B-physics and LVL1 di-muon trigger in the ATLAS experiment at the LHC

Abstract

Many interesting physics processes in the ATLAS experiment at the LHC will be characterized by the presence of pairs of muons in the final state. For this reason, the ATLAS first level muon trigger has been designed to allow the selection of di-muon events. However, in order to increase the trigger acceptance of the muon spectrometer, several regions of overlap between the trigger chambers are foreseen in the detector layout. A muon crossing one of these regions may generate two separate triggers, thus producing a false di-muon trigger. The trigger system must therefore be aware of the geometrical overlaps, in order to resolve such fake double triggers. The overlap solving mechanism of the ATLAS LVL1 muon trigger has been intensively studied with the final detector layout. The overlap flags, needed by the trigger logic to properly handle fake double triggers due to geometrical overlap of the trigger detectors, have been set on a strip-by-strip basis. The chosen method consisted mainly in simulating the propagation of single-muons through the ATLAS spectrometer. The simulated response of the trigger system was then analyzed in order to locate the events for which two muon triggers were generated. Two studies have been performed to evaluate the impact of the overlap flags on the performance of the LVL1 muon trigger and its impact on B-physics: a single muon sample was used to check the proper removal of the fake double triggers and a di-muon B physics sample was used to check the amount of real double triggers that are lost due to the overlap removing mechanism. In addition the chosen B physics channel was the Λb → Λμμ, a rare decay channel with a particular muon trigger interest because of the di-muon topology.