The molecular nature of some exotic hadrons

TL;DR

This paper reviews recent experimental and theoretical advances in understanding exotic hadrons, focusing on meson-baryon interactions, potential molecular states, and predictions of new heavy baryon resonances across charm and bottom sectors.

Contribution

It introduces a unitarized coupled-channel model to interpret exotic heavy baryons as meson-baryon molecules and predicts new doubly-charmed and bottom sector states.

Findings

Identification of a potential N* resonance around 2 GeV as a meson-baryon quasi-bound state.

Interpretation of some Omega_c states as hadron molecules.

Prediction of doubly-charmed Xi_cc quasibound states.

Abstract

The exciting discovery by LHCb of the $P_c(4312)^+$ and $P_c(4450)^+$ pentaquarks, or the suggestion of a tetraquark nature for the $Z_c(3900)$ state seen at BESIII and Belle, have triggered a lot of activity in the field of hadron physics, with new experiments planned for searching other exotic mesons and baryons, and many theoretical developments trying to disentangle the true multiquark nature from their possible molecular origin. After a brief review of the present status of these searches, this paper focusses on recently seen or yet to be discovered exotic heavy baryons that may emerge from a conveniently unitarized meson-baryon interaction model in coupled channels. In particular, we will show how interferences between the different coupled-channel amplitudes of the model may reveal the existence of a $N^*$ resonance around 2 GeV having a meson-baryon quasi-bound state nature. We also discuss the possible interpretation of some of the $\Omega_c$ states recently discovered at LHCb as being hadron molecules. The model also predicts the existence of doubly-charmed quasibound meson-baryon $\Xi_{cc}$ states, which would be excited states of the ground-state $\Xi_{cc}(3621)$ MeV, whose mass has only been recently established. Extensions of these results to the bottom sector will also be presented.