A di-muon selection for the second level trigger in the ATLAS detector

Abstract

Di-muon selection for the second level trigger in the ATLAS experiment. The B production cross section at the LHC is very high, but it is just a small fraction of the total inelastic cross section (0.7%), therefore the trigger for B physics is required to be very selective. The output rate of the first level trigger is expected to contain 20 KHz of events with at least one energetic muon with pT above 6 GeV. Most of these events are from K and ? decays, and only 4 KHz are b events. At the second level trigger (level 2) this rate of events must be reduced to 2-10 KHz. Currently this goal is achieved by confirming a muon in the Inner Detector in addition to confirming it in the Muon Spectrometer. This selection criterion removes K and ? decays, but many interesting b events are likely to be filtered out as well. Acceptable first level trigger (level 1) rates in ATLAS have been reduced due to staging of high level trigger processing power. Envisioned trigger menus now include 2 low pT ? or one higher pT ?. This would reduce the number of available b-physics events. Algorithms are developed for recovery of the most promising b events at level 2. We developed an algorithm which achieves high efficiency for the gold channels (J/?) at level 2 using the identification of relatively low pT muons that decay from J/?. Physics and time performance of the algorithm will be presented here.