As proton bunches collide at the LHC, there are several sources of backgrounds that accompany the physics being produced from the "hard", high-pt pp collisions: additional pp interactions, "cavern neutron/photon haze", beam halo, beam gas interactions, etc. The ATLAS simulation would like to model these backgrounds as accurately as possible.
We soon expect collisions at the LHC, and will use real data from "random" events to measure these backgrounds, and directly include simulated events "overlaid" on top of the real data background events. Your job would be to study the results of these overlaid data/MC events to help verify that the simulation is doing things correctly, and to compare the real data to simulated backgrounds, for instance.
Another possible project is to study additional MC generators, such as DPMJET3, and study what they predict for very far forward interactions of pp colision products with the beampipe and the resulting effects on the cavern haze backgrounds, etc.