B-Physics Event Selection for the ATLAS High Level Trigger

The ATLAS B-physics trigger relies on the partial reconstruction of B decays in order to select semi-exclusively specific channels of interest. The trigger is initiated by a muon from the LVL1 trigger which is confirmed at LVL2, firstly in the muon detector and then in association with an Inner Detector (ID) track. The next step is an un-guided search for tracks in the ID. In this note various candidatealgorithms are assessed for implementation in the LVL2 trigger and Event Filter (EF). The track search can be initiated from the inside of the ID, using information from the Pixel detector or from the outside using information from the Transition Radiation Tracker (TRT).The result of the initial search is a set of tracks the parameters of which form seeds for extrapolation into the Semi-Conductor Tracker (SCT) and subsequently into the TRT, for tracks initiated at the inside, or into the Pixel detector for tracks initiated from the TRT.Three channels of interest for physics studies are used as examples ofthe trigger selection process: Bd->pi+pi- and B decays including the subsequent decays Ds->phi(K+K-)pi and J/psi->e+e-. The overall performance of the track reconstruction algorithms and LVL2 and EF selections are characterized in terms of efficiency for the signal channel, trigger rate and execution time.