8. Statistical Analysis and Results 109
8.4. Combined Results
8.4. Combined Results
The OS dilepton t¯tV channel is combined with the three-lepton, or trilepton, final state, and with the final state with two leptons with same-sign charge, or SS dilepton. In the SS dilepton channel, only the di-muon events are used, motivated by a negligible contribution of background processes where the charge of one of the leptons is misidentified. The estimation of the charge lepton misidentification in electrons is difficult and non-negligible, and as a result, for the purposes of the conference note, the ee and eµ channels were not included in the SS dilepton result.
Table 9.1 summarises the characteristics of the trilepton and SS di-muon final states, com-paring them with those from the OS dilepton channel. The trilepton final state is split into two regions, 3`Z and 3`Zveto, similarly to the OS dilepton channel, based on the invariant mass of the same-flavour opposite-sign lepton pair, so that each region has sensitivity to t¯tZ or t¯tW, respectively. The main t¯tV decay modes contributing to the 3`Z region are: t¯t decaying into lepton+jets, and theZ-boson decaying into two leptons. For the 3`Zveto region,t¯tdecays pre-dominantly into a dilepton final state, and the W-boson decays leptonically. The SS di-muon channel is sensitive tot¯tW, since the main decay mode contributing to that selection criteria is t¯tdecaying into lepton+jets, and the W-boson decaying leptonically, so that both leptons have same-sign charge.
The main difference in the analysis strategy of the trilepton and SS di-muon channels, com-pared to the OS dilepton channel, is that the former have comparable signal and background contributions, so that the S/B is large enough to perform a counting experiment. Therefore, the OS dilepton channel is the only multivariate analysis, and the fit regions provided by the trilepton and SS di-muon channels contain only event counting information. No “control re-gions” from the trilepton and SS di-muon channels are included in the fit, though they are used to estimate the largest background contributions.
Trilepton Same-sign Opposite-sign di-muon dilepton Channels 3`Z 3`Zveto 2µSS 2`OSZveto 2`OSZ
Signal t¯tZ t¯tW t¯tW t¯tZ andttW¯ t¯tZ Main Bkgds tZ, W Z, ttZ¯ ,t¯tH t¯t+jets Z+jets
lepton misID
Fit regions 3 1 1 3 3
Table 8.9.: Overview of the final state channels included in the combination of the t¯tV mea-surements at√
s= 8 TeV. The suffices “Z” and “Zveto” refer to the lepton invari-ant mass requirement |m``−mZ| < 10 GeV and |m``−mZ| > 10 GeV, respec-tively, corresponding in the trilepton case, to the invariant mass of the same-flavour opposite-sign lepton pair.
A simultaneous fit to the data is performed under the signal plus background hypothesis using the distributions of the discriminating variable in each of the six fit regions in the OS dilepton channel, and the event counts from the one fit region in the SS dilepton channel and the four fit regions in the trilepton channel. TheRooStatsproject [198] allows to perform the combination of fit models easily by combining the correspondingworkspaces, where the data and the fit model (processes, fit regions, fit bins) is stored.
Before performing the combination, the list of systematic uncertainties that are correlated
8. Statistical Analysis and Results
across channels is specified, leaving the rest uncorrelated across channels.
All uncertainties grouped as “Physics Objects”-related in Table 8.2 are correlated across chan-nels, except for the systematics related to theb, cand light-tagging efficiency, since the trilepton and SS di-muon channel use the corresponding envelope uncertainty 6. From the uncertain-ties grouped as “Background Modelling”, only the t¯tH and tZ cross section uncertainties are correlated. The reason for this is the different background composition in each channel, and the different treatment of them within each analysis. All “Signal Modelling” uncertainties are correlated. The signal PDF uncertainty was not included in the combined fit.
Table 8.10 summarises the observed signal strength µwith respect to the NLO QCD predic-tion, and the corresponding observed and expected significance,σ, for the three 1-parameter-of-interest measurements presented in Section 8.3.1.
1-parameter-of-interest combined fit results Process Signal Strength Observed σ Expectedσ
t¯tV 0.89+0.23−0.22 4.9 4.9
ttW¯ 1.25+0.57−0.48 3.1 2.4
t¯tZ 0.73+0.29−0.26 3.2 3.8
Table 8.10.: The observed signal strength relative to the SM prediction and its total uncer-tainty, and the observed and expected significance of thettV¯ ,t¯tW andt¯tZ signals for the combination of all channels. The combined t¯tV result assumes SM ratio oft¯tZ tottW¯ cross sections, thet¯tW result assumes SMt¯tZ production rate and thet¯tZ result assumes SMttW¯ production rate.
Table 8.11 provides a breakdown of the total uncertainty on the measured signal strengths of thettZ¯ and t¯tW processes, for the 1-parameter-of-interest fit. In both cases, the measurement is limited by the statistical uncertainty, followed by the uncertainties due to modelling of the detector effects.
The result of the simultaneous fit of the signal strengths of both ttZ¯ and t¯tW processes compared to the NLO QCD expectation, is shown in Figure 8.24. The results are shown for the fit to all channels combined in black, for the fit to the OS dilepton channel in magenta, and for the fit to the trilepton and SS di-muon channels in red. The NLO QCD expectation is shown with a blue cross. The solid and dashed curves show the 68% and 95% CL uncertainty contours.
The overlaid contours demonstrate the relative sensitivity to t¯tZ and ttW¯ production for each individual measurement.
The results from the fit shown in Figure 8.24 for the individual channels and their combination are summarised in Table 8.12. The observed and expected significances are calculated with respect to a no-ttZ¯ and no-ttW¯ background hypothesis.
Table 8.13 shows the fitted signal strengths translated in measured t¯tZ and ttW¯ cross sec-tions, and their uncertainties, split into the statistical and systematic contribusec-tions, as well as the observed and expected signal significances for t¯tZ and t¯tW separately, from the simulta-neous combined fit. The significances for each signal process are calculated assuming the null hypothesis for one of them and treating the other as a free parameter in the fit.
6Since these channels do not rely on shape information in each fit region, there is no need to use the breakdown in sub-components of these systematics.
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8.4. Combined Results
Uncertainty µt¯tZ µt¯tW
Detector 0.07 0.13
Background from simulation 0.08 0.09
MisID lepton 0.03 0.09
Signal modelling <0.01 < 0.01
Total systematics 0.10 0.24
Statistics +0.26−0.24 +0.49−0.43
Total +0.29−0.26 +0.57−0.48
Table 8.11.: Breakdown of uncertainties on the measured signal strength ofttZ¯ andttW¯ pro-cesses from the corresponding 1-parameter-of-interest fits. Systematic uncertain-ties are symmetrised.
W) t
SM (t / σ σ
-4 -2 0 2 4 6 8
Z)t (tSMσ / σ
-1 0 1 2 3 4
5 ATLAS Preliminary ATLAS Best Fit Trilepton+SS Best Fit OS Dilepton Best Fit ATLAS 68% CL ATLAS 95% CL NLO calculation*
Z Theory uncertainty t
t
W Theory uncertainty t
t
L dt = 20.3 fb-1
∫
= 8 TeV s
* Campbell(2012),Kardos(2011),Garzelli(2011,2012)
Figure 8.24.: The result of the combined simultaneous fit of thettZ¯ andt¯tW signal strengths along with the 68% CL and 95% CL uncertainty contours compared to the fit results in the OS dilepton and trilepton plus SS di-muon channels. The χ2 quantile for two degrees of freedom is used to define the likelihood contours. The dashed area corresponds to the 22% uncertainty on the NLO QCD theoretical calculations ofσ(t¯tZ) andσ(t¯tW).
The measured signal strengths and significances in the simultaneous fit are close to those obtained from the fits using 1-parameter-of-interest, due to the very small correlation between thet¯tZ and t¯tW measurements, as seen in Figure 8.24. In both types of measurements, with 1 or 2 parameters-of-interest in the fit, thettZ¯ and t¯tW processes are observed with significance larger than 3 standard deviations from the background-only hypothesis, providingevidencefor both t¯tZ and t¯tW production for the first time with the ATLAS experiment.
8. Statistical Analysis and Results
Channel µttZ¯ µt¯tW Observed σ Expectedσ
Trilepton and SS di-muon 0.70+0.30−0.28 1.37+0.62−0.51 4.1 4.1
OS dilepton 0.77±0.65 0.71±2.41 0.4 0.6
Combination 0.71+0.28−0.26 1.30+0.59−0.48 4.4 4.4
Table 8.12.: The observed signal strength fort¯tZ andttW¯ production from the simultaneous fit of two parameters of interest, and the observed and expected significance of the signals for each individual channel and the combination. The signal significance is calculated with respect to a no-t¯tZ and no-t¯tW background hypothesis. The result for thet¯tW (t¯tZ) signal strength is obtained treating thettZ¯ (t¯tW) signal strength as a nuisance parameter.
Summary of combined simultaneous fit results
Process Measured cross-sections Observed σ Expectedσ t¯tZ 150+58−54(total) = 150+55−50(stat.)±21(syst.) fb 3.1 3.7 t¯tW 300+140−110(total) = 300+120−100(stat.)+70−40(syst.) fb 3.1 2.3
Table 8.13.: The measured cross-sections and total uncertainty, and the observed and expected significance of the individualttW¯ and t¯tZ signals from the simultaneous fit of 2-parameters-of-interest for the combination of all channels. The significances for each signal process is calculated assuming the null hypothesis for one of them and treating the other as a free parameter in the fit.
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