Wing Sheung Chan

Event selection and classification 73 in Z → τ τ → `τ had - vis + 3 ν events, CRZ τ τ , is defined. Events in CRZ τ τ are required to pass the same selection criteria as those for the SR, except the cut on m T ( τ had - vis , E miss T ) . Events are required to have m T ( τ had - vis , E miss T ) > 35 GeV and m T ( `, E miss T ) < 40 GeV instead, which can be justified by the expected m T distributions as shown in Figure 4.1. Additionally, the so-called collinear mass, m coll ( ` , τ ), of the event must be between 70 GeV and 110 GeV for the event to be considered. The collinear mass is the invariant mass of the ` – τ had - vis – E miss T system, assuming that the E miss T has a virtual z -component such that its pseudorapidity is that same as that of the τ had - vis candidate. In the case of a signal event, the collinear mass is a good proxy for the true invariant mass of the ` – τ had - vis – ν system, which is expected to be the Z boson mass. Another control region, CRZ `` , is defined for measuring corrections to the modelled ` → τ had - vis misidentification rates. The region consists of dominantly Z → `` events and is defined using outputs of the neural network classifiers. The event selection is the same as that for the SR, except that the last criterion in Table 4.2 is inverted and both the outputs of the NN Z ττ and NN Wjets classifiers (see Section 4.2) must be larger than 0.8. These cuts select events that are classified by the neural networks to be likely a Z → `` event but not likely a Z → τ τ or W +jets event. Furthermore, four regions known as the fakes-enriched regions (FR) are defined. The FRs are a special kind of control region. These regions are enriched in jet → τ had - vis fakes and are essential for deriving the data-driven prediction of fakes in other regions, including the SR (see Section 5.5) . The four FRs are: FRW Dominated by W ( → τν ) +jets events, which contribute the most to the total amount of fakes in the SR. The region is defined by inverting the cut on m T ( τ had - vis , E miss T ) for the SR and requiring m T ( `, E miss T ) > 40 GeV . FRQ Dominated by QCD multijet events, which are the second largest contribution to all the fakes in the SR. This region requires events to satisfy the SR selection but with an inverted isolation requirement for the light lepton. Events with m T ( `, E miss T ) > 40 GeV are excluded to suppress the W +jets background. Moreover, the ` – τ had - vis pair is required to carry same-sign (SS) electric charges instead of OS charges to reduce contamination from Z → τ τ events. FRZ Dominated by Z ( → `` ) +jets events. The event selection for this region is the same as that for the SR, except that exactly two same-flavour, opposite-sign-charged light leptons are required instead of just one. The invariant mass of the light lepton pair must fall within the interval (81 GeV , 101 GeV) . The event is still required to have a τ had - vis candidate. FRT Dominated by t ¯ t events. Events in the region must satisfy the SR selection but with two or more b -jets identifie d † . † Events from single-top or Wt productions also contribute to the total fakes background. However, the contribution from these events is smaller than that from t ¯ t events by orders of magnitude in the SR. Therefore, the FRT is only optimised to include t ¯ t events and requires two or more b -jets instead of at least one b -jet.