Chiral extrapolation of light resonances from one and two-loop unitarized chiral perturbation theory versus lattice results

Abstract

We study the pion mass dependence of the ρ(770) and f_0(600) masses and widths from one and two-loop unitarized chiral perturbation theory ( ChPT). We show the consistency of one-loop calculations with lattice results for the M_ρ, f_π and the isospin-2 scattering length a-20. Then, we develop and apply the modified inverse amplitude method formalism for two-loop ChPT. In contrast to the f_0(600), the ρ(770) is rather sensitive to the two-loop ChPT parameters-our main source of systematic uncertainty. We thus provide two-loop unitarized fits constrained by lattice information on M_ρ, f_π, by the q q̅ leading 1/N_c behavior of the ρ and by existing estimates of low-energy constants. These fits yield relatively stable predictions up to m_π ̴̲ 300-350 MeV for the rho coupling and width as well as for all the f_0(600) parameters. We confirm, to two loops, the weak m_π dependence of the rho coupling and the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin relation, and the existence of two virtual f_0(600) poles for sufficiently high m_π. At two loops one of these poles becomes a bound state when m_π is somewhat larger than 300 MeV.