First observation of the $B_c^+$ meson in PbPb and pp collisions at 5.02 TeV at CMS

About

Studying the interaction of heavy quarks in the quark-gluon plasma (QGP) created in PbPb collisions at the LHC sheds light on the behaviour of QCD at very high temperature. The $B_c^+$ meson is a novel probe of the QGP, that provides original insights on the effects of the medium on the heavy-quark bound states, such as the recombination, Debye screening and energy loss processes. We report here the first observation of the $B_c^+$ meson in heavy ion collisions, via the $B_c^+ \to (J/\psi\to\mu^+\mu^-) \mu^+ \nu_\mu$ decay channel, with a significance well above 5$\sigma$. The $B_c^+$ yields in proton-proton (pp) and lead-lead (PbPb) collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\textrm{NN}}}= 5.02$ TeV, respectively from 2017 and 2018 CMS data, are measured. The yields and the nuclear modification factor $R_{\textrm{AA}}(B_c^+)$ are shown in two bins of the trimuon $p_T$ and in two ranges of centrality. The observed suppression is significantly less than that of $B^+$ and $D$ mesons and heavy quarkonia, which may point at the importance of heavy-quark coalescence in the production of $B_c^+$ mesons in heavy ion collisions.

LHC Seminar - CMS: https://cern.zoom.us/j/68063414718?pwd=dXNpQ1JobWRwQzgzMVJSdlRZeEVCdz09.

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