Hybrid Mesons
  The SU(3)_flavor constituent quark model has been quite successful to explain the properties as well as the observed spectrum of mesons with pseudoscalar and vector quantum numbers. Many radial and orbital excitations of quark-antiquark systems predicted by the model, however, have not yet been observed experimentally or assigned unambiguously. In addition, a much richer spectrum of mesons is expected from QCD, in which quarks interact which each other through the exchange of colored self-interacting gluons. Owing to this particular structure of QCD, configurations are allowed in which an excited gluonic field contributes to the quantum numbers J^{PC} of the meson. States with a valence color-octet qqbar' pair neutralized in color by an excited gluon field are termed hybrids. The observation of such states, however, is difficult because they will mix with ordinary qqbar' states with the same quantum numbers, merely augmenting the observed spectrum for a given J^{PC}. Since the gluonic field may carry quantum numbers other than 0^{++}, however, this can give rise to states with "exotic" quantum numbers J^{PC}=0^{--}, 0^{+-}, 1^{-+}, 2^{+-},... The lowest-lying hybrid multiplet is expected to contain a state with exotic quantum numbers J^{PC}=1^{-+}. The identification of such a state is considered a "smoking gun" for the observation of non-qqbar mesons. The search for hybrid states has been a central goal of hadron spectroscopy in the last 20 years. Ongoing and upcoming high-statistics experiments are expected to shed new light on the existence of such states in nature. In this paper, theoretical predictions for masses and decay modes as well as recent experimental evidence for hybrid meson states and future experimental directions are discussed. 
Bernhard Ketzer
hep-ex hep-ph nucl-ex