Chiral effective theory with a light scalar and lattice QCD

We extend the usual chiral perturbation theory framework (χ  PT) to allow the inclusion of a light dynamical isosinglet scalar. Using lattice QCD results, and a few phenomenological inputs, we explore the parameter space of the effective theory. We discuss the S-wave pion–pion scattering lengths, extract the average value of the two light quark masses and evaluate the impact of the dynamical singlet field in the low-energy constants l¯1, l¯3 and l¯4 of χPT. We also show how to extract the mass and width of the sigma resonance from chiral extrapolations of lattice QCD data.