Dalitz decay. π0 → γe+e− and the π0 electromagnetic transition form factor

We have studied the π 0 Dalitz decay π 0 → γ e + e − using the pion beam and a magnetic pair spectrometer at the Saclay Linear Accelerator. A shape analysis of the (e + e − ) invariant mass squared x , carried out on 32 000 events, gives a direct measurement of the π 0 electromagnetic transition form factor squared | F ( x )| 2 , and of its slope parameter a in the expression F ( x ) = 1 + ax . We find a = −0.11±0.03(stat.)±0.08(syst.) m −2 π 0 .