CMS-BPH-19-001

CMS-BPH-19-001 ; CERN-EP-2020-146
Measurement of $\mathrm{B_c(2S)^{+}}$ and $\mathrm{B^{*}_c(2S)^{+}}$ cross section ratios in proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
19 August 2020
Phys. Rev. D 102 (2020) 092007
Abstract: The ratios of the $\mathrm{B_c(2S)^{+}}$ to $\mathrm{B_c^{+}}$ , $\mathrm{B^{}_c(2S)^{+}}$ to $\mathrm{B_c^{+}}$ , and $\mathrm{B^{}_c(2S)^{+}}$ to $\mathrm{B_c(2S)^{+}}$ production cross sections are measured in proton-proton collisions at $\sqrt{s} =$ 13 TeV, using a data sample collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 143 fb $^{-1}$ . The three measurements are made in the $\mathrm{B_c^{+}}$ meson phase space region defined by the transverse momentum ${p_{\mathrm{T}}} >$ 15 GeV and absolute rapidity $\|y\| <$ 2.4, with the excited $\mathrm{B_c(2S)^{+}}$ states reconstructed through the ${ \mathrm{B_c^{+}} \pi^{+}\pi^{-}}$ , followed by the ${ \mathrm{B_c^{+}} \! \to \! \mathrm{J/\psi} \, \pi^{+}}$ and ${ \mathrm{J/\psi} \! \to \! \mu^{+}\mu^{-}}$ decays. The $\mathrm{B_c(2S)^{+}}$ to $\mathrm{B_c^{+}}$ , $\mathrm{B^{}_c(2S)^{+}}$ to $\mathrm{B_c^{+}}$ , and $\mathrm{B^{}_c(2S)^{+}}$ to $\mathrm{B_c(2S)^{+}}$ cross section ratios, including the unknown $\mathrm{B^{()}_c(2S)^{+}} \!\to\! \mathrm{B_{c}^{()+}} \pi^{+}\pi^{-}$ branching fractions, are (3.47 $\pm$ 0.63 (stat) $\pm$ 0.33 (syst))%, (4.69 $\pm$ 0.71 (stat) $\pm$ 0.56 (syst))%, and 1.35 $\pm$ 0.32 (stat) $\pm$ 0.09 (syst), respectively. None of these ratios shows a significant dependence on the ${p_{\mathrm{T}}}$ or $\|y\|$ of the $\mathrm{B_c^{+}}$ meson. The normalized dipion invariant mass distributions from the decays ${ \mathrm{B^{()}_c(2S)^{+}} \!\to\! \mathrm{B_{c}^{()+}} \pi^{+}\pi^{-}}$ are also reported.
Links: e-print arXiv:2008.08629 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of the $\mathrm{B_c^{+}} \, \to \, \mathrm{J}/\psi \, \pi^{+}$ candidates, after applying all event selection criteria [1]. The fitted contributions are shown by the stacked distributions, the solid line representing their sum. The vertical dashed lines indicate the mass window used to select the $\mathrm{B_c^{+}}$ candidates for the $\mathrm{B_c(2S)^{+}}$ reconstruction.
png pdf	Figure 2: Invariant mass distribution of the ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{()+} \pi^{+}\pi^{-}}$ candidates [1]. The ${\mathrm{B_c} ^{}\mathrm {(2S)}^{\,+}}$ corresponds to the lower-mass peak, the ${\mathrm{B_c} \mathrm {(2S)}^{\,+}}$ to the higher. The fitted contributions are shown by the stacked distributions, the solid line representing their sum.
png pdf	Figure 3: The $R^{+}$ and $R^{+}$ (upper), and $R^{+}/R^{+}$ (lower) cross section ratios, including the ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{(*)+} \pi^{+}\pi^{-}}$ branching fractions, as functions of the $\mathrm{B_c^{+}}$ ${p_{\mathrm {T}}}$ (left) and ${\| y \|}$ (right). The horizontal bars show the bin widths. The markers are shown at the average $\mathrm{B_c^{+}}$ ${p_{\mathrm {T}}}$ or ${\| y \|}$ values of the events contributing to each bin, in the background-subtracted distributions, and the vertical bars represent the statistical uncertainties only. The systematic uncertainties are essentially independent of the $\mathrm{B_c^{+}}$ kinematics.
png pdf	Figure 3-a: The $R^{+}$ and $R^{+}$ (upper), and $R^{+}/R^{+}$ (lower) cross section ratios, including the ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{(*)+} \pi^{+}\pi^{-}}$ branching fractions, as functions of the $\mathrm{B_c^{+}}$ ${p_{\mathrm {T}}}$ . The horizontal bars show the bin widths. The markers are shown at the average $\mathrm{B_c^{+}}$ ${p_{\mathrm {T}}}$ values of the events contributing to each bin, in the background-subtracted distributions, and the vertical bars represent the statistical uncertainties only. The systematic uncertainties are essentially independent of the $\mathrm{B_c^{+}}$ kinematics.
png pdf	Figure 3-b: The $R^{+}$ and $R^{+}$ (upper), and $R^{+}/R^{+}$ (lower) cross section ratios, including the ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{(*)+} \pi^{+}\pi^{-}}$ branching fractions, as functions of the $\mathrm{B_c^{+}}$ ${\| y \|}$ . The horizontal bars show the bin widths. The markers are shown at the average $\mathrm{B_c^{+}}$ ${\| y \|}$ values of the events contributing to each bin, in the background-subtracted distributions, and the vertical bars represent the statistical uncertainties only. The systematic uncertainties are essentially independent of the $\mathrm{B_c^{+}}$ kinematics.
png pdf	Figure 4: The dipion invariant mass distributions from ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{(*)+} \pi^{+}\pi^{-}}$ decays in data, normalized to unity. The inner and outer tick marks designate the statistical and total uncertainties, respectively. The lines show the corresponding predictions from phase space simulations.

Tables
png pdf	Table 1: Ratios of the reconstruction efficiencies relevant for the determination of the $R^{+}$ , $R^{+}$ , and $R^{+}/R^{+}$ cross section ratios. The central values are followed by the several uncertainties presented in the text.
png pdf	Table 2: Relative systematic uncertainties (in %) in the cross section ratios, including the ${\mathrm{B_c(2S)^{+}} \,\to \, \mathrm{B_c} ^{(*)+} \pi^{+}\pi^{-}}$ branching fractions, corresponding to the sources described in the text. The total uncertainty is the sum in quadrature of the individual terms.

Summary

The ratios of the

$\mathrm{B_c(2S)^{+}}$ to

$\mathrm{B_c^{+}}$ ,

$\mathrm{B^{*}_c(2S)^{+}}$ to

$\mathrm{B_c^{+}}$ , and

$\mathrm{B^{*}_c(2S)^{+}}$ to

$\mathrm{B_c(2S)^{+}}$ production cross sections,

$R^{+}$ ,

$R^{*+}$ , and

$R^{*+}/R^{+}$ , respectively, have been measured in proton-proton collisions at

$\sqrt{s} =$ 13 TeV. Data set used in the analysis corresponds to an integrated luminosity of 143 fb

$^{-1}$ collected by the CMS experiment at the LHC between 2015 and 2018.

The

$\mathrm{B_c(2S)^{+}}$ mesons were reconstructed through the decays

${ \mathrm{B^{(*)}_c(2S)^{+}} \!\to\! \mathrm{B_{c}^{(*)+}} \pi^{+}\pi^{-}}$ , followed by the

${ \mathrm{B_c^{+}} \! \to \! \mathrm{J/\psi} \, \pi^{+}}$ and

${ \mathrm{J/\psi} \! \to \! \mu^{+}\mu^{-}}$ . The measured cross section ratios, including the (unknown)

${ \mathrm{B^{(*)}_c(2S)^{+}} \!\to\! \mathrm{B_{c}^{(*)+}} \pi^{+}\pi^{-}}$ branching fractions, are

$\ \ \ \ \ \ \ \ \ \ \ R^{+} =$ (3.47

$\pm$ 0.63 (stat)

$\pm$ 0.33 (syst))%

$\ \ \ \ \ \ \ \ \ \ R^{*+} =$ (4.69

$\pm$ 0.71 (stat)

$\pm$ 0.56 (syst))%

$R^{*+}/R^{+} =$ 1.35

$\pm$ 0.32 (stat)

$\pm$ 0.09 (syst)
No significant dependences on the transverse momentum

$p_{\mathrm{T}}$ or rapidity

$|y|$ of the

$\mathrm{B_c^{+}}$ mesons have been observed for any of these three ratios. The normalized dipion invariant mass distributions for the

${ \mathrm{B^{(*)}_c(2S)^{+}} \!\to\! \mathrm{B_{c}^{(*)+}} \pi^{+}\pi^{-}}$ decays are also reported. These results, obtained in the phase space region defined by

$\mathrm{B_c^{+}}$ meson

$p_{\mathrm{T}} >$ 15 GeV and

$|y| <$ 2.4, may provide new important input to improve the theoretical understanding of the nature of the bc heavy-quarkonium states and their production processes.

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