  We consider dephasing by interactions in a one-dimensional chiral fermion
system (e.g. a Quantum Hall edge state). For finite-range interactions, we
calculate the spatial decay of the Green's function at fixed energy, which sets
the contrast in a Mach-Zehnder interferometer. Using a physically transparent
semiclassical ansatz, we find a power-law decay of the coherence at high
energies and zero temperature (T=0), with a universal asymptotic exponent of 1,
independent of the interaction strength. We obtain the dephasing rate at T>0
and the fluctuation spectrum acting on an electron.
