$1^{-+}$ exotic meson at light quark masses

Abstract

The mass of the $1^{-+}$ exotic meson, created with hybrid interpolating fields, is explored at light quark masses approaching 25 MeV ($m_π/ m_ρ\simeq 1/3$). Access to such light quark masses is facilitated by the use of the Fat-Link Irrelevant Clover (FLIC) fermion action. Additionally, we make use of large ($20^3 \times 40$) lattices to obtain good control of statistical and finite volume errors. Our results indicate that the $1^{-+}$ exotic exhibits significant curvature close the chiral limit, indicating previous linear extrapolations, far from the chiral regime, have overestimated the mass of the $1^{-+}$. We find for the first time in lattice studies a $1^{-+}$ mass in agreement with the $π_1 (1600)$ candidate. We also find a strangeness $\pm$1 $J^P = 1^-$ state with a mass close to 2 GeV.