$\sigma(500)$ resonance pole positions as function of $m_\pi$: analysis with a unitary coupled-channel model

Abstract

Resonance pole positions of the $f_0(500)$ alias $\sigma(500)$ meson are computed and plotted as a continuous function of pion mass in the framework of a unitary and analytic coupled-channel model for scalar mesons as dynamical $q\bar{q}$ states. The $\sigma$ is described with a light and a strange $q\bar{q}$ seed, mixing with each other mainly through the common $\pi\pi$, $K\bar{K}$, and $\eta\eta$ meson-meson channels. The few model parameters are fitted to experimental $S$-wave $\pi\pi$ phase shifts up to 1 GeV, yielding, in the case of the physical pion mass, resonance poles at $(460-i222)$ MeV for the $\sigma(500)$ and $(978-i37)$ MeV for the $f_0(980)$. Resonance, bound-state, and virtual-state pole trajectories are shown as a function of $m_\pi$ running from 139.57 MeV to 1 GeV. These are compared to recent lattice QCD computations that use interpolating fields corresponding to the model's channels, i.e., for a few discrete $m_\pi $values.