πe3Form Factorf-Near the Mass Shell

Abstract

The generalized Ward-Takahashi identity (gWTI) in the pion sector for broken isotopic symmetry is derived and used for the model-independent calculation of the longitudinal form factorf-of theπe3vector vertex. The on-shellf-is found to be proportional to the mass difference of the pions and the difference between the vector isospinT=1and scalar isospinT=2pion radii. A numerical estimate of the form factor yields a value two times higher than the previous estimate from the quark model. Off-shell form factors are known to be ambiguous because of the gauge dependence and the freedom in the parameterization of the fields. The near-mass-shellf-appears to be an exception, allowing for experimental verification of the consequences of the gWTI. We calculate the near-mass-shellf-using the gWTI and dispersion techniques. The results are discussed in the context of the conservation of vector current (CVC) hypothesis.