A new measurement of the rare decay ! 0 with the Crystal Ball/TAPS detectors at the Mainz Microtron

A new measurement of the rare, doubly radiative decay � ! � 0 was conducted with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. New data on the dependence of the partial decay width, ( � ! � 0 ), on the two-photon invariant mass squared, m 2 (), as well as a new, more precise value for the decay width, ( � ! � 0 ) = (0.33 ± 0.03tot) eV, are based on analysis of 1.2 × 10 3 � ! � 0 decays from a total of 6 × 10 7 � mesons produced in the p ! �p reaction. The present results for d( � ! � 0 )/dm 2 () are in good agreement with previous measurements and recent theoretical calculations for this dependence.