Radiative transitions of low-lying positive-parity mesons

Abstract

Experiments soon to be performed, such as Coulomb dissociation of hadrons and $e^{\pm }{e}^{-}\to e^{\pm }{e}^{-}+\mathrm{hadrons}$, will be able to measure radiative decays of mesons with $J^{\mathrm{PC}}=2^{++}, 1^{++}, 0^{++}, \mathrm{and} 1^{+-}$. In advance of these experiments, predictions are made for rates and angular distributions in such processes as $A_{1,2}\to \pi \gamma$, $f_0\to \rho \gamma$, and $f_0\to \gamma \gamma$. These predictions are based only on vector dominance and single-quark selection rules, but will permit the first tests of such hypotheses for excited mesons. Results include: (a) relatively large partial widths for $f_0\to \gamma \rho$ and $f_0\to \gamma \gamma$: $\Gamma (f_0\to \gamma \rho )\simeq 1.5$ MeV and $\Gamma (f_0\to \gamma \gamma )\simeq 8$ keV; (b) the suppression of $\lambda =0, \pm 1$ decays relative to $\lambda =\pm 2$ for these processes; (c) relative phases for helicity amplitudes in these processes, implying definite angular distributions in $\gamma \gamma \to f_0\to \pi \pi$ and $f_0\to \gamma \rho \to \gamma {\pi }^{+}{\pi }^{-}$; and (d) the nearly complete suppression of $\gamma \gamma$ couplings of $0^{++}$ $q\overline{q}$ states.