Evidence for an η_c(1S) π^- resonance in B⁰→ η_c(1S) K⁺π^- decays

A Dalitz plot analysis of B0→ηc(1S)K+π- decays is performed using data samples of pp collisions collected with the LHCb detector at centre-of-mass energies of s=7,8 and 13TeV, corresponding to a total integrated luminosity of 4.7fb-1. A satisfactory description of the data is obtained when including a contribution representing an exotic ηc(1 S) π- resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are 4096±20-22+18MeV and 152±58-35+60MeV, respectively. The spin-parity assignments JP= 0 + and JP= 1 - are both consistent with the data. In addition, the first measurement of the B0→ηc(1S)K+π- branching fraction is performed and gives B(B0→ηc(1S)K+π-)=(5.73±0.24±0.13±0.66)×10-4,where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.