Hadronic molecules in B decays

TL;DR

This paper investigates various hadronic molecular states involving D and K mesons, calculating their properties and production rates, and supports their identification with several experimentally observed exotic states.

Contribution

It constructs interpolating currents for these molecules, computes their masses and decay constants via QCD sum rules, and analyzes their production and decay characteristics, providing theoretical support for their experimental interpretations.

Findings

Supports the molecular interpretation of X(3872), Zc(3900), Zc(4020), and X0(2900).

Identifies Zcs(3985), Zcs(4000), Zcs(4220) as strange partners of known states.

Estimates the lifetime of weakly-coupled molecular states.

Abstract

There are eighteen possibly existing $D^{(*)} \bar D^{(*)}$, $D^{(*)} \bar K^{(*)}$, and $D^{(*)} D_s^{(*)-}$ hadronic molecular states. We construct their corresponding interpolating currents, and calculate their masses and decay constants using QCD sum rules. Based on these results, we calculate their relative production rates in $B$ and $B^*$ decays through the current algebra, and calculate their relative branching ratios through the Fierz rearrangement, as summarized in Table III. Our results support the interpretations of the $X(3872)$, $Z_c(3900)$, $Z_c(4020)$, and $X_0(2900)$ as the molecular states $D \bar D^*$ of $J^{PC} = 1^{++}$, $D \bar D^*$ of $J^{PC} = 1^{+-}$, $D^* \bar D^*$ of $J^{PC} = 1^{+-}$, and $D^* \bar K^*$ of $J^P = 0^{+}$, respectively. Our results also suggest that the $Z_{cs}(3985)$, $Z_{cs}(4000)$, and $Z_{cs}(4220)$ are strange partners of the $X(3872)$, $Z_c(3900)$, and $Z_c(4020)$, respectively. In the calculations we estimate the lifetime of a weakly-coupled composite particle $A = |BC\rangle$ to be $1/t_A \approx 1/t_B + 1/t_C + \Gamma_{A \to BC} + \cdots$, with $\cdots$ partial widths of other possible decay channels.