The decays ${\chi }_{c1}\to J/\psi {\mu }^{+}{\mu }^{-}$ and ${\chi }_{c2}\to J/\psi {\mu }^{+}{\mu }^{-}$ are observed and used to study the resonance parameters of the ${\chi }_{c1}$ and ${\chi }_{c2}$ mesons. The masses of these states are measured to be $m({\chi }_{c1})=3510.71\pm 0.04(\mathrm{stat})\pm 0.09(\mathrm{syst})\mathrm{MeV}$ and $m({\chi }_{c2})=3556.10\pm 0.06(\mathrm{stat})\pm 0.11(\mathrm{syst})\mathrm{MeV}$, where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference $m({\chi }_{c2})-m({\chi }_{c1})=45.39\pm 0.07(\mathrm{stat})\pm 0.03(\mathrm{syst})\mathrm{MeV}$. The natural width of the ${\chi }_{c2}$ meson is measured to be $\mathrm{\Gamma }({\chi }_{c2})=2.10\pm 0.20(\mathrm{stat})\pm 0.02(\mathrm{syst})\mathrm{MeV}$. These results are in good agreement with and have comparable precision to the current world averages.

• Received 13 September 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.221801

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

© 2017 CERN, for the LHCb Collaboration