How to understand the $X(2900)$?

TL;DR

This paper models the interactions of $ar{D}^ ext{*}K^ ext{*}$ to explain the observed $X(2900)$ states, proposing they are hadronic molecules with specific quantum numbers, and suggests more detailed measurements are needed to resolve their substructure.

Contribution

It provides a coupled-channel formalism analysis that reproduces the $X(2900)$ states as molecular states with specific quantum numbers, offering a unified explanation for experimental observations.

Findings

Two states with $I(J^P)=0(0^+)$ and $0(1^-)$ are identified.

Masses match experimental data, but widths differ.

The $X_1(2900)$ is interpreted as a $P$-wave excitation of $X_0(2900)$.

Abstract

In this work, the $S$- and $P$-wave $\bar{D}^\ast K^\ast$ interactions are studied in a coupled-channel formalism to understand the recently observed $X_0(2900)$ and $X_1(2900)$ at LHCb. The experimental event distributions can be well described, and two states with $I(J^P)=0(0^+)$ and $0(1^-)$ are yielded in an unified framework. The masses of the $0^+$ and $1^-$ states are consistent with the experimental data, but the width of the $0^+$ state is larger than that of the $1^-$ one. The $X_1(2900)$ can be interpreted as the $P$-wave excitation of the ground-state $X_0(2900)$ in the hadronic molecular picture. The $S$- and $P$-wave multiplets in the $\bar{D}^\ast K^\ast$ system have many members, so the present peak in the $D^-K^+$ invariant mass distributions might contain multi subpeaks. In order to probe the fine structures behind the single whole peak now, more refined measurements in the $B^+\to D^+D^-K^+$ decay channel are necessary.