Anomalous Sudakov Form Factors

While radiative corrections of infrared origin normally depress high energy amplitudes (Sudakov form factors), we find that in some cases resummation of leading effects produces exponentials with positive exponents, giving rise to amplitudes that grow indefinitely with energy. The effect happens in broken gauge theories like the electroweak sector of the Standard Model, and is related to the existence of amplitudes that do not respect the gauge symmetry. Contrary to expectations, these amplitudes, although mass suppressed, do not vanish in the very high energy limit, but rather become dominant. As a working example we consider a model with two chiral abelian gauge groups U (1)⊗U(1) with large mass splitting MZ′ ≫ MZ , and we compute leading radiative corrections corrections to the decay of the heavy extra Z boson into light fermions. The chirality breaking magnetic dipole moment becomes the dominant contribution to the Z width at very high energies.