Analysis of $\pi^+\pi^-$ mass spectrum in $\chi_{c1}(3872)\to\pi^+\pi^- J/\psi$ decays

The $\pi^+\pi^-$ mass spectrum in $\chi_{c1}(3872)\to\pi^+\pi^- J/\psi$ decays is analyzed to probe for its resonant content. This can shed light at the nature of this exotic hadron candidate. The data sample of $B^{\pm}\to K^{\pm}\chi_{c1}(3872)$ events is selected for a good control of the backgrounds and for the optimal mass resolution. The data were obtained with the LHCb detector, and corresponds to 9 fb$^{-1}$ of the integrated luminosity in $pp$ collisions at the LHC.