Evidence of a J/ψ KS0 Structure in B0 →j/ψφ KS0 Decays

An amplitude analysis of B0→J/ψφKS0 decays is performed using proton-proton collision data, corresponding to an integrated luminosity of 9 fb-1, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. Evidence with a significance of 4.0 standard deviations of a structure in the J/ψKS0 system, named Tψs1θ(4000)0, is seen, with its mass and width measured to be 3991-10+12 -17+9 MeV/c2 and 105-25+29 -23+17 MeV, respectively, where the first uncertainty is statistical and the second systematic. The Tψs1θ(4000)0 state is likely to be the isospin partner of the Tψs1θ(4000)+ state, previously observed in the J/ψK+ system of the B+→J/ψφK+ decay. When isospin symmetry for the charged and neutral Tψs1θ(4000) states is assumed, the signal significance increases to 5.4 standard deviations.