First exotic hadron with open heavy flavor: $𝑐𝑠$$𝑢̄𝑑̄ tetraquark

The LHCb Collaboration has reported resonant activity in the channel D+K-, identifying two components: X0(2900) with JP=0+ at 2866 ± 7 MeV, Γ0 = 57 ± 13 MeV and X1(2900) with JP = 1- at 2904 ± 7 MeV, Γ1 = 110 ± 12 MeV. We interpret the X0(2900) component as a $cs$$ūd̄ isosinglet compact tetraquark, calculating its mass to be 2863 ± 12 MeV. This is the first exotic hadron with open heavy flavor. The analogous $bs$$ūd̄ tetraquark is predicted at 6213 ± 12 MeV. We discuss possible interpretations of the heavier and wider X1(2900) state and examine potential implications for other systems with two heavy quarks.