The ATLAS High Level Trigger reconstructs the position and shape of the interaction region (the "beam spot") in real time using a dedicated algorithm that analyses the distribution of primary vertices in the Inner Detector. The characteristics of the interaction region and its time-evolution are of great interest to the LHC operators as they have a direct impact on the luminosity generated for the experiment.
Independent Outside of the detector, there is a wealth of information available from the beam instrumentation of the LHC itself, including beam pickups, beam position monitors, synchrotron light monitors etc. These per-beam and per-bunch measurements can be directly related to the luminous region observed in ATLAS. Both LHC and ATLAS measurements are recorded in real-time in the so-called LHC Logging Database.
...