a Measurement of the Neutral Pion Form Factor by Means of the Reaction Neutral Pion ---> Electron Positron Gamma.

The slope parameter "a" of the (pi)('0) - electromagnetic transition form factor was obtained from a measurement of the partial branching ratio of the Dalitz decay (pi)('0) (--->) e('+)e('-)(gamma) into high invariant-mass electron -positron pairs. The experiment was carried out at the TRIUMF cyclotron laboratory. A beam of (pi)('-) mesons was stopped in a liquid hydrogen target to produce neutral pions in the reaction (pi)('-) p (--->) n (pi)('0). The electron-positron pairs were detected with two delay-line chamber telescopes, each followed by a large NaI crystal. A third NaI crystal measured the number of (pi)('0)'s in the target by counting the photons from the decay (pi)('0) (--->) (gamma) (gamma). Data was taken at three detector opening angles 60(DEGREES), 130(DEGREES) and 156(DEGREES). The 60(DEGREES) measurement confirms our normalization. A value. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). where the first error is due to statistics and the second is an estimate of systematic effects, was obtained from the 130(DEGREES) geometry. This value does not support the result of the most recent previous experiment and is in accord with most theoretical calculations indicating that there is no unexpected structure in the (pi)('0) decay form factor.