Hadronic Spectrum - Multiquark States

Abstract

Many newly discovered mesons behave like c c ¯ charmonium states in that they preferentially decay into final states that contain a c- and a c ¯ -quark, but do not fit expectations for any of the unfilled levels of the conventional c c ¯ spectrum. There is a growing suspicion that at least some of these states are exotic, i.e. have a substructure that is more complex than the quark-antiquark mesons of the classical constituent quark model. Some of these candidate states have a non-zero electric charge and, thus, a minimal quark content of c c ¯ u d ¯ or c c ¯ d u ¯ . In addition, states with similar properties have been observed in the b- and s-quark sectors. In this report, the experimental situation is briefly reviewed.