EMI2001 Symposium on Electromagnetic Interactions in Hadrons and Nuclei

Can the scalar mesons a0/f0(980) be described by a kaon-antikaon state?

The scalar mesons f₀(980) and the a₀(980) are two of the best-verified states in the excited meson spectrum. They appear as narrow resonances in the 2 and mass spectra, respectively, observed in hadroproduction reactions and in the decays of heavier particles such as the J/ and the Z⁰. Their relatively low mass and narrow width do not match well with quark-model expectations for the lightest scalar multiplet, fueling speculation that these states might be different in nature from conventional q,qbar mesons. A variety of theoretical models have been put forward to explain these states, most notably the so-called "2-kaon molecule" model in which they appear as kaon-antikaon bound states. Within this picture, the radiative decays of the (1020) meson two two-pseudoscalar final states are expected to be dominated by f₀ and a₀ intermediate states, in spite of their appearance very near the edge of the available phase space. Recent experimental results from e+e- colliders have recently provided striking verification of this effect. Meanwhile, new work within a more general theoretical approach to the two-meson scattering problem has shown that the appearance of narrow scalar resonances near the two-kaon threshold is not a unique prediction of the molecular model, but are a consequence of the general principles of unitarity and chiral symmetry. Faced with this embarrassment of success, an attempt is made to synthesize some conclusions regarding the structure of the light scalars.

(Powerpoint slides, 670KB)
(conference proceedings, postscript, 320KB)