Evidence of a J/ψK0S Structure in B0→J/ψϕK0S Decays

An amplitude analysis of B0→J/ψϕK0S 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/ψK0S system, named Tθψs1(4000)0, is seen, with its mass and width measured to be 3991+12−10 +9−17  MeV/c2 and 105+29−25 +17−23  MeV, respectively, where the first uncertainty is statistical and the second systematic. The Tθψs1(4000)0 state is likely to be the isospin partner of the Tθψs1(4000)+ state, previously observed in the J/ψK+ system of the B+→J/ψϕK+ decay. When isospin symmetry for the charged and neutral Tθψs1(4000) states is assumed, the signal significance increases to 5.4 standard deviations.