# Observation of an Excited ${B}_{c}^{+}$ State

12 June 2019

Abstract: Using $pp$ collision data corresponding to an integrated luminosity of $8.5\text{}\text{}{\mathrm{fb}}^{-1}$ recorded by the LHCb experiment at center-of-mass energies of $\sqrt{s}=7$, 8, and 13 TeV, the observation of an excited ${B}_{c}^{+}$ state in the ${B}_{c}^{+}{\pi }^{+}{\pi }^{-}$ invariant-mass spectrum is reported. The observed peak has a mass of $6841.2±0.6\left(\mathrm{stat}\right)±0.1\left(\mathrm{syst}\right)±0.8\left({B}_{c}^{+}\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$, where the last uncertainty is due to the limited knowledge of the ${B}_{c}^{+}$ mass. It is consistent with expectations of the ${B}_{c}^{*}\left(2{{}^{3}S}_{1}{\right)}^{+}$ state reconstructed without the low-energy photon from the ${B}_{c}^{*}\left(1{{}^{3}S}_{1}{\right)}^{+}\to {B}_{c}^{+}\gamma$ decay following ${B}_{c}^{*}\left(2{{}^{3}S}_{1}{\right)}^{+}\to {B}_{c}^{*}\left(1{{}^{3}S}_{1}{\right)}^{+}{\pi }^{+}{\pi }^{-}$. A second state is seen with a global (local) statistical significance of $2.2\sigma$ ($3.2\sigma$) and a mass of $6872.1±1.3\left(\mathrm{stat}\right)±0.1\left(\mathrm{syst}\right)±0.8\left({B}_{c}^{+}\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$, and is consistent with the ${B}_{c}\left(2{{}^{1}S}_{0}{\right)}^{+}$ state. These mass measurements are the most precise to date.

Published in: Physical Review Letters 122 (2019)