Exotic states and their properties from large-$N_{\mathrm{c}}$ QCD

Abstract

The analysis of two-ordinary-meson scattering amplitudes in the limit of a large number $N_{\mathrm{c}}$ of the colour degrees of freedom of quantum chromodynamics, with suitably decreasing strong coupling and all quarks transforming according to the gauge group's fundamental representation, enables us to establish a set of rigorous consistency conditions for the emergence of a tetraquark (i.e., a bound state of two quarks and two antiquarks) as a pole in these amplitudes. For genuinely flavour-exotic tetraquarks, these constraints require the existence of two tetraquark states distinguishable by their preferred couplings to two ordinary mesons, whereas, for cryptoexotic tetraquarks, our constraints may be satisfied by a single tetraquark state, which then, however, may mix with ordinary mesons. For elucidation of the tetraquark features, the consideration of the subleading contributions proves to be mandatory: for both variants of tetraquarks, their decay widths fall off like $1/N_{\mathrm{c}}^2$ for large $N_{\mathrm{c}}.$

Comments: 4 pages, 4 figures, talk presented at "The European Physical Society Conference on High Energy Physics - EPS-HEP 2017" (5 - 12 July 2017, Venice, Italy)