Observation and investigation of the $T_{c\bar{c}1}(4430{)}^{+}$ structure in $B^{+}\to \psi (2S)K_{\text{S}}^0{\pi }^{+}$ decays

The first four-dimensional amplitude analysis of the $B^{+}\to \psi (2S)K_{\text{S}}^0{\pi }^{+}$ decay is performed with proton-proton collision data collected by the LHCb experiment at $\sqrt{s}=13\mathrm{T}\mathrm{e}\mathrm{V}$, corresponding to an integrated luminosity of $5.4\mathrm{f}{\mathrm{b}}^{-1}$. The data cannot be fully explained by $B^{+}\to \psi (2S)K^{\ast +}$ contributions alone. A significantly better description of the data is obtained by adding a $T_{c\bar{c}}^{+}$ contribution decaying to $\psi (2S){\pi }^{+}$. The properties of the $T_{c\bar{c}}^{+}$ structure are consistent with the exotic state $T_{c\bar{c}1}(4430{)}^{+}$ reported in the isospin-related ${\bar{B}}^0\to \psi (2S)K^{-}{\pi }^{+}$ decay. Effects of a possible $T_{c\bar{c}1}(4430{)}^{+}\to {\bar{D}}_1^{\ast }(2600{)}^0{D}^{+}$ decay mode on the $T_{c\bar{c}1}(4430{)}^{+}\to \psi (2S){\pi }^{+}$ mass distribution are investigated through a Flatté parametrization, providing constraints on the relative decay strength. A description of the $T_{c\bar{c}1}(4430{)}^{+}$ structure using the triangle singularity mechanism is studied and also found to be consistent with the data.