Abstract
Abstract Low energy γγ → π 0 π 0 is a process that is finite at lowest order in chiral perturbation theory and is consequently regarded as a key prediction of this theory. Existing experimental data from Crystal Ball are, however, at variance with this prediction. Here we present a dispersive calculation embodying the low energy theorems of QED and PCAC, together with data on ππ scattering, to compute γγ → π 0 π 0 at low energies. We predict a cross-section in very good agreement with the Crystal Ball results. The dispersive approach naturally includes the non-perturbative aspects of hadronic final state interactions that lowest order chiral perturbation theory omits. Consequently, future measurements of DAΦNE of γγ → π + π - and π 0 π 0 near threshold may be able to reduce the present uncertainly in low energy ππ phase shifts.
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