Study of D_s1(2460)⁺→D_s⁺π⁺π^- in B→D¯^(*)D_s⁺π⁺π^- decays

An amplitude analysis of the D_s1(2460)⁺→D_s⁺π⁺π^- transition is performed simultaneously in B⁰→D^-D_s⁺π⁺π^-, B⁺→D¯⁰D_s⁺π⁺π^-, and B⁰→D^*-D_s⁺π⁺π^- decays. The study is based on a data sample of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of s=7,8, and 13 TeV, corresponding to a total integrated luminosity of 9fb^-1. A clear double-peak structure is observed in the m(π⁺π^-) spectrum of the D_s1(2460)⁺→D_s⁺π⁺π^- decay. The data can be described either with a model including f₀500,f₀980, and f₂(1270) resonances, in which the contributions of f₀(980) and f₂(1270) are unexpectedly large, or with a model including f₀(500), a doubly charged open-charm tetraquark state T_cs¯⁺⁺ and its isospin partner T_cs¯⁰. If the former is considered implausible, the T_cs¯ states are observed with high significance, and the data are consistent with isospin symmetry. When imposing isospin constraints between the two T_cs¯ states, their mass and width are determined to be (2327±13±13) MeV and (96±16_-23⁺¹⁷⁰) MeV, respectively, where the first uncertainty is statistical and the second is systematic. The mass is slightly below the DK threshold, and a spin-parity of 0⁺ is favoured with high significance.