The XYZ states: Experimental and theoretical status and perspectives
Accepted version
Peer-reviewed
Repository URI
Repository DOI
Change log
Authors
Abstract
The quark model was formulated in 1964 to classify mesons as bound states
made of a quark-antiquark pair, and baryons as bound states made of three
quarks. For a long time all known mesons and baryons could be classified within
this scheme. Quantum Chromodynamics (QCD), however, in principle also allows
the existence of more complex structures, generically called exotic hadrons or
simply exotics. These include four-quark hadrons (tetraquarks and hadronic
molecules), five-quark hadrons (pentaquarks) and states with active gluonic
degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic
hadrons have been systematically searched for in numerous experiments for many
years. Remarkably, in the past fifteen years, many new hadrons that do not
exhibit the expected properties of ordinary (not exotic) hadrons have been
discovered in the quarkonium spectrum. These hadrons are collectively known as