Tagged Control Plots and Event Yields

6.3. Tagged Control Plots and Event Yields

The final number of events for both MC and data in both the e + jets and µ + jets channels after the event selection are found in Table 6.2. Control plots in Figure 6.4 and Figure 6.5 show the MC to data comparison after event selection. Several object prop-erties are plotted including pT, η and φ of the objects. Also included are the number of jets and number of b-tagged jets in each event. The uncertainties on MC expectation include the MC statistical uncertainty, 100 % QCD estimation uncertainty and systemat-ical uncertainties from the b-tagging scale factor uncertainties along with the scale factor uncertainties from reconstruction, trigger and identification of the leptons. These scale factors are applied to the MC, and as a result, their uncertainties are included in the MC expectation. In the case of theµ+ jets, the trigger was not applied directly to MC, instead the trigger efficiency was applied to objects based on p_T and η of the muon. The overall W + jets normalization is obtained from data. The overall agreement of MC to data is very good, however in both channels, the MC expectation is lower than the observed data.

The signal over background (S/B) for this event selection is found to be 2.6±1.2 in the µ + jets channel and 2.8±1.2 in the e + jets channel when considering only the tt¯ as signal. Since the single top event yield is dependent on the top mass, it is also used as signal in the templates. The S/B in this case is 3.3±1.8 in the µ+ jets channel and 3.6±1.9 in the e+ jets channel.

Channel with R

L dt = 1.04 fb⁻¹ µ + jets e + jets t¯t (mtop = 172.5 GeV) 6300±230 4250±160 Single top (mtop = 172.5 GeV) 360±14 260±10

W + jets 1300±920 840±590

Z + jets 140±17 110±13

Diboson 22 ±2 14 ±1

QCD Multijets 520±520 270±270

Signal + background 8600±1100 5700±700

Data 9114 5832

Table 6.2.: Observed numbers of events in data compared to the expected numbers of MC signal and different background channels for only the object selection cuts listed beforehand. The tighter object definition in the e+ jets channel lowers the number of events in comparison to theµ+ jets channel. The QCD multijets backgroundin both channels is estimated using the Matrix Method.

Errors for MC yields include the MC statistics and b-tagging event weight scale factor uncertainties. The data-driven estimates are obtained from the estimate itself, including 100 % for QCD multijets.

The overall agreement for object kinematics between model and data is very good. The inclusive jet p_T spectrum, which is of importance for top quark mass measurements is well described by the MC and data-driven estimates in both channels. Both the hard and soft ends of the p_T spectrum are well described in MC when compared to the observed data. The missing segment of the calorimeter for some of the data taking period is also well modeled in both the jet φ and electron φ. The overall normalization agrees in both channels.

6. Model to Data Comparison

Figure 6.4.: µ+ jets channel control plots. They include the number of jets, number of b-tags along with kinematics (pT,η andφ) of the jets and muons. The un-certainties include the MC statistics, scale factors and data-driven estimate uncertainties.

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6.3. Tagged Control Plots and Event Yields

Figure 6.5.: e + jets channel control plots. They include the number of jets and b-tags along with the kinematics (pT, η and φ) of the jets and electrons.

The uncertainties include the MC statistics, scale factors and data-driven estimate uncertainties.

6. Model to Data Comparison

The leading jet is expected to come on average from the hard process, containing the highest pT jet. As a result, it is expected that the pT and η of the leading jet must have a good agreement in the model when compared to data. The leading jet p_T and η spectrums for data are found in the subsequent Figure 6.6 and show an overall agreement of the model expectation.

Figure 6.6.: (Top left): Leading Jet pT spectrum for µ + jets and (top right): e + jets channels. (Bottom left): Leading Jet η spectrum for µ+ jets and (bottom right): e + jets channels. The agreement for both channels is very good showing a good description of the data by the MC used.

6.3.1. Pileup

All MC is re-weighted based on the difference of pileup in MC and data. Since the MC number of primary vertices does not match that of data, a re-weighting is necessary in MC. To check the output of this re-weighting, the number of primary vertices is plotted for both MC and data in Figure 6.7. The overall agreement is very good. The agreement of number of primary vertices is required as a first step to verify the effects of pileup are modeled properly in the MC. The top quark mass as a function of the number of primary vertices or position in the bunch train is checked in the systematics of the measurement

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6.3. Tagged Control Plots and Event Yields

and an appropriate error is associated to the outcome of this reweighting procedure.

Number of Primary Vertices After Reweight

2 4 6 8 10 12

Number of Primary Vertices After Reweight

2 4 6 8 10 12

Figure 6.7.: (Left): µ + jets channel and (right): e + jets channel plotted number of primary vertices in data and the reweighted number of primary vertices in MC. The reweighted MC shows very good agreement with the data period used for this analysis.

6.3.2. b-Tagging

The weight of the JetFitterCombNN on inclusive 4-jet events (after event selection with and without including at least one b-tag in this selection) is found in Figure 6.8. The large amount of events contain jets with a weight<0. A significant portion of these events are jets from background events. It can be clearly seen that after the b-tag requirement, a significant portion of the background events are discarded. Not only are a large amount of background events eliminated by the b-tag requirement, but also there are few background events remaining with large jet weights. Most of the events which contain at least one jet above the jet weight cut of 0.35 are signal. This can also be seen in Figure 6.4 and 6.5, in the top right corner where number of b-tags is shown. In the events with 1 b-tag, the majority of events are signal, however a large number of background events still remain.

For a much more pure signal sample, one could require 2 b-tags. The 2 b-tag requirement however, is not necessary in the lepton + jets channel since the background sample is also fit when making the data measurement and does not constitute a large uncertainty.

6. Model to Data Comparison

Figure 6.8.: (Left plots): µ + jets channel and (right plots): e + jets channel for the jet weight from the JetFitterCombNN tagger. The top plots show the jet weight of all jets after the event selection (without b-tagging). It can be seen that a very large portion of jets from background processes, such as W + jets, have a low Jet Weight. The cut at 0.35 diminishes this amount, as seen in the bottom two plots after it is required that at least one jet per event has a weight greater than 0.35.

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