Spectrum of light hybrid mesons with exotic quantum numbers
In plain words
Hybrid mesons contain a quark, an antiquark and an excited gluon field, and some have spin-parity combinations impossible for a plain quark-antiquark pair. A few candidates exist, and whether they form the complete family predicted by lattice QCD is open.
Precise statement
Lattice QCD predicts a lightest hybrid multiplet including a $J^{PC} = 1-+$ nonet between about $1.6\text{ and }2.0\ \mathrm{GeV}$. Establish whether $\pi1(1600)$ is the only isovector $1-+$ state below $2\ \mathrm{GeV}$ (a 2019 JPAC analysis described $\pi1(1400)$ and $\pi1(1600)$ as one pole), and whether $\eta1(1855)$ (BESIII, 2022) and an isoscalar partner complete the nonet. Answer: masses, widths and decay patterns of the $1-+$ nonet members.
What would settle it
Partial-wave analyses of GlueX photoproduction, COMPASS/AMBER pion-beam data and BESIII $J/\psi$ decays establishing all members of the $1-+$ nonet.
Status in the literature
Unverified note
The single-pole pi1 interpretation (2019) and the eta1(1855) observation (2022) awaited independent confirmation as of 2026.