Precision dispersive approaches versus unitarized chiral perturbation theory for the lightest scalar resonances sigma/f(0)(500) and kappa/K*(0)(700)

Abstract

For several decades, the sigma/f(0)(500) and kappa/K*(0)(700) resonances have been subject to long-standing debate. Both their existence and properties were controversial until very recently. In this tutorial review, we compare model-independent dispersive and analytic techniques versus unitarized Chiral Perturbation Theory, when applied to the lightest scalar mesons sigma/f(0)(500) and kappa/K*(0)(700). Generically, the former have settled the long-standing controversy about the existence of these states, providing a precise determination of their parameters, whereas unitarization of chiral effective theories allows us to understand their nature, spectroscopic classification and dependence on QCD parameters. Here we review in a pedagogical way their uses, advantages and caveats.