A study of the lineshape of the ${\chi }_{c1}(3872)$ state is made using a data sample corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ collected in $pp$ collisions at center-of-mass energies of 7 and 8 TeV with the LHCb detector. Candidate ${\chi }_{c1}(3872)$ and $\psi (2S)$ mesons from $b$-hadron decays are selected in the $J/\psi {\pi }^{+}{\pi }^{-}$ decay mode. Describing the lineshape with a Breit-Wigner function, the mass splitting between the ${\chi }_{c1}(3872)$ and $\psi (2S)$ states, $\mathrm{\Delta }m$, and the width of the ${\chi }_{c1}(3872)$ state, ${\mathrm{\Gamma }}_{\mathrm{BW}}$, are determined to be $\genfrac{}{}{0ex}{}{\mathrm{\Delta }m=185.598\pm 0.067\pm 0.068\mathrm{MeV},}{{\mathrm{\Gamma }}_{\mathrm{BW}}=1.39\pm 0.24\pm 0.10\mathrm{MeV},}$ where the first uncertainty is statistical and the second systematic. Using a Flatté-inspired model, the mode and full width at half maximum of the lineshape are determined to be $\genfrac{}{}{0ex}{}{\mathrm{mode}=3871.69_{-0.04-0.13}^{+0.00+0.05}\mathrm{MeV},}{\mathrm{FWHM}=0.22_{-0.06-0.13}^{+0.07+0.11}\mathrm{MeV}.}$ An investigation of the analytic structure of the Flatté amplitude reveals a pole structure, which is compatible with a quasibound $D^0{\overline{D}}^{*0}$ state but a quasivirtual state is still allowed at the level of 2 standard deviations.

• Received 29 May 2020
• Accepted 21 September 2020

DOI:https://doi.org/10.1103/PhysRevD.102.092005

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© 2020 CERN, for the LHCb Collaboration