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

TL;DR

This paper reports the first precise measurement and resolution of the $ ext{χ}_b1$(3P) and $ ext{χ}_b2$(3P) bottomonium states' masses through their decay to $ ext{Υ}$(3S) and a photon, using CMS data at 13 TeV.

Contribution

It provides the first individual mass measurements of the $ ext{χ}_b1$(3P) and $ ext{χ}_b2$(3P) states with high resolution, resolving their mass splitting.

Findings

Masses of $ ext{χ}_b1$(3P) and $ ext{χ}_b2$(3P)$ measured precisely.

First resolution of $J=1$ and $J=2$ states in this energy region.

Mass splitting between the states determined as 10.60 MeV.

Abstract

The $\chi_\mathrm{b1}$(3P) and $\chi_\mathrm{b2}$(3P) states are observed through their $\Upsilon$(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 $\Upsilon$(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 $\chi_\mathrm{b}$(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: 10$\,$513.42 $\pm$ 0.41 (stat) $\pm$ 0.18 (syst) MeV and 10$\,$524.02 $\pm$ 0.57 (stat) $\pm$ 0.18 (syst) MeV; they are determined with respect to the world-average value of the $\Upsilon$(3S) mass, which has an uncertainty of 0.5 MeV. The mass splitting is measured to be 10.60 $\pm$ 0.64 (stat) $\pm$ 0.17 (syst) MeV.