Observation of the B${}_{\mathrm{c}}^{+}$ Meson in Pb-Pb and pp Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV and Measurement of its Nuclear Modification Factor

The B${}_{\mathrm{c}}^{+}$ meson is observed for the first time in heavy ion collisions. Data from the CMS detector are used to study the production of the B${}_{\mathrm{c}}^{+}$ meson in lead-lead (Pb-Pb) and proton-proton (pp) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV , via the B${}_{\mathrm{c}}^{+}$ → (J/ψ → μ${}^{+}$μ${}^{-}$)μ${}^{+}$ν${}_{\mu }$ decay. The B${}_{\mathrm{c}}^{+}$ nuclear modification factor, derived from the PbPb-to-pp ratio of production cross sections, is measured in two bins of the trimuon transverse momentum and of the PbPb collision centrality. The B${}_{\mathrm{c}}^{+}$meson is shown to be less suppressed than quarkonia and most of the open heavy-flavor mesons, suggesting that effects of the hot and dense nuclear matter created in heavy ion collisions contribute to its production. This measurement sets forth a promising new probe of the interplay of suppression and enhancement mechanisms in the production of heavy-flavor mesons in the quark-gluon plasma.