When the LHC beams cross in ATLAS, each crossing at design luminosity will have on average over 20 hadronic interactions. They are mostly low-p+T+ and "uninteresting" events. However, they will necessarily be superimposed on the high-pT and "interesting" events which we normally simulate. While these background events could in principle be simulated, it is far simpler to use real data with a zero-bias trigger. It naturally includes all background sources in the right ratio. It can track changes in these backgrounds. Overlay refers to the mechanism of adding these real-data background events to the high-pT one in order to simulate what we actually observe. We are developing tools to validate that it is done correctly.
Outline of overlay
The first step is to acquire luminosity weighted zero-bias events. Select a trigger which (1) scales with luminosity, (2) has high enough rate so it is statistically precise, and (3) is robust against changing beam conditions. When the trigger is satisfied, wait one revolution of the LHC beams, and record the next crossing of the same bunches, whether or not any trigger is satisfied on this subsequent crossing.
When we need N simulated events for a certain time period, we select N zero-bias events recorded during this time and look up the detector conditions at those N discrete times. These conditions could be the same for some period; they could also be different for every event. We then generate and simulate signal events according to these conditions, whatever those conditions might be.
After the signal events have been simulated, we add in the hits recorded in the corresponding zero-bias event to give the complete signal plus background sample. This step is referred to as overlay.
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