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 B ⁰→D ^-D _s ⁺π ⁺π ^-, B ⁺→D‾ ⁰D _s ⁺π ⁺π ^-, and B ⁰→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 f ₀500,f ₀980, and f ₂(1270) resonances, in which the contributions of f ₀(980) and f ₂(1270) are unexpectedly large, or with a model including f ₀(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.