  We investigate experimentally the effects of light assisted collisions on the
coherence between momentum states in Bose-Einstein condensates. The onset of
superradiant Rayleigh scattering serves as a sensitive monitor for matter wave
coherence. A subtle interplay of binary and collective effects leads to a
profound asymmetry between the two sides of the atomic resonance and provides
far bigger coherence loss rates for a condensate bathed in blue detuned light
than previously estimated. We present a simplified quantitative model
containing the essential physics to explain our experimental data and point at
a new experimental route to study strongly coupled light matter systems.
