A study of the lineshape of the $\chi_{ c1}(3872)$ state is made using a data sample corresponding to an integrated luminosity of $3\,$fb$^{ -1}$ collected in $pp$ collisions at centre-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 { \mbox{ lineshape}} with a Breit--Wigner function, the mass splitting between the $\chi_{ c1}(3872)$ and $\psi(2S)$ states, $\Delta m$, and the width of the $\chi_{ c1}(3872)$ state, $\Gamma_{ \mathrm{ BW}}$, are determined to be \begin{ eqnarray*} \Delta m & = & 185.598 \pm 0.067 \pm 0.068\, \mathrm{ MeV} \,, \\ \Gamma_{ \mathrm{ BW}} & = & \phantom{ 00}1.39\phantom{ 0} \pm 0.24\phantom{ 0} \pm 0.10\phantom{ 0} \mathrm{ MeV} \,, \end{ eqnarray*} where the first uncertainty is statistical and the second systematic. Using a Flatt\'e-inspired model, the mode and full width at half maximum of the lineshape are determined to be \begin{ eqnarray*} \mathrm{ mode} & = 3871.69^{ \,+\,0.00\,+\,0.05}_{ \,-\,0.04\,-\,0.13} &\mathrm{ MeV} \\ \mathrm{ FWHM} & = 0.22^{ \,+\,0.07\,+\,0.11}_{ \,-\,0.06\,-\,0.13}& \mathrm{ MeV} . \end{ eqnarray*} An investigation of the analytic structure of the Flatt\'e amplitude reveals a pole structure, which is compatible with a quasi-bound $D^0\bar{ D}^{ *0}$ state but a quasi-virtual state is still allowed at the level of $2$ standard deviations.
Categories: hep-ex