Wing Sheung Chan

62 Object reconstruction and identification reconstruction is mainly to recover acceptance in the region covered by the MS but not by the ID ( 2 . 5 < | η | < 2 . 7 ). In cases where an ID track is shared by multiple muon candidates of different types, preference is first given to CB muons, then to ST muons, and finally to CT muons. MS tracks that are shared by ME muons are given to the candidate with better fit quality and larger hit multiplicity. Three sets of identification criteria with increasing efficiencies, Tight , Medium and Loose , are defined to identify muons produced directly from hard-scattering vertices or decays of heavy resonances (prompt muon). The major background for prompt muon identification is muons from pion and kaon decays. The identification criteria consider the fact that non-prompt muons typically have tracks with a poor quality of fit and inconsistent momentum measured in the ID and the MS. In addition, an extra set of identification criteria, High-pT , is defined specifically for optimised momentum resolution for muons with p T > 100 GeV . Another type of muon identification background is cosmic muons, which are muons from cosmic rays that penetrate the atmosphere and land surface, and reach the detector. Cosmic muons can be effectively rejected by requiring the muon tracks to have small impact parameters with respect to the interaction vertex. Similar to electrons, isolation WPs are also defined for muons to veto muons originated from jets. These WPs are defined using variables similar to those described in Section 3.3 for electrons. The performance of the muon reconstruction, identification and isolation is summarised in Figure 3.7. 3.5. Missing transverse momentum As neutral particles that only interact weakly, neutrinos are never detected by tracking detectors, nor do they deposit energy in the calorimeters. The transverse energy of these invisible particles, nonetheless, can still be inferred. At the LHC, the centre of mass of the colliding protons can be expected to be stationary in the transverse plane. This implies that the total vector sum of the transverse momenta of all outgoing particles must have a vanishing magnitude. That is, X all particles p T = 0 . (3.9) If some of the particles are invisible to the detector, either because they are neutrinos or some hypothetical inert particles, their total transverse momentum can then be calculated