Observation and Investigation of the T_cc1(4430)⁺ Structure in B⁺ → ψ(2S)K⁰ _Sπ⁺ Decays

The first four-dimensional amplitude analysis of the B⁺ → ψ(2S)K⁰ _Sπ⁺ decay is performed with proton-proton collision data collected by the LHCb experiment at √s = 13 TeV, corresponding to an integrated luminosity of 5.4 fb⁻¹. The data cannot be fully explained by B⁺ → ψ(2S)K*⁺ contributions alone. A significantly better description of the data is obtained by adding a T⁺ _cc contribution decaying to ψ(2S)π⁺. The properties of the T⁺ _cc structure are consistent with the exotic state T_cc1(4430)⁺ reported in the isospin-related B⁰ → ψ(2S)K−π⁺ decay. Effects of a possible T_cc1(4430)⁺ → D*₁ (2600)⁰D⁺ decay mode on the T_cc1(4430)⁺ → ψ(2S)π⁺ mass distribution are investigated through a Flatté parametrization, providing constraints on the relative decay strength. A description of the T_cc1(4430)⁺ structure using the triangle singularity mechanism is studied and also found to be consistent with the data.