An amplitude analysis of $B^0\to J/\psi \varphi K_{\text{S}}^0$ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, 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 $J/\psi K_{\text{S}}^0$ system, named $T_{\psi s1}^{\theta }(4000{)}^0$, is seen, with its mass and width measured to be ${3991}_{-10}^{+12}{}_{-17}^{+9}\mathrm{MeV}/{\mathrm{c}}^2$ and ${105}_{-25}^{+29}{}_{-23}^{+17}\mathrm{MeV}$, respectively, where the first uncertainty is statistical and the second systematic. The $T_{\psi s1}^{\theta }(4000{)}^0$ state is likely to be the isospin partner of the $T_{\psi s1}^{\theta }(4000{)}^{+}$ state, previously observed in the $J/\psi K^{+}$ system of the $B^{+}\to J/\psi \varphi K^{+}$ decay. When isospin symmetry for the charged and neutral $T_{\psi s1}^{\theta }(4000)$ states is assumed, the signal significance increases to 5.4 standard deviations.

• Received 17 January 2023
• Accepted 21 March 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.131901

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