First exotic hadron with open heavy flavor: csu¯d¯ tetraquark

The LHCb Collaboration has reported resonant activity in the channel $D^{+}{K}^{-}$, identifying two components: $X_0(2900)$ with $J^P=0^{+}$ at $2866\pm 7\mathrm{MeV}$, ${\mathrm{\Gamma }}_0=57\pm 13\mathrm{MeV}$ and $X_1(2900)$ with $J^P=1^{-}$ at $2904\pm 7\mathrm{MeV}$, ${\mathrm{\Gamma }}_1=110\pm 12\mathrm{MeV}$. We interpret the $X_0(2900)$ component as a $cs\overline{u}\overline{d}$ isosinglet compact tetraquark, calculating its mass to be $2863\pm 12\mathrm{MeV}$. This is the first exotic hadron with open heavy flavor. The analogous $bs\overline{u}\overline{d}$ tetraquark is predicted at $6213\pm 12\mathrm{MeV}$. We discuss possible interpretations of the heavier and wider $X_1(2900)$ state and examine potential implications for other systems with two heavy quarks.