Study of Ds1(2460)+ → Ds+ π+ π− in B → anti-D(*) Ds+ π+ π− decay

An amplitude analysis of the Ds1(2460)+ → Ds+ π+ π− transition is performed simultaneously in B0 → D− Ds+ π+ π−, B+ → anti-D0 Ds+ π+ π−, and B0 → D*− Ds+ 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 9 fb^−1. A clear double-peak structure is observed in the m(π+ π−) spectrum of the Ds1(2460)+ → Ds+ π+ π− decay. The data can be described either with a model including f0(500), f0(980) 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++_{c anti-s} and its isospin partner T0_{c anti-s}. If the former is considered implausible, the T_{c anti-s} states are observed with high significance, and the data are consistent with isospin symmetry. When imposing isospin constraints between the two T_{c anti-s} 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 D K threshold, and a spin-parity of 0+ is favoured with high significance.