The decays ${\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c1}\to J/\mathrm{<ruby><rb>\psi </rb><rt>ぷさい</rt></ruby>}{\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}^{+}{\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}^{-}$ and ${\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2}\to J/\mathrm{<ruby><rb>\psi </rb><rt>ぷさい</rt></ruby>}{\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}^{+}{\mathrm{<ruby><rb>\mu </rb><rt>みゅー</rt></ruby>}}^{-}$ are observed and used to study the resonance parameters of the ${\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c1}$ and ${\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2}$ mesons. The masses of these states are measured to be $m({\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c1})=3510.71\pm 0.04(\mathrm{stat})\pm 0.09(\mathrm{syst})\mathrm{MeV}$ and $m({\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2})=3556.10\pm 0.06(\mathrm{stat})\pm 0.11(\mathrm{syst})\mathrm{MeV}$, where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference $m({\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2})-m({\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c1})=45.39\pm 0.07(\mathrm{stat})\pm 0.03(\mathrm{syst})\mathrm{MeV}$. The natural width of the ${\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2}$ meson is measured to be $\mathrm{<ruby><rb>\Gamma </rb><rt>がんま</rt></ruby>}({\mathrm{<ruby><rb>\chi </rb><rt>かい</rt></ruby>}}_{c2})=2.10\pm 0.20(\mathrm{stat})\pm 0.02(\mathrm{syst})\mathrm{MeV}$. 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

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© 2017 CERN, for the LHCb Collaboration