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CMS-BPH-18-007 ; CERN-EP-2019-014
Observation of two excited $ \mathrm{B^{+}_{c}} $ states and measurement of the ${\mathrm{B^{+}_{c}} \mathrm{(2S)}}$ mass in pp collisions at $\sqrt{s} = $ 13 TeV
Phys. Rev. Lett. 122 (2019) 132001
Abstract: Signals consistent with the ${\mathrm{B^{+}_{c}} \mathrm{(2S)}}$ and ${\mathrm{B^{*+}_{c}} \mathrm{(2S)}}$ states are observed in proton-proton collisions at $\sqrt{s} = $ 13 TeV, in an event sample corresponding to an integrated luminosity of 143 fb$^{-1}$, collected by the CMS experiment during the 2015-2018 LHC running periods. These excited $\mathrm{\bar{b}c} $ states are observed in the ${\mathrm{B^{+}_{c}} \pi^{+} \pi^{-}}$ invariant mass spectrum, with the ground state $ \mathrm{B^{+}_{c}} $ reconstructed through its decay to ${\mathrm{J}/\psi \pi^{+}} $. The two states are well resolved from each other and are observed with a significance exceeding five standard deviations. The mass of the ${\mathrm{B^{+}_{c}} \mathrm{(2S)}}$ meson is measured to be 6871.0 $\pm$ 1.2 (stat) $\pm$ 0.8 (syst) $\pm$ 0.8 ($\mathrm{B^{+}_{c}} $) MeV, where the last term corresponds to the uncertainty in the world-average $ \mathrm{B^{+}_{c}} $ mass.
Figures Summary References CMS Publications

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Figure 1:
Transitions between the lightest ${\mathrm {B_{c}}}$ states, with solid and dashed lines indicating the emission of photons and pion pairs, respectively [2].

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Figure 2:
The invariant mass distribution of the ${{\mathrm {B_{c}}} ^{+}}$ candidates. The vertical dashed lines indicate the mass window retained for the reconstruction of the ${{{\mathrm {B_{c}}} ^{+} \mathrm {(2S)}}}$ and ${{{\mathrm {B_{c}}} ^{*+}} \mathrm {(2S)}}$ candidates. The vertical bars on the points represent the statistical uncertainty in the data. The contributions from various sources are shown by the stacked distributions. The solid line represents the result of the fit.

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Figure 3:
The $M({\mathrm{B^{+}_{c}} \pi^{+} \pi^{-}} ) - M({{\mathrm {B_{c}}} ^{+}}) + m_{{{\mathrm {B_{c}}} ^{+}}}$ distribution. The ${{{\mathrm {B_{c}}}} ^{+} \mathrm {(2S)}}$ is assumed to be the right-most peak. The vertical bars on the points represent the statistical uncertainty in the data. The contributions from the various sources are shown by the stacked distributions. The solid line represents the result of the fit.
In summary, signals consistent with the ${\mathrm{B^{+}_{c}} \mathrm{(2S)}}$ and ${\mathrm{B^{*+}_{c}} \mathrm{(2S)}}$ states have been separately observed for the first time by investigating the ${\mathrm{B^{+}_{c}}} \pi^{+}\pi^{-}$ invariant mass spectrum measured by CMS. The analysis is based on the entire LHC sample of proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to a total integrated luminosity of 143 fb$^{-1}$. The two peaks are well resolved, with a measured mass difference of $\Delta M = $ 29.1 $\pm$ 1.5 (stat) $\pm$ 0.7 (syst) MeV. The ${\mathrm{B^{+}_{c}} \mathrm{(2S)}}$ mass is measured to be 6871.0 $\pm$ 1.2 (stat) $\pm$ 0.8 (syst) $\pm$ 0.8 (${\mathrm{B^{+}_{c}}}$) MeV, where the last term is the uncertainty in the world-average ${\mathrm{B^{+}_{c}}}$ mass. Because the low-energy photon emitted in the ${\mathrm{B^{*+}_{c}} \to {\mathrm{B^{+}_{c}}} \gamma}$ radiative decay is not reconstructed, the observed ${\mathrm{B^{*+}_{c}} \mathrm{(2S)}}$ peak has a mass lower than the true value, which remains unknown. These measurements contribute significantly to the detailed characterization of heavy meson spectroscopy and provide a rich source of information on the nonperturbative QCD processes that bind heavy quarks into hadrons.
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