The light scalars, especially the sigma, in charm decay and elsewhere

Abstract

We discuss how the lightest scalars, in particular the broad sigma resonance, can be understood as unitarized (qbar q) states within a unitarized quark model (UQM). The bare (qbar q) scalars are strongly distorted by hadronic mass shifts, and the (ubar u+ dbar d) state becomes a very broad resonance, with its pole at 470-i250 MeV. This is the sigma meson required by models for spontaneous breaking of chiral symmetry. We also discuss the less well known phenomenon that with a large coupling there can appear two physical resonance poles on the second sheet although only one bare quark-antiquark state is put in. The f_0(980) and f_0(1370) resonance poles can thus be two manifestations of the same (sbar s) quark state. Both of these are dominant in the E791 Dalitz plot of D_s-> 3pi, where (sbar s) intermediate states should be dominant. Recently this light sigma has clearly been observed in D-> sigma pi-> 3pi by the E791 experiment at Fermilab. We discuss how this decay channel can be predicted in a Constituent Quark Meson Model (CQM), which incorporates heavy quark and chiral symmetries.