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

Abstract

Signals consistent with the B+c(2S) and B*+c(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 B+cπ+π− invariant mass spectrum, with the ground state B+c reconstructed through its decay to J/ψπ+. 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 B+c(2S) meson is measured to be 6871.0 ±1.2 (stat) ± 0.8 (syst) ± 0.8 (B+c) MeV, where the last term corresponds to the uncertainty in the world-average B+c mass.