Observation of Two Excited Bc+ States and Measurement of the Bc+ (2S) Mass in pp Collisions at s =13 TeV


Signals consistent with the Bc +(2S) and Bc ?+(2S) states are observed in proton-proton collisions at ? 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 bc states are observed in the Bc + ? +? ? invariant mass spectrum, with the ground state Bc + reconstructed through its decay to J/? ?+. The two states are reconstructed as two well-resolved peaks, separated in mass by 29.1 ± 1.5 (stat) ± 0.7 (syst) MeV. The observation of two peaks, rather than one, is established with a significance exceeding five standard deviations. The mass of the Bc +(2S) meson is measured to be 6871.0 ± 1.2 (stat) ± 0.8 (syst) ± 0.8 (Bc +) MeV, where the last term corresponds to the uncertainty in the world-average Bc + mass


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Physical Review Letters 122, 132001 (2019)