Physical Review D
The pion form factor is calculated in the framework of the charge-conjugation invariant covariant spectator theory. This formalism is established in Minkowski space, and the calculation is set up in momentum space. In a previous calculation we included only the leading pole coming from the spectator quark (referred to as the relativistic impulse approximation). In this paper we also include the contributions from the poles of the quark which interacts with the photon and average over all poles in both the upper and lower half-planes in order to preserve charge conjugation invariance (referred to as the C-symmetric complete impulse approximation). We find that for small pion mass these contributions are significant at all values of the four-momentum transfer Q(2) but, surprisingly, do not alter the shape obtained from the spectator poles alone.
Biernat, E. P., Gross, F., Peña, M. T., & Stadler, A. (2015). Charge-conjugation symmetric complete impulse approximation for the pion electromagnetic form factor in the Covariant Spectator Theory. Physical Review D, 92(7), 076011.