Probing Meson Structure with Photons

The scalar mesons f(980) and a(985) are two of the best-verified states in the excited meson spectrum. They appear as narrow resonances in the 2 and mass spectra, respectively, as observed in hadroproduction reactions and in the decays of heavier particles such as the J/ and the Z. Although they were once thought to be members of the fundamental nonet of scalar qq' mesons, conventional wisdom is now inclined against that assignment in favor of an exotic (i.e. non-qq') interpretation for these states. These two models are quite different in terms of their charge and current distributions, and so it was suggested early in the 90's (see reference article below) that an experiment using the photon as a probe might distinguish empirically between them. The Radphi experiment at TJNAF will measure the radiative decays of the (1020) meson to f and a in a run scheduled for this summer. First results for these decays were published in 1998 by experiments at VEPP-2M in Novirsibirsk, Russia and seem to support a non-quark-model assignment. Under experimental conditions that are different from collider experiments, Radphi will provide an independent check of this important result.

1. Introduction: three models

 

2. Primary physics issue: structure of the f(980) and a(985)

 

3. Experimental technique: photoproduction

 

4. Radphi apparatus

 

5. Present status and future plans