TRIUMF: Canada's national laboratory for particle and nuclear physics

A Measurement of the Neutral Pion Form Factor by Means of the Reaction π° → e⁺e-. Gumplinger, Peter


The slope parameter "a" of the π° - electromagnetic transition form factor was obtained from a measurement of the partial branching ratio of the Dalitz decay π° → e⁺e- into high invariant-mass electron-positron pairs. The experiment was carried out at the TRIUMF cyclotron laboratory. A beam of π- mesons was stopped in a liquid hydrogen target to produce neutral pions in the reaction π- p → nπ° The electron-positron pairs were detected with two delay-line chamber telescopes, each followed by a large Nal crystal. A third Nal crystal measured the number of π°’s in the target by counting the photons from the decay π°→ y y. Data was taken at three detector opening angles 60°, 130° and 156°. The 60° measurement confirms our normalization. A value a = -0.01 ± 0.035 +0.07, -0.05 where the first error is due to statistics and the second is an estimate of systematic effects, was obtained from the 130° 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 π° decay form factor.

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