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CMS-PAS-HIN-20-004

Observation of the ${\rm B_c^+}$ meson in PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV

CMS Collaboration

May 2021

Abstract: The ${\rm B_c^+}$ meson is observed with the CMS detector in proton-proton and lead-lead collisions at a center-of-mass energy per nucleon pair of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV, via the ${\rm B_c^+} \rightarrow ({\rm J}/\psi \rightarrow \mu^+ \mu^-) \mu^+ \nu_\mu$ decay. Its cross section and nuclear modification factor are measured in two bins of the trimuon transverse momentum, and in two ranges of collision centrality. As the ${\rm B_c^+}$ is the only meson containing two different flavored heavy quarks, it provides a unique bridge between charmonia, bottomonia, and open heavy mesons. The insights gained from these results will further the understanding of the interplay of suppression and enhancement mechanisms in the production of heavy mesons in the hot and dense matter created in heavy ion collisions.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures
png pdf	Figure 1: 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 panels show the pull between data and the fitted distributions.
png pdf	Figure 2: 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 panels show the pull between data and the fitted distributions.
png pdf	Figure 3: $\mathrm{B^{+}_{c}}$ meson production pp -equivalent cross-section times branching fraction of the studied decay in pp and PbPb collisions. Two bins of the trimuon ${p_{\mathrm {T}}}$ are shown, which also correspond to different rapidity ranges. The solid and lighter rectangles show the fit uncertainty and the total uncertainty, respectively. The bin-to-bin correlation factor $\rho _{1\textrm {-}2}$ is also displayed. The markers of the ${p_\text {T}^{\mu \mu \mu}}$ bins are placed according to the Lafferty-Wyatt prescription [39].
png pdf	Figure 4: $\mathrm{B^{+}_{c}}$ meson nuclear modification factor in ${p_\text {T}^{\mu \mu \mu}}$ bins corresponding to different rapidity ranges (left), and in centrality bins integrated over the studied kinematic range (right). The solid and lighter rectangles, respectively, show the bin-to-bin-uncorrelated uncertainty and the total uncertainty, such that the uncertainty on the difference of the two bins is the quadratic sum of uncorrelated uncertainties. The bin-to-bin correlation factor $\rho _{1\textrm {-}2}$ is also displayed. The markers of the ${p_\text {T}^{\mu \mu \mu}}$ bins are placed as the average of the Lafferty-Wyatt prescription [39] applied on the pp and PbPb spectra. The centrality bin markers are placed at the minimum-bias average number of participants $N_{\textrm {part}}$.
png pdf	Figure 4-a: $\mathrm{B^{+}_{c}}$ meson nuclear modification factor in ${p_\text {T}^{\mu \mu \mu}}$ bins corresponding to different rapidity ranges. The solid and lighter rectangles, respectively, show the bin-to-bin-uncorrelated uncertainty and the total uncertainty, such that the uncertainty on the difference of the two bins is the quadratic sum of uncorrelated uncertainties. The bin-to-bin correlation factor $\rho _{1\textrm {-}2}$ is also displayed. The markers of the ${p_\text {T}^{\mu \mu \mu}}$ bins are placed as the average of the Lafferty-Wyatt prescription [39] applied on the pp and PbPb spectra.
png pdf	Figure 4-b: $\mathrm{B^{+}_{c}}$ meson nuclear modification factor in centrality bins integrated over the studied kinematic range. The solid and lighter rectangles, respectively, show the bin-to-bin-uncorrelated uncertainty and the total uncertainty, such that the uncertainty on the difference of the two bins is the quadratic sum of uncorrelated uncertainties. The bin-to-bin correlation factor $\rho _{1\textrm {-}2}$ is also displayed. The centrality bin markers are placed at the minimum-bias average number of participants $N_{\textrm {part}}$.

Summary

In summary, the first observation of the ${\rm B_c^+}$ meson in heavy ion collisions is presented. The production cross sections in PbPb and pp collisions, along with their ratios, are measured in two bins of ${p_{\mathrm {T}}}$ and two bins of centrality. This unique meson, a hybrid beauty-charm state as well as the heaviest ground-state B meson, helps disentangle the enhancement and suppression mechanisms at play in the evolution of heavy quarks through the QGP.

Additional Figures
png pdf	Additional Figure 1: Distribution of the BDT variable for the pp samples. The ROC curve showing the discriminative power is also shown on the right.
png pdf	Additional Figure 2: Distribution of the BDT variable for the PbPb samples. The ROC curve showing the discriminative power is also shown on the right.
png pdf	Additional Figure 3: Trimuon mass distribution for simulated decays in the studied kinematic region, without any other selection.
png pdf	Additional Figure 4: Acceptance times efficiency of the trigger, reconstruction and selection of the signal, as a function of the trimuon ${p_{\mathrm {T}}}$ and absolute rapidity, in pp collision. It is estimated from the simulated signal sample corrected with the final measured ${p_\text {T}^{\mu \mu \mu}}$ distribution. The black lines show the regions chosen as the two kinematic bins.
png pdf	Additional Figure 5: Acceptance times efficiency of the trigger, reconstruction and selection of the signal, as a function of the trimuon ${p_{\mathrm {T}}}$ and absolute rapidity, in PbPb collision. It is estimated from the simulated signal sample corrected with the final measured ${p_\text {T}^{\mu \mu \mu}}$ distribution. The black lines show the regions chosen as the two kinematic bins.
png pdf	Additional Figure 6: Template fit in pp for candidates with 6 $ < {p_\text {T}^{\mu \mu \mu}} < $ 11 GeV and 1.3 $ < | {y^{\mu \mu \mu}} | < $ 2.3. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 7: Template fit in pp for candidates with 11 $ < {p_\text {T}^{\mu \mu \mu}} < $ 35 GeV and $| {y^{\mu \mu \mu}} | < $ 2.3. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 8: Template fit in PbPb for candidates with 6 $ < {p_\text {T}^{\mu \mu \mu}} < $ 11 GeV and 1.3 $ < | {y^{\mu \mu \mu}} | < $ 2.3. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 9: Template fit in PbPb for candidates with 11 $ < {p_\text {T}^{\mu \mu \mu}} < $ 35 GeV and $| {y^{\mu \mu \mu}} | < $ 2.3. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 10: Template fit in PbPb for candidates in the 0-20% centrality range, integrated over the two studied kinematic regions. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 11: Template fit in PbPb for candidates in the 20-90% centrality range, integrated over the two studied kinematic regions. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 12: Template fit in pp for candidates with 6 $ < {p_\text {T}^{\mu \mu \mu}} < $ 11 GeV and 1.3 $ < | {y^{\mu \mu \mu}} | < $ 2.3, with a BDT variable decorrelated from the trimuon mass. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 13: Template fit in PbPb for candidates with 6 $ < {p_\text {T}^{\mu \mu \mu}} < $ 11 GeV and 1.3 $ < | {y^{\mu \mu \mu}} | < $ 2.3, with a BDT variable decorrelated from the trimuon mass. The trimuon mass distributions of data and of the fitted signal and background sources are shown in the three BDT bins. The lower panels show the pull between data and the fitted distributions.
png pdf	Additional Figure 14: Post-fit yield and its uncertainty, divided by the nominal post-fit yield, for all fit method variations. The pp and PbPb yields as well as the ratio PbPb /pp are shown for both ${p_{\mathrm {T}}}$ bins. For each bin, a vertical dotted line at 1 indicates the position of the nominal value. The horizontal dotted lines indicate groups of similar variations. The names of all variations are mentioned in the left column. The variations included in the calculation of the uncertainty are in orange; the others, only used as checks, are in violet.
png pdf	Additional Figure 15: $-2\Delta \ln\mathcal {L}$ versus the parameters of interest $r_1$ and $r_2$ that multiply the signal normalisation in the low-${p_{\mathrm {T}}}$ and high-${p_{\mathrm {T}}}$ bin, respectively. Only the regions with $-2\Delta \ln\mathcal {L} < 25$ are coloured, which approximately corresponds to a 5$\sigma $ contour. The first (blue) colour level corresponds to a 1$\sigma $ contour.
png pdf	Additional Figure 16: Contribution of the various sources to the relative uncertainties in the cross sections and in the ${R_{\textrm {PbPb}}}$, compared to the total uncertainty, for the ${p_{\mathrm {T}}}$ dependence. Bin1 is at low-${p_{\mathrm {T}}}$ and bin2 is at high-${p_{\mathrm {T}}}$. The average of the lower and higher uncertainty is shown, except for the uncertainties from other decays where, for the cross sections, only the lower (non-zero) uncertainty is shown.
png pdf	Additional Figure 17: Contribution of the various sources to the relative uncertainties in the cross sections and in the ${R_{\textrm {PbPb}}}$, compared to the total uncertainty, for the centrality dependence. Bin1 is at low-${p_{\mathrm {T}}}$ and bin2 is at high-${p_{\mathrm {T}}}$. The average of the lower and higher uncertainty is shown, except for the uncertainties from other decays where, for the cross sections, only the lower (non-zero) uncertainty is shown.
png pdf	Additional Figure 18: Comparison of theoretical predictions of the nuclear modification factor of in the two ${p_{\mathrm {T}}}$ bins. The kinematics of the prediction are the ones of the full, whereas the trimuon kinematics are used for our measurement.
png pdf	Additional Figure 19: Comparison of theoretical predictions of the nuclear modification factor of in the two centrality bins. The kinematics of the prediction are the ones of the full, whereas the trimuon kinematics are used for our measurement.
png pdf	Additional Figure 20: Comparison of the modification to the nuclear modification factors measured with CMS for heavy quarkonia ground and excited states. Only the total uncertainty is shown for the, whereas all other results show the statistical uncertainties as bars and the systematic uncertainties as shaded boxes. The modification is binned in the trimuon ${p_{\mathrm {T}}}$.
png pdf	Additional Figure 21: Comparison of the modification to the nuclear modification factors measured with CMS for various open heavy flavour mesons and for light hadrons. Only the total uncertainty is shown for the, whereas all other results show the statistical uncertainties as bars and the systematic uncertainties as shaded boxes. The modification is binned in the trimuon ${p_{\mathrm {T}}}$.
png pdf	Additional Figure 22: Comparison and ratio of the BDT distributions of data and of the sum of all post-fit templates (signal MC plus the three backgrounds) for the first $p_{\mathrm{T}}$ bin in PbPb. The templates result from the final fit, after the second $p_{\mathrm{T}}$ spectrum correction of the signal MC. The displayed uncertainties are purely statistical.
png pdf	Additional Figure 23: Comparison and ratio of the BDT distributions of data and of the sum of all post-fit templates (signal MC plus the three backgrounds) for the second $p_{\mathrm{T}}$ bin in PbPb. The templates result from the final fit, after the second $p_{\mathrm{T}}$ spectrum correction of the signal MC. The displayed uncertainties are purely statistical.
png pdf	Additional Figure 24: Comparison and ratio of the BDT distributions of data and of the sum of all post-fit templates (signal MC plus the three backgrounds) for the first $p_{\mathrm{T}}$ bin in pp. The templates result from the final fit, after the BDT distribution correction and the second $p_{\mathrm{T}}$ spectrum correction of the signal MC. The displayed uncertainties are purely statistical.
png pdf	Additional Figure 25: Comparison and ratio of the BDT distributions of data and of the sum of all post-fit templates (signal MC plus the three backgrounds) for the second $p_{\mathrm{T}}$ bin in pp. The templates result from the final fit, after the BDT distribution correction and the second $p_{\mathrm{T}}$ spectrum correction of the signal MC. The displayed uncertainties are purely statistical.

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