ar X iv : h ep - p h / 95 04 37 2 v 1 2 4 A pr 1 99 5 HU - SEFT R 1995 - 05 ( for ZPC ) UNDERSTANDING THE SCALAR MESON qq̄ NONET

It is shown that one can fit the available data on the a 0 (980), f 0 (980), f 0 (1300) and K * 0 (1430) mesons as a distorted 0 ++ q ¯ q nonet using very few (5-6) parameters and an improved version of the unitarized quark model. This includes all light two-pseudoscalar thresholds, constraints from Adler zeroes, flavour symmetric couplings, unitarity and physically acceptable analyticity. The parameters include a bare u¯ u or d ¯ d mass, an overall coupling constant, a cutoff and a strange quark mass of 100 MeV, which is in accord with expectations from the quark model. It is found that in particular for the a 0 (980) and f 0 (980) the K ¯ K component in the wave function is large, i.e., for a large fraction of the time the q ¯ q state is transformed into a virtual K ¯ K pair. This K ¯ K component, together with a similar component of η ′ π for the a 0 (980), and ηη, ηη ′ and η ′ η ′ components for the f 0 (980), causes the substantial shift to a lower mass than what is naively expected from the q ¯ q component alone. Mass, width and mixing parameters, including sheet and pole positions, of the four resonances are given, with a detailed pedagogigal discussion of their meaning.