. In PbPb vs pp a high suppression of their production is observed, with B0s mesons being slightly less suppressed than B+ mesons. At low pT, B0s RAA is compatible with the hypothesis of quark coalescence.