$X_0(2900)$ and $X_1(2900)$: hadronic molecules or compact tetraquarks

Abstract

Very recently the LHCb Collaboration reported their observation of the first two fully open-flavor tetraquark states, the $X_0(2900)$ of $J^P=0^{+}$ and the $X_1(2900)$ of $J^P=1^{-}$. We study their possible interpretations using the method of QCD sum rules, paying special attention to an interesting feature of this experiment that the higher resonance $X_1(2900)$ has a width significantly larger than the lower one $X_0(2900)$. Our results suggest that the $X_0(2900)$ can be interpreted as the $S$-wave $D^{\ast -}{K}^{\ast +}$ molecule state of $J^P=0^{+}$, and the $X_1(2900)$ can be interpreted as the $P$-wave $\bar{c}\bar{s}ud$ compact tetraquark state of $J^P=1^{-}$. Mass predictions of their bottom partners are also given.

Comments: revised version, 5 pages, 2 figures, published in Chinese Physics Letters (Express Letter)