CMS-HIN-20-004

CMS-HIN-20-004 ; CERN-EP-2021-259
Observation of the $\mathrm{B_c^{+}}$ meson in PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
7 January 2022
Phys. Rev. Lett. 128 (2022) 252301
Abstract: The $\mathrm{B_c^{+}}$ meson is observed for the first time in heavy ion collisions. Data from the CMS detector are used to measure the production of the $\mathrm{B_c^{+}}$ meson in lead-lead (PbPb) and proton-proton (pp) collisions at a center-of-mass energy per nucleon pair of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV, via the $\mathrm{B_c^{+}} \to (\mathrm{J}/\psi \to \mu^{+} \mu^{-}) \mu^{+} \nu_{\mu}$ decay. The $\mathrm{B_c^{+}}$ 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 $\mathrm{B_c^{+}}$ meson is shown to be less suppressed than other 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 first observation of the $\mathrm{B_c^{+}}$ meson in heavy ion collisions sets forth a new promising probe of the interplay of suppression and enhancement mechanisms in the production of heavy-flavor mesons in the quark-gluon plasma.
Links: e-print arXiv:2201.02659 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Template fit of the trimuon mass distributions in the three BDT bins, for the pp (top row) and PbPb (bottom row) data samples integrated over the two studied kinematic regions. The lower panels show the pull between the data and the fitted distributions.
png pdf	Figure 1-a: Template fit of the trimuon mass distributions in the three BDT bins, for the pp data sample integrated over the two studied kinematic regions. The lower panel shows the pull between the data and the fitted distributions.
png pdf	Figure 1-b: Template fit of the trimuon mass distributions in the three BDT bins, for the PbPb data sample integrated over the two studied kinematic regions. The lower panel shows the pull between the data and the fitted distributions.
png pdf	Figure 2: The $\mathrm{B_c^{+}}$ meson production in pp and PbPb collisions. Left: The pp -equivalent cross-section times branching fraction of the studied decay in pp and PbPb collisions. Middle and right: The nuclear modification factor in ${p_\text {T}^{\mu \mu \mu}}$ bins (middle), and in centrality bins integrated over the studied kinematic range (right). See the text for details.
png pdf	Figure 2-a: The $\mathrm{B_c^{+}}$ meson production in pp and PbPb collisions. The pp -equivalent cross-section times branching fraction of the studied decay in pp and PbPb collisions. See the text for details.
png pdf	Figure 2-b: The $\mathrm{B_c^{+}}$ meson production in pp and PbPb collisions. The nuclear modification factor in ${p_\text {T}^{\mu \mu \mu}}$ bins. See the text for details.
png pdf	Figure 2-c: The $\mathrm{B_c^{+}}$ meson production in pp and PbPb collisions. The nuclear modification factor in centrality bins integrated over the studied kinematic range. See the text for details.
png pdf	Figure A1: Nuclear modification factor of the $\mathrm{B_c^{+}}$ meson compared to that of light charged hadrons, and $\mathrm{B_c^{+}}$, $\mathrm{B_s}$ and ${\mathrm{D^0}}$ mesons, as a function of the measured transverse momentum. The total uncertainty is shown for the $\mathrm{B_c^{+}}$ meson, whereas the statistical (bars) and systematic (filled rectangles) uncertainties are shown for the other hadrons.
png pdf	Figure A2: Nuclear modification factor of the $\mathrm{B_c^{+}}$ meson compared to that of the ground and first excited states of charmonia and bottomonia, as a function of the measured transverse momentum. The total uncertainty is shown for the $\mathrm{B_c^{+}}$ meson, whereas the statistical (bars) and systematic (filled rectangles) uncertainties are shown for the other hadrons.

Summary

In summary, the first observation of the $\mathrm{B_c^{+}}$ meson in heavy ion collisions is presented, using the $\mathrm{B_c^{+}} \to (\mathrm{J}/\psi \to \mu^{+} \mu^{-}) \mu^{+} \nu_{\mu}$ decay. The production cross sections in lead-lead and proton-proton collisions and the nuclear modification factor derived from their ratio are measured in two bins of the trimuon transverse momentum, and in two ranges of the heavy-ion centrality. This unique beauty-charm state can help disentangle the enhancement (possibly dominant in central events at low-$p_{\mathrm{T}}$) and suppression (dominant at high-$p_{\mathrm{T}}$) mechanisms at play in the evolution of heavy quarks through the quark-gluon plasma.

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