How can one understand the lightest scalars , especially the σ

We discuss how the a 0 (980), f 0 (980), K * 0 (1430) and particularly the broad σ resonance can be understood within a coupled channel framework, which includes all light two-pseudoscalar thresholds together with constraints from Adler zeroes, flavour symmetric couplings, unitarity and physically acceptable analyticity. All q ¯ q scalars are, when uni-tarized, strongly distorted by hadronic mass shifts, and the nonstrange isoscalar state becomes a very broad resonance, with its pole at 470-i250 MeV. We believe this is the sigma meson required by models for spontaneous breaking of chiral symmetry. Recently this light resonance has clearly been observed in D → σπ → 3π by the E791 experiment at Fermilab, and we discuss how this decay channel can be predicted in a Constituent Quark Meson Model (CQM), which incorporates heavy quark and chiral symmetries. 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 s¯ s quark state. Both of these states are seen clearly in D s → 3π by the E791 experiment, where s¯ s intermediate states are expected to be dominant. This talk is mainly based on earlier papers [1,2] on the light scalars and on a more recent one [3] on the σ in charm decay, including a few new comments. First we shall discuss the evidence for the light σ and explain how one can understand the controversial light scalar mesons with a unitarized quark model, which includes most well established theoretical constraints: • Adler zeroes as required by chiral symmetry, • all light two-pseudoscalar (PP) thresholds with flavor symmetric couplings in a coupled channel framework • physically acceptable analyticity, and • unitarity. 2 A unique feature of this model is that it simultaneously describes the whole scalar nonet and one obtains a good representation of a large set of relevant data. Only six parameters, which all have a clear physical interpretation, are needed. After describing our understanding of the q ¯ q nonet, we discuss the recently measured D → σπ → 3π decay, where the σ is clearly seen as the dominant peak. 2. The problematic scalars and the existence of the σ The interpretation of the nature …