Physics Overview

The study of the radiative decays of the heavier vector mesons, J/psi (charmonium: c-cbar) and upsilon (bottomonium: b-bbar), soon after their discovery, played an important role in understanding quarkonium spectroscopy. Interestingly enough, although the phi (strangeonium: s-sbar) was discovered earlier, much less is known about its radiative decays. One can enumerate seven energetically allowed decays phi -> X gamma, where X is a particle. Only two have been observed: phi -> pi⁰ gamma and phi -> eta gamma. Although phase space would favor the former, the branching ratio for the latter is greater by an order of magnitude, reflecting the s-sbar contribution to the quark content of the eta. Two of the energetically allowed decays, phi -> rho gamma and phi -> omega gamma are forbidden since they are C-violating. However, the experimental limits on the branching ratios (< few percent) for these modes are poorly determined. Another allowed decay involves a pseudoscalar: phi -> eta' gamma, as yet unobserved, should provide information on the quark/gluon content of the eta'. Radiative phi decays into the isoscalar f₀(980) and isovector a₀(980) could shed light on the substructure of these states. Originally assigned to the scalar nonet, there is mounting evidence that these are not q-qbar states. There is much theoretical speculation about what these states might be and possible interpretations include four-quark states and molecular states (K-Kbar). The measurement of the ratio: (phi -> a₀ gamma)/(phi -> f₀ gamma) should allow one to select among interpretations. The decays into these scalar states are also important since they provide information on phi -> K_short K_long gamma, a potentially insidious background to CP-studies at an e⁺e^- phi factory if the very low energy radiated photon goes undetected. A phi factory is predicated on a correlated source of neutral kaons since phi -> K_short K_long.

Up to now, information on radiative decays comes from e⁺e^- -> phi -> X gamma and one of the reasons for the paucity of data on these radiative decays is that the radiated photon is low in energy (Eg). Incidentally, the decays V -> P gamma and V -> S gamma (where V = vector, P = pseudoscalar and S = scalar) are E1 or M1 transitions and the resulting E_g³ strongly damps the decay rate. At TJNAF, the photoproduction process will be used in this experiment to produce a fairly well collimated source of phi's, defined in energy and with a rate ( 30 phi's/sec) comparable to an e⁺e^- luminosity of 10³¹/cm²/sec. This is comparable to what is currently available in colliders but the boost factor (as high as 12 for 6 GeV electrons) provides an important advantage. This experiment will use the tagged photon beam in Hall B. Photons in the range from 4 to 6 GeV will be incident on a Be target. A 624-element lead glass detector will be used to look at decays:

• phi -> pi⁰ (or eta) gamma -> 3 gamma
• phi ->omega gamma -> pi⁰ gamma gamma -> 4 gamma
• phi -> f₀ gamma -> pi⁰ pi⁰ gamma -> 5 gamma; and
• phi -> a₀ gamma -> pi⁰ eta gamma -> 5 gamma