Observation of the $\chi_{b1}(3P)$ and $\chi_{b2}(3P)$ and measurement of their masses

Abstract

The $\chi_{b1}(3P)$ and $\chi_{b2}(3P)$ states are observed through their $\gamma(3S)_\gamma$ decays, using an event sample of proton-proton collisions collected by the CMS experiment at the CERN LHC. The data were collected at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of $80.0  fb^{−1}$. The$\gamma(3S)$ mesons are identified through their dimuon decay channel, while the low-energy photons are detected after converting to $e^+e ^−$ pairs in the silicon tracker, leading to a $\chi_{b1}(3P)$ mass resolution of 2.2 MeV. This is the first time that the J=1 and 2 states are well resolved and their masses individually measured: $10513.42±0.41(stat)±0.18(syst)  MeV$ and $10524.02±0.57(stat)±0.18(syst)  MeV$; they are determined with respect to the world-average value of the $\gamma(3S)$ mass, which has an uncertainty of 0.5 MeV. The mass splitting is measured to be $10.60±0.64(stat)±0.17(syst)  MeV$.