Abstract
The search for mesons with non-quark-antiquark (exotic) quantum numbers has gone on for nearly thirty years. There currently is experimental evidence of three isospin one states, the $\pi_{1}(1400)$, the $\pi_{1}(1600)$ and the $\pi_{1}(2015)$. For all of these states, there are questions about the identification of these state, and even if some of them exist. In this article, we will review both the theoretical work and the experimental evidence associated with these exotic quantum number states. We find that the $\pi_{1}(1600)$ could be the lightest exotic quantum number hybrid meson, but observations of other members of the nonet would be useful.
Highlights
The quark model describes mesons as bound states of quarks and antiquarks, much akin to positronium (e+e−)
In the PDG notation, the parity and charge conjugation determine the name of the hybrid, which is taken as the name of the normal meson of the same JP C and isospin
The first predictions for exotic quantum number mesons came from calculations in the Bag model [5, 6]
Summary
The quark model describes mesons as bound states of quarks and antiquarks (qq), much akin to positronium (e+e−). Observation of states with exotic quantum numbers has been of great experimental interest as it would be clear evidence for mesons beyond the simple qqpicture. Moving beyond the simple quark-model picture of mesons, there have been predictions for states with these exotic quantum numbers. Bag-model calculations [2] referred to states with qqand gluons as “hermaphrodite mesons”, and predicted that the lightest nonet (JP C = 1−+) might have masses near 1 GeV as well as distinctive decay modes. They might be relatively stable, and observable. A recent review on the related topic of glueballs can be found in reference [4]
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