1−+exotic meson at light quark masses

The mass of the $1^{-+}$ exotic meson, created with hybrid interpolating fields, is explored in numerical simulations of quenched QCD on large (${20}^3\times 40$) lattices to obtain good control of statistical and finite volume errors. Using the Fat-Link Irrelevant Clover (FLIC) fermion action, the properties of the $1^{-+}$ are investigated at light quark masses approaching 25 MeV ($m_{\pi }/m_{\rho }\simeq 1/3$). Under the standard assumption that the coupling to the quenched $a_1{\eta }^{\prime }$ channel comes with a negative metric, our results indicate that the $1^{-+}$ exotic exhibits significant curvature close to the chiral limit, suggesting previous linear extrapolations have overestimated the mass of the $1^{-+}$. We find for the first time in lattice studies a $1^{-+}$ mass consistent with the ${\pi }_1(1600)$ candidate. We also find a strangeness $\pm 1$ $J^P=1^{-}$ state with a mass close to 2 GeV.