First exotic hadron with open heavy flavor: $𝑐⁢𝑠$⁢$\bar{𝑢}$$\bar{𝑑}$ tetraquark

The LHCb Collaboration has reported resonant activity in the channel D+K-, identifying two components: X0(2900) with J^P=0⁺ at 2866 ± 7 MeV, Γ₀ = 57 ± 13 MeV and X₁(2900) with J^P = 1^- at 2904 ± 7 MeV, Γ₁ = 110 ± 12 MeV. We interpret the X₀(2900) component as a $cs$$\bar{u}$$\bar{d}$ isosinglet compact tetraquark, calculating its mass to be 2863 ± 12 MeV. This is the first exotic hadron with open heavy flavor. The analogous $bs$$\bar{u}$$\bar{d}$ 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.