Wing Sheung Chan

60 Object reconstruction and identification needs of various analyses. For the search for Z → `τ decays, where a high jet rejection is desired and a constant efficiency is not required, the FCTight WP that is based on fixed cuts on both the calorimeter- and track-based isolation variables is used. The performance of the electron reconstruction, identification and isolation is sum- marised in Figure 3.6. 3.4. Muons The muon is a rather special particle in terms of detection. Muons have a relatively long lifetim e † , and without decaying, their interactions with the calorimeter materials are very limited due to their minimum-ionising nature. It is precisely for this reason that an entire detector subsystem, the muon spectrometer and the toroidal magnets, is dedicated for the detection of muons. The performance of their reconstruction and identification is also quite different from other particles [86] . Muons are reconstructed primarily via tracking. Reconstructions are first carried out separately in the ID and the MS. In the ID, muons are reconstructed as tracks in the same way as any other charged particles. In the MS, segments are first constructed by identifying certain hit patterns. Segments in the different layers of the MS are then fit together to form muon track candidates. For each candidate, a χ 2 fit of the associated hits is performed. The quality of the χ 2 fit is used as a criterion for accepting the candidate. The information provided by the ID, MS and calorimeters is then combined using a number of algorithms to construct the muon candidates. Four types of reconstructed muons are defined, depending on the algorithm and subdetector information used: Combined (CB) muons The separately reconstructed tracks in the ID and the MS are matched and their hits are refitted to form combined tracks. The matching is con- ducted in an outside-in manner, where matching ID tracks are found by extrapolating the MS tracks. Segment-tagged (ST) muons ID tracks are extrapolated to the MS where matching MS segments are found. If at least one segment in the MDTs or CSCs are matched, the track is considered a muon candidate. ST muons are mostly muons with low p T or in regions with reduced MS acceptance, such that only one layer of the MS is crossed. Calorimeter-tagged (CT) muons An ID track is also considered a muon candidate if it can be matched to energy deposits in the calorimeters that are compatible with a minimum-ionising particle. CT muons have the highest misreconstruction rate amongst all types of muon candidates, but are useful in recovering acceptance in the crack region of the MS ( | η | < 0 . 1 ). Extrapolated (ME) muons MS tracks that, when extrapolated, are compatible with the nominal interaction point are also reconstructed as a muon candidate. This type of † In the laboratory frame of reference, where they are typically ultrarelativistic.