  We study theoretically electronic Mach-Zehnder interferometers built from
integer quantum Hall edge states, showing that the results of recent
experiments can be understood in terms of multiparticle interference effects.
These experiments probe the visibility of Aharonov-Bohm (AB) oscillations in
differential conductance as an interferometer is driven out of equilibrium by
an applied bias, finding a lobe pattern in visibility as a function of voltage.
We calculate the dependence on voltage of the visibility and the phase of AB
oscillations at zero temperature, taking into account long range interactions
between electrons in the same edge for interferometers operating at a filling
fraction $\nu=1$. We obtain an exact solution via bosonization for models in
which electrons interact only when they are inside the interferometer. This
solution is non-perturbative in the tunneling probabilities at quantum point
contacts. The results match observations in considerable detail provided the
transparency of the incoming contact is close to one-half: the variation in
visibility with bias voltage consists of a series of lobes of decreasing
amplitude, and the phase of the AB-fringes is practically constant inside the
lobes but jumps by $\pi$ at the minima of the visibility. We discuss in
addition the consequences of approximations made in other recent treatments of
this problem. We also formulate perturbation theory in the interaction strength
and use this to study the importance of interactions that are not internal to
the interferometer.
