Observation and Investigation of the T _cc1(4430) ⁺ Structure in B ⁺ → ψ(2S)K ⁰ _Sπ ⁺ Decays

The first four-dimensional amplitude analysis of the B ⁺ → ψ(2S)K ⁰ _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 B ⁰ → 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.