The Bc+ meson in heavy-ion collisions with the CMS detector

This thesis addresses how heavy-quark hadronisation and high-energy partons are affected by the quark-gluon plasma (QGP), a hot and dense medium created in lead-lead (PbPb) collisions at the LHC. Data from the CMS detector are analysed to achieve the first observation of Bc+ mesons in heavy-ion collisions. Building on an existing scaling law from a model of radiative energy loss in the QGP, a second scaling law is also brought to light. The analysis of CMS data from 2017 proton-proton and 2018 PbPb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair leads to the observation of Bc+ -> (J/psi -> mu+mu−) mu+ nu_mu decays, and to the measurement of the Bc+ nuclear modification factor in two bins of the trimuon transverse momentum (pT) or of the PbPb collision centrality. It is presented in this thesis and summarised in Ref. [1]. Three main backgrounds are described either with simulation or with specifically-designed data-driven samples. A boosted decision tree (BDT) is trained on the selected background and simulated signal candidates. A likelihood fit is run on signal and background templates, binned in BDT, trimuon invariant mass, and pT or centrality. The acceptance and efficiency of the selection chain are evaluated iteratively in each pT or centrality bin with the simulated signal, whose pT spectrum is first corrected with the one measured in a preliminary analysis. The Bc+ meson is found to be less suppressed than all measured open and hidden heavy flavour mesons, except the B0s meson. The results also hint at a softening of the pT spectrum in PbPb collisions. These may indicate that heavy-quark recombination is a significant Bc+ production mechanism. At high pT (> 10−15 GeV), radiative energy loss should be the dominant source of suppression of hadrons in the QGP. An existing model for the radiative energy loss of partons, based on the BDMPS medium-induced gluon spectrum, predicts a universal pT-dependence of the nuclear modification factor [2]. This fits collected measurements in systems of various geometric configurations and energies, from which the corresponding mean energy losses are determined [3]. A new scaling law is found consistent with the gathered measurements: it links the extracted mean energy loss with the average path length in the medium and the charged particle multiplicity. This leads to the extraction of medium expansion and diffusion properties, and could allow for a prediction of the azimuthal asymmetry coefficient v2 at high pT. This thesis is structured with an introduction (chapter 1) travelling from general physics considerations to concepts needed in the following, then a first part concerning the Bc analysis, and a second part concerning my work on the phenomenology of energy loss. Chapter 2 motivates the experimental search for Bc mesons in heavy ion collisions. Chapter 3 presents the LHC and the CMS detector, and the overall analysis strategy (section 3.3). Chapter 4 deals with the backgrounds blurring the Bc signal, and chapter 5 details the candidate selection established to discriminate against them. Chapter 6 shows the extraction of the signal yields from a template fit of the trimuon mass distributions. Chapter 7 computes the correction of the observed yields for the acceptance and efficiency of the signal reconstruction and selection. Chapter 8 summarises the sources of uncertainties and how they are estimated. Chapter 9 shows the results and their interpretation, including the Bc nuclear modification factor. In the phenomenology part, chapter 10 describes the energy loss model I start from, and chapter 11 shows two scaling laws it results in. Chapter 12 shows some improvements and additions to the model. Lists of abbreviations, acronyms, and some numerical values, can be found at the end of the document. [1] CMS Collaboration, Observation of the Bc+ meson in PbPb and pp collisions at sqrt(sNN) = 5.02 TeV, arXiv:2201.02659, Phys. Rev. Lett. 128 (2022) 252301 [2] F. Arleo, Quenching of Hadron Spectra in Heavy Ion Collisions at the LHC. Phys. Rev. Lett., 119(6), 062302, 2017. doi: 10.1103/PhysRevLett.119.062302. [3] F. Arleo and G. Falmagne, Quenching of hadron spectra in XeXe and PbPb collisions at the LHC. PoS, HardProbes2018, 075, 2019. doi: 10.22323/1.345.0075