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

The LHCb Collaboration has reported resonant activity in the channel $D^+ K^-$,identifyingtwocomponents:$X_0(2900)$with$J^P = 0^+$at$2866 {\pm} 7$MeV,$\Gamma_0=57{\pm} 13$MeVand$X_1(2900)$with$J^P = 1^-$at$2904 {\pm} 7$MeV,$\Gamma_1=110{\pm} 12$MeV\mathrm{.}Weinterpretthe$X_0(2900)$componentasa$cs \bar u\bar d$ isosinglet compact tetraquark, calculating its mass to be $2863\pm 12$ MeV. This is the first exotic hadron with open heavy flavor. The analogous $bs\bar{u}\bar{d}$ tetraquark is predicted at $6213 {\pm} 12$MeV\mathrm{.}Wediscusspossibleinterpretationsoftheheavierandwider$X_1(2900)$ state and examine potential implications for other systems with two heavy quarks.