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2. Physics and Detectors at LHC
from the TRT, and works inwards adding silicon hits as it progresses. This recovers tracks from secondaries, such as photon conversions and long-lived hadron decays. The association between the stand-alone and inner-detector tracks is performed using a χ2 -test, defined from the difference between the respective track parameters weighted by their combined covariance matrices. The parameters are evaluated at the point of closest approach to the beam axis. The combined track parameters are derived either from a statistical combination of the two tracks (Staco algorithm) or from a refit of the full track (MuId algorithm). An alternative approach is used to recover muons whose reconstruction in the muon spectrometer failed, either because of too low muon pT or because of the acceptance: this approach is based on the extrapolation of an inner detector track to the inner or middle stations of the muon spectrometer; the muon hypothesis is confirmed by a match to a track segment in these stations, not associated to a MS track. These muons are indicated as tagged muons. Finally another type of muon identification is still possible which doesn’t use the Muon Spectrometer information. This is based on Inner Detector tracks extrapolated to the calorimetric system. If the energy deposit associated to the track is compatible with the hypothesis of a minimum ionizing particle this is tagged as a muon. In the following this will be referred as calo-muons. The level-one muon barrel trigger covers the region between −1.05 < η < 1.05. It is designed to provide three “low-pT thresholds” between 4 ≤ pT ≤ 10 GeV , and three “high-pT thresholds” between 10 < pT ≤ 40 GeV . Each trigger threshold selects muons with a pT greater that its value. Because of the arrangement of detector services in the muon spectrometer, the feet of the detector, as well as support for the toroid coils, the barrel trigger coverage is ∼ 85% of the region −1.05 < η < 1.05. The trigger electronics uses signals coming from three layers of Resistive Plate Chambers (RPC). The RPCs are packed together with the MDT chambers. There are two layers of chambers around the MDT middle stations, of which the innermost layer is referred to as the low-pT plane and the outer is known as the pivot plane. The third RPC layer is on the MDT outer station and is called the high-pT plane. The trigger logic is seeded by the pivot plane: if there is an hit on that plane, the trigger logic checks for hits on the low-pT plane which are within a road defined around a track emerging from the interaction point and bending in the magnetic field and which are in the same time window of 25 ns (1 BC) ; if so, a low-pT trigger is issued. To further reduce the fake rate, it is also required to have hits in at least 3 of the 4 layers (2 pivot and 2 low-pT ) involved in the trigger decision (called trigger majority). When a low-pT trigger is issued, the high-pT trigger logic looks for hits in the high-pT plane fulfilling the trigger logic requirement; one hit out of two gas-gaps in addition to the low-pT trigger. This implies that an high-pT trigger always requires a low-pT , and in case of multiple triggers the systems forward to the subsequent trigger elements the highest pT threshold issued. This is relevant for the efficiency estimation: in fact, the inefficiency in the high-pT trigger contains the inefficiency of the low-pT ones. The level-one end-caps muon trigger is based on signals provided by TGC