Measurement of the LHC Beam Parameters Using the Distributed High Level Trigger System of the ATLAS Detector during 2009 and 2010

Exploring the first collision data from the LHC recorded at sqrt(s)=7 TeV in 2010, we present the first online measurement of the LHC luminous region parameters. This is achieved using highly optimized software algorithms running on a large farm of commercial processors in the high-level trigger system of the ATLAS experiment. A dedicated fast vertex fitting algorithm allows for event-by-event reconstruction of interaction vertices with a time constraint of a few milliseconds. The three-dimensional distribution of these vertices enables the characterization of interaction-point beam parameters and is used to extract the luminous region position, orientation and size. These results are then fed back to high-level trigger algorithms that depend on the precise knowledge of impact parameter or decay length, such as b-tagging. Changes in the transverse centroid position due to IP-orbit drifts, and longitudinal shifts arising from RF phase changes, are accounted for, and the performance of these algorithms is optimized in near real-time. Lastly, by counting the number of primary vertices online, we use this same algorithm to monitor the instantaneous luminosity of the accelerator. These capabilities were available during the first collision runs of 2009 and are being extended for the high energy running of 2010.