document: States outside the constituent quark model have been hypothesized to exist almost since the introduction of color [1–4]. Hybrid mesons, qq̄ states with an admixture of gluons, and glueballs, states with no quark content, rely on the self interaction property of gluons due to their color charge. Looking for glueballs would be the most obvious way to find evidence for states with constituent gluons; however, the search is hindered by the fact that these states may significantly mix with regular qq̄-mesons in the region where the lightest are predicted to occur. As such, they may not be observable as pure states and disentangling the observed spectra may be a very difficult task. Instead, hybrid mesons (qq̄gn) may be a better place to search for evidence of resonances outside the constituent quark model, especially since the lightest of theses states are predicted to have exotic quantum numbers of spin, parity, and charge conjugation, JPC, that is, combinations that are unattainable by regular qq̄-mesons.
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