Wing Sheung Chan

Event selection and classification 69 Furthermore, depending on the search channel ( eτ or µτ ), the leading- p T electron or muon in the event must match the object that “fired” the trigger(s). This ensures that the effect of the trigger efficiency can be properly taken into account in the signal and background modelling. 4.1.3. Signal region The signal Z → `τ events have a number of unique features that can be exploited to differentiate them from the SM background. To begin with, the event is a resonance, meaning that the ` – τ pair, before decaying or radiating, has an invariant mass close to the Z boson rest mass. This also implies that the light lepton and τ lepton are typically boosted due to the massiveness of the parent Z boson. Furthermore, for electric charge and momentum to be conserved, the leptons must carry opposite-sign (OS) charges and travel almost back-to-back in the transverse plane. Since the τ lepton is typically boosted, the τ had - vis and neutrino from its decay are expected to travel in almost the same direction in the laboratory frame of reference. Hence, the directions of the reconstructed τ had - vis and E miss T can be expected to be almost collinear. Utilising the characteristics of the signal events, a set of criteria is defined to select potential signal events with high efficiency while rejecting background events effectively. The region in the parameter space defined by these selection criteria is the so-called signal region (SR) of the analysis. Maximum-likelihood fits (see Section 6.1) to the measurements in the SR are used to extract possible evidence of Z → `τ decays. Events in the SR are required to have exactly one reconstructed light lepton and at least one τ had - vis candidate. The light lepton and the τ had - vis candidate with leading p T must satisfy the object selection criteria aforementioned in Table 3.2. The flavour of the light lepton determines which search channel will the event be considered in ( eτ or µτ ). Events with additional τ leptons are not vetoed, since doing so would complicate the estimation of events with misidentified τ had - vis candidates, as it mixes per-event and per-object misidentification rates. It has been estimated that only roughly 0.1% of the events in the SR have more than one τ had - vis candidate, and the leading- p T τ had - vis candidate in signal events is correctly reconstructed from the signal τ lepton 99.8% of the time. For the sake of convenience, from this point onwards, “ the τ had - vis candidate” of an event would always refer to the leading- p T τ had - vis in the event unless the context clearly implies otherwise. The light lepton and the τ had - vis candidate are required to carry OS charges. The visible invariant mass, m vis ( `, τ ) ≡ m ( `, τ had - vis ) = p 2 p T ( ` ) p T ( τ had - vis ) [cosh∆ η ( `, τ had - vis ) − cos ∆ φ ( `, τ had - vis )] , (4.1) is required to be larger than 60 GeV to reject background events with ` – τ had - vis pairs that are unlikely to originate from Z → `τ decays. While an upper cut on m vis ( `, τ ) can also reject background events, it has been found that keeping events with high m vis ( `, τ ) are