Pre-selection Criteria and event yields

As mentioned in the beginning of this chapter, it is essential to apply pre-selection criteria to the simulation samples used in the analysis. The purpose of the pre-selection is to ensure that the quality of the objects used in the analysis (jets,leptons etc) is optimal, and depending on the process we are studying, reduce the corresponding background, while minimizing the number of signal events that are being rejected. To obtain good quality Particle Flow (PF) jets, a minimum p of 30 GeV/c is required. The lepton p cut is looser, requiring

jets. The rest pre-selection criteria ensure that the final state objects correspond to those of thet¯tH, H→b¯bdileptonic decay, which consists of four b-jets, two leptons and missing energy due to the presence of neutrinos. In order to identify jets that originate from b-quark decays, the CSVv2 b-tagging discriminator is used. There are three selection criteria which are often called working or operating points (WP), the loose (CSVv2>0.460), the medium (CSVv2>0.800) and the tight (CSVv2>0.935) WP. For the purpose of this analysis, the medium WP is used. The event pre-selection criteria are analytically listed below.

• #P F jets >3 and #leptons >1

For the four leadingpT jets and two leadingpT leptons :

• #b−tagged P F jets= 4

• b-tag at medium WP (>0.8)

• pT >30 GeV/c all PF jets

• pT >25 GeV/c leading lepton

• pT >15 GeV/c subleading lepton

• |η| <2.4 for leptons and≥2 jets

• M ET >40 GeV (in same flavour channels)

• m_ll>20 GeV (in same flavour channels)

• Z veto (76 GeV< mll <106 GeV ) (in same flavor channels)

5.2.1 Sanity check

In order to cross-check our code, ntuple production and resulting event yields we compare the resulting event yields against the HIG-16-038 analysis using the same pre-selection, listed in Figure 5.14 [34]. The events yields of the two analyses, as well as their ratio can be observed in Table 5.3.

Figure 5.1: Object and event selection criteria used by the HIG-16-038 analysis for the t¯tH, H→b¯bdileptonic decay channel.

Process Event yields HIG-16-038 analysis

Event yields Ratio

t¯t+jets,

all decays 1438 1676 0.86

ttH,¯

all decays 59 68 0.87

Table 5.3: Event yield comparison for the signal and main background using the same pre- selection scaled to 200f b⁻¹luminosity. The second column corresponds to the event yields calculated for this thesis when reproducing the HIG-16-038 analysis event selection, while the event yields of the third column, are taken from the HIG-16-038 analysis and scaled to 200f b⁻¹. The third column corresponds to the ratio of the second over the third column.

According to the last column of Table 5.3, we where able to reproduce the event yields of the HIG-16-038 analysis within a∼15% agreement. The∼15% difference appears due to us not yet applying the recommended DATA/MC scale factors, trigger and b-tagging efficiencies etc. After this brief sanity check, that ensured the validity of our code, ntuple production etc, we will proceed with extrapolating the event yields for a luminosity of 200f b⁻¹ using the optimized pre-selection discussed in section 5.2.

5.2.2 Event yields

After applying the event pre-selection described in section 5.2, the resulting event yields for Lumi= 200f b⁻¹ can be observed in Table 5.4 with respect to the signal samples and in Table 5.5 with respect to the background samples. In order to calculate the event yield of each process, we multiply the absolute number of events that pass the pre-selection with a corresponding weight that is defined as:

weight= Lumi×σprocess

NM C

(5.1) where Lumiis the luminosity we choose to scale the events,σ_process is the cross section of the process andN_{M C} corresponds to the number of events produced by theM C generator prior to applying any event cuts.

Signal Samples

Process Event yield

t¯tH, H→b¯b 50

t¯tH, H → non b¯b 2

Total Signal 52

Table 5.4: Event yield of signal samples using the pre-selection described in section 5.2. The

Background Samples

Process Event yield

t¯t+jets, all decays 1155 Z/γ^?+jets, 10GeV < M <50GeV 0

Z/γ^?+jets, M >50GeV 0

Single t 14

W +jets, W →lv 0

W W 0

W Z 0

ZZ 3

tt¯+W, W →lv 2

tt¯+W, W →qq 2

t¯t+Z, Z →qq 15

Total Background 1191

Table 5.5: Event yield of background samples using the pre-selection described in section 5.2. The event yields are scaled to 200f b⁻¹ luminosity.

According to the resulting yields forLumi= 200f b⁻¹, we expect 50 signal events compared to approximately 1200 background events. As expected, the most dominant background is thet¯t+jets process, which constitutes∼ 97% of the total background.

5.2.3 Higgs mass distributions

The next step after calculating the event yields is by using the model independent mass re- construction method, to reconstruct the mass of the two b jets, assigned to the Higgs boson for both the signal and background samples. Regarding the signal samples, we expect the m_b¯b distribution to be similar with that produced using generator level events, but with a smeared peak around 125 GeV instead of a delta function. On the other hand, regarding the background samples, we do not expect the m_b¯b distribution to form a peak around 125 GeV.

For each event that fulfils the event selection criteria, the four leading p_T jet four-vectors, two leading p_T lepton four-vectors as well as the missing energy in the x and y axes (M ET_x, M ET_y) are considered as inputs for the method. In Figure 5.2 we can observe the Higgs mass distribution for the two signal samples, weighted with their corresponding weights. As expected, the distribution of the t¯tH, H → b¯b process forms a peak around 125 GeV and is significantly larger that the distribution of the t¯tH, H → non b¯bprocess.

In Figure 5.4 we can observe the different m_b¯b distributions of the background samples.

The most significant contributions comes from thett¯+jetsprocess, which as expected has a different shape from the signal. Consequently, the following studies will be performed considering as background only thett¯+jetsprocess contribution.

Figure 5.2: Higgs mass distribution of thettH, H→b¯b(blue) andttH, H→non b¯b(green) processes, weighted with their corresponding weights.

Figure 5.3: m_b¯b distribution of the the processes that contribute as backgrounds weighted with their corresponding weights. We again observe that t¯t+jets background (black) is dominant.