Radiative Decays of Hadronic Molecules

It is argued that radiative decays of scalars a0/f0(980) can serve as a decisive tool in establishing the nature of the latter. In particular, predictions for the widths of the radiative decays S → γV (S = a0/f0(980), V = ω/ρ/γ) are given in the framework of the molecule model of the scalars. Finite–range corrections are discussed in detail for the two-gamma decays of hadronic molecules, with a special attention payed to the interplay of various scales involved in the problem and to the gauge invariance of the amplitude. The results are applied to the two-photon decay of the f0(980), and the existing experimental data on this decay are argued to support the molecule assignment for the scalar f0(980). The problem of the structure of light scalar mesons is of a fundamental importance for understanding the properties of the entire scalar sector, that is, the sector of states with the quantum numbers of the vacuum, including purely gluonic excitations. In particular, the identification of the a0(980) and f0(980) mesons, together with the experimental studies of the lightest scalars (σ and κ), will allow one to establish the structure of multiplets of scalars and to find the signature of the scalar glueball in the spectrum of physical states. There are several models for the a0(980) and f0(980). The latter can be considered as P0 quark–antiquark states [1] strongly coupled to the mesonic continuum and thus strongly distorted with the unitarisation process. However, due to the proximity of the KK̄ threshold, it is natural to assume a considerable admixture of the four–quark component in the wave functions of these mesons, either as a compact four–quark with hidden strangeness [2, 3], or as a KK̄ molecule. These might be t-channel exchanges to be responsible for the formation of such a molecule [4, 5, 6, 7]. It is therefore important to establish a test which would allow one to distinguish between these models and thus to reveal the actual nature of these scalars (in particular, efficient methods to discriminate between the molecule and compact states are strongly needed — for the recent progress