Interaction of exotic states in a hadronic medium: the Z _c (3900) case

Abstract

We investigate the interactions of the charged exotic state $Z_c(3900)$ in a hadronic medium composed of light mesons. We study processes such as $Z_c \pi \to D\bar{D}$, $Z_c \pi \to D^*\bar{D}^*$, $Z_c \pi \to D\bar{D}^*$ and the inverse ones. Using effective Lagrangians and form factors calculated with QCD sum rules (treating the $Z_c(3900)$ as a tetraquark) we estimate the vacuum and thermally-averaged cross-sections of these reactions. We find that the $Z_c(3900)$ has relatively large interaction cross sections with the constituent particles of the hadronic medium.
After that, we use the production and suppression cross sections in a rate equation to estimate the time evolution of the $Z_c$ multiplicity. We include the $Z_c$ decay and regeneration terms. The coalescence model is employed to compute the initial $Z_c$ multiplicity for the compact tetraquark configuration. Our results indicate that the combined effects of hadronic interactions, hydrodynamical expansion, decay and regeneration affect the final yield, which is bigger than the initial value. Besides, the dependence of the $Z_c$ final yield with the centrality, center-of-mass energy and the charged hadron multiplicity measured at midrapidity $[dN_{ch}/d\eta \,(\eta<0.5)]$ is also investigated.