(It's really just the beam smashing into the collimator just before the detector.)
The circular views are cross-sections of the ATLAS detector (so the beam is coming into/out of the page). Green indicates splashes of energy in the calorimeter (calorimeter = energy detector; think high school chemistry class with a styrofoam cup). In the second picture, the red dots are where muons crossed the muon detectors ("hits").
And, for a laugh: