a Measurement of the Pion Electromagnetic Charge Radius by Pion-Electron Scattering.

A pion-electron scattering experiment has been performed at Fermilab with a 200 GeV/c incident pion beam and with various target materials. This allowed a measurement of the cross section in the momentum transfer region given by .02 < q('2) < .16 (units of GeV('2)). By comparing the shape of the observed differential cross section to that of a point coulomb cross section, an electromagnetic charge radius for the pion was determined. That result was <r(,(pi))('2)> = .157 (+OR-) .06 (+OR-) .08 fermi('2) (statistical and systematic errors respectively). While the shape of the q('2) distribution indicated the above result, we observed an absolute cross section which was smaller than the prediction. In light of this discrepancy, a more conservative analysis of the data was undertaken. From that analysis an upper limit on the square of the pion radius was obtained. That limit was <r(,(pi))('2)> < .47 fermi('2).