Abstract: A peaking structure in the J/?fmass spectrum near threshold is observed in B±?J/?fK±decays, produced in pp collisions at vs=7TeVcollected with the CMS detector at the LHC. The data sample, selected on the basis of the dimuon decay mode of the J/?, corresponds to an integrated luminosity of 5.2fb-1. Fitting the structure to an S-wave relativistic Breit–Wigner lineshape above a three-body phase-space nonresonant component gives a signal statistical significance exceeding five standard deviations. The fitted mass and width values are m =4148.0 ±2.4 (stat.) ±6.3 (syst.) MeVand G=28+15-11(stat.) ±19 (syst.) MeV, respectively. Evidence for an additional peaking structure at higher J/?fmass is also reported.

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