Observation of the Open-Charm Tetraquark State $T_{cs0}^\ast(2870)^0$ in the $B^- \to D^- D^0 K_S^0$ Decay

An amplitude analysis of $B^- \to D^- D^0 K_S^0$ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of $9~\text{fb}^{-1}$, collected with the LHCb detector at center-of-mass energies of $7$, $8$, and $13~\text{TeV}$.

A resonant structure of spin-parity $0^+$ is observed in the $D^0 K_S^0$ invariant-mass spectrum with a significance of $5.3\sigma$. The mass and width of the state, modeled with a Breit−Wigner lineshape, are determined to be

$2883 \pm 11 \pm 6~\text{MeV}/c^2 \quad \text{and} \quad 87^{+22}_{-47} \pm 6~\text{MeV},$

where the first uncertainties are statistical and the second systematic.

These properties and the quark content are consistent with those of the open-charm tetraquark state $T_{cs0}^\ast(2870)^0$ observed previously in the $D^+ K^-$ final state of the $B^- \to D^- D^+ K^-$ decay.

This result confirms the existence of the $T_{cs0}^\ast(2870)^0$ state in a new decay mode. The $T_{cs1}^\ast(2900)^0$ state, reported in the $B^- \to D^- D^+ K^-$ decay, is also searched for in the $D^0 K_S^0$ invariant-mass spectrum of the $B^- \to D^- D^0 K_S^0$ decay, without finding evidence for it.