Spectral functions of nucleon form factors: Three-pion continua at low energies
N. Kaiser🇩🇪 · E. Passemar🇺🇸
We study the imaginary parts of the isoscalar electromagnetic and isovector axial form factors of the nucleon close to the -threshold in covariant baryon chiral perturbation theory. At the two-loop level, the contributions arising from leading and next-to-leading order chiral -vertices, as well as pion-induced excitations of virtual -isobars, are calculated. It is found that the heavy baryon treatment overestimates substantially these -continua. From a phenomenological analysis, that includes the narrow -resonance or the broad -resonance, one can recognize small windows near threshold, where chiral -dynamics prevails. However, in the case of the isoscalar electromagnetic form factors , the radiative correction provided by the -intermediate state turns out to be of similar size.