Study of the lineshape of the χc1(3872) state
A study of the lineshape of theχc1ð3872Þstate is made using a data sample corresponding to anintegrated luminosity of3fb−1collected inppcollisions at center-of-mass energies of 7 and 8 TeV withthe LHCb detector. Candidateχc1ð3872Þandψð2SÞmesons fromb-hadron decays are selected in theJ=ψπþπ−decay mode. Describing the lineshape with a Breit-Wigner function, the mass splitting betweentheχc1ð3872Þandψð2SÞstates,Δm, and the width of theχc1ð3872Þstate,ΓBW, are determined to beΔm¼185.5980.0670.068MeV;ΓBW¼1.390.240.10MeV;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 bemode¼3871.69þ0.00þ0.05−0.04−0.13MeV;FWHM¼0.22þ0.07þ0.11−0.06−0.13MeV:An investigation of the analytic structure of the Flatt ́e amplitude reveals a polestructure, which is compatible with a quasiboundD0 ̄D0state but a quasivirtual state is still allowed at thelevel of 2 standard deviations.