Evidence of a $J/$$\psi$$K_S^0$ Structure in $B^0$ → $J/$$\psi$$\phi$$K_S^0$
An amplitude analysis of $B^0$ → $J/$$\psi$$\phi$$K_S^0$ 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/$$\psi$$K_S^0$ system, named $T_{ψs1}^{θ}$(4000) 0 , is seen, with its mass and width measured to be 3991$_{–10 –17}^{+12 +9}$ MeV/c 2 and 105$_{–25 –23}^{+29 +17}$ MeV, respectively, where the first uncertainty is statistical and the second systematic. The $T_{ψs1}^{θ}$(4000) 0 state is likely to be the isospin partner of the $T_{ψs1}^{θ}$(4000) + state, previously observed in the $J/ψK^+$ system of the $B^+$→$J/ψ$$\phi$$K^+$ decay. When isospin symmetry for the charged and neutral $T_{ψs1}^{θ}$(4000) states is assumed, the signal significance increases to 5.4 standard deviations.