Event selection and categorisation

As mentioned in Section 3.2.6, the analysis is performed exclusively on the events passing the trigger selection, which requires each event to include at least one electron or one muon. Various triggers are combined together with a logical OR, depending on the transverse mo- mentum of the lepton candidates. Different requirements are also applied for 2015 and 2016 datasets, due to different data taking conditions (see Table 6.1)1.

Trigger name (2015) Object minimum pT [GeV]

e24_lhmedium_L1EM20VH electron 24

e60_lhmedium electron 60

e120_lhloose electron 120

mu_20_iloose_L1MU15 muon 20

mu_50 muon 50

Trigger name (2016) Object minimum p_T [GeV]

e26_lhtight_nod0_ivarloose electron 26

e60_lhmedium_nod0 electron 60

e120_lhloose_nod0 electron 120

mu_26_ivarmedium muon 26

mu_50 muon 50

Table 6.1: Triggers used for the event selection. The first column corresponds to the trigger name. The name includes information about the identification and isolation criteria applied on the objects. The ID working points used for electrons are identified by the strings "lhtight", "lhmedium" and "lhloose". Muons always require a medium identification. The tag "nod0" means that the cuts on the impact parameters have been removed, while "ivarloose" and "ivarmedium" refer to the isolation working points, described in Ref. [115].

The number of events passing the trigger selection is reduced further by imposing the quality criteria described in Chapter 4. These include a Tight identification working point for elec- trons and a Medium identification WP for muons. They also require both kind of leptons to pass the Gradient isolation and to satisfy a certain number of cuts on their impact parame- ters, to reduce the amount of fake leptons.

Finally, a set of cuts is applied at analysis level. Given the large number of expected jets (and b-jets) in the signal final state, the analysis is performed solely on events that contain at least five jets, of which at least two are b-tagged, with a minimum transverse momentum of

1

A different set of low-pT triggers is used for the estimate of the fake background. Details are given in

Appendix B.

Event selection and categorisation

25 GeV. A cut on the transverse momentum is also applied on leptons, which are required to have a p_T of at least 27 GeV. Both leptons and jets are required to be within |η| ≤ 2.5. More forward objects are neglected. Events containing more than one electron or muon are vetoed, preventing possible overlaps with the dilepton analysis. Events that include tau leptons contribute to the analysis in the rare occasion in which the leptons produced by their decays survive to the quality requirements or when another lepton is found in the events. Events are then categorised into regions, using the multiplicity of the jets and b-jets. A total of six regions is defined: two control regions (CRs), dominated by t¯t + light events, and

four signal regions (SRs), enriched in signal and dominated by t¯t + HF events. The region

definitions are given in Table 6.2: the control regions correspond to events with either five jets and two b-jets (5j2b) or at least six jets and two b-jets (≥6j2b). The four signal regions require either three or more than three b-jets (5j3b, 5j≥4b, ≥6j3b, ≥6j≥4b).

2 b-jets 3 b-jets ≥ 4b-jets

5 jets CR SR SR

≥ 6 jets CR SR SR

Table 6.2: Control and signal regions used in the analysis.

The signal over background ratio (S/B) of the event yields in the six regions is presented in Figure 6.1, for the 18 signal mass hypotheses (the signal events are normalised to 1 pb). The sensitivity to the H+ production is expected to come from the regions with the highest (relative) amount of signal, i.e. the 5j≥4b and ≥6j≥4b regions in the low-mass range, and the ≥6j3b and ≥6j≥4b regions in the high mass range.

Search for ¯tbH+(H+→ t¯b)

Figure 6.1: Signal over background ratio (S/B) as a function of the H+ mass in the control and signal regions. The signal is normalised to 1 pb for all mass points, while the backgrounds are normalised to the SM cross-sections [13].

Event selection and categorisation

The expected background composition for the six regions is shown in Figure 6.2. The yields of the t¯t + jets background are split into the three flavour subcategories: t¯t + light, t¯t + ≥1c

and t¯t + ≥1b. The t¯tW , t¯tZ and t¯tH processes are included in the t¯tX category, while all

other backgrounds are classified as non−t¯t. The dominant background in the control regions

is produced by t¯t + light events, while the 5j≥4b and ≥6j≥4b signal regions are dominated

by t¯t + ≥ 1b events. The 5j3b and ≥6j3b signal regions consist of a mixture of the three

categories: 50% of 5j3b is made of t¯t + light events, with equal amounts of t¯t + ≥ 1c and t¯t + ≥1b. The percentage of t¯t + HF events slightly increases in the ≥6j3b region, which is

almost evenly divided between the three flavour categories. The analysis is missing a region dominated by t¯t + ≥1c, which is the most difficult t¯t + jets category to control. Observed

and expected event yields for the six regions are presented in Figure 6.3. The signal event yields are shown for two mass hypotheses, at 200 and 800 GeV, normalised to 1 pb and superimposed to the distribution of the SM backgrounds. Data and predictions agree within the systematic uncertainties, however the SM background is slightly underestimated in the signal regions.

Figure 6.2: Background composition of control and signal regions. The t¯t + jets background

is divided in the three flavour categories t¯t + light, t¯t + ≥1c and t¯t + ≥1b. The t¯tX category

includes the t¯tZ, t¯tW and t¯tH processes. All other backgrounds are included in the non−t¯t

Search for ¯tbH+(H+→ t¯b)

Figure 6.3: Pre-fit comparison of the predicted and observed event yields in control and signal regions. Each background is normalised to the predicted SM cross-section. Two signal hypotheses (m_H+ = 200 GeV and m_H+ = 800 GeV) are shown as dashed lines, normalised

to 1 pb and superimposed to the background distribution. The lower panel shows the ratio between the observed yields and predicted yields for the SM backgrounds. The error bands include all systematic and statistical uncertainties.

Multivariate methods