The Zc structures in a coupled-channels model

The Zc( 3900) +/- / Zc( 3885) +/- and Zc( 4020) +/- are two charmonium- like structures discovered in the p J/. and D * D (*)+ h. c. invariant mass spectra. Their nature is puzzling due to their charge, which forces its minimal quark content to be cc ud ( cc du ). Thus, it is necessary to explore four- quark systems in order to understand their inner structure. Additionally, their strong coupling to channels such as p J/. and the closeness of their mass to D * D (*)- thresholds stimulates both a molecular interpretation or a coupled- channels threshold effect. In this work we perform a coupled- channels calculation of the I G( J PC) = 1 + ( 1 +-) sector including D(*) D *+ h. c., p J/. and..c channels in the framework of a constituent quarkmodel which satisfactorily describes awide range of properties of ( non-) conventional hadrons containing heavy quarks. The meson- meson interactions are dominated by the non- diagonal p J/. - D * D (*) and..c - D * D (*) couplings which indicates that the Zc( 3900) +/- / Zc( 3885) +/- and Zc( 4020) +/- are unusual structures. The study of the analytic structure of the S- matrix allows us to conclude that the point- wise behavior of the line shapes in the p J/. and D D * invariant mass distributions is due to the presence of two virtual states that produce the Zc peaks.