Systematic Uncertainties Applied to the Leptons, Scale Factors and Cor-

11.7. Systematic Uncertainties Applied to the Leptons, Scale Factors and Corrections to the E 6

The final set of systematic uncertainties are those applied to the leptons, 6E_T, and b-tagging scale factors. The lepton resolution in MC is smeared to match that of data as previously stated in Section 4.1.5 and Section 4.1.8. The uncertainty on the lepton smearing is quantified as a source of systematic uncertainty. The next uncertainty is due to the scale factors applied to the b-tagging weight. The efficiency and rejection of the b-tagger contain an uncertainty. The last uncertainty is due to out-of cone corrections coming from energy depositions which are not located inside the reconstructed cone. These uncertainties are expected to have a minimal effect on the top mass estimation.

Lepton Smearing To match the resolution of the lepton pT (ET) in the µ + jets (e + jets) channels, the lepton is smeared in MC to match the resolution found in 2010 data. The overview of the resolution smearing is further explained in Section 4.1.5 and Section 4.1.8. In the electron case, the energy is smeared by about 3-4 %, with the largest uncertainty for low energy electrons, whereas for the muon it is the p_T components of the combined algorithm, including the inner detector and muon spectrometer. The full muon resolution for different detector components used to determine the smearing can be found in [160]. The resulting systematic uncertainty is minimal since the lepton is not used to estimate the top quark mass.

Lepton Energy Scale In addition to the smearing, the lepton energy offset in thee+ jets channel is measured from 2010 data in the Z →eepeak [120]. The scale factors are less than 2 % within |η|<2.5. The offset error is used to estimate the uncertainty due to the lepton energy scale. The systematic uncertainty has no visible effect on the top quark mass estimation.

b-tag Scale Factors b-tagging scale factors are applied to MC to accommodate for the difference in the b-tagging efficiency and mistag rates when compared to data. A scale-factor is given to each jet, along with its associated error. The scale factor depends on jet p_T, η, and flavour [161]. The b-tagging scale factor uncertainty is estimated by shifting in the uncertainty up and down in each jet, resulting in an event weight which is systematically shifted up or down. The combination of all event weights shifted up and down are combined into shifted systematic templates.

The largest difference, either up or down, in each channel is taken as the systematic uncertainty.

Out-of-Cone 6E_T Corrections Uncertainties due to the 6E_T arise from the corrections in the 6E_T. Soft jets with 7 < p_T < 20 GeV are used for the calculation of the 6E_T, however they are not used as physics objects. They are 100 % correlated with the uncertainty from the energy in the calorimeter not associated to objects used for the analysis. As a result, the systematic uncertainty is the addition of shifted soft jets and unassociated cells, defined as a cell out term, fluctuated up and down. The maximum difference to the nominal template is taken as the systematic. Both signal and background are fluctuated for this systematic. Since again the 6E_T is not used to estimate the top mass, its effect is minimal.

The results of the measurement on data as well as the quantified systematic uncertainties are found in the following chapter.

12. Top Mass Measurement Results

12.1. Mass Measurement on Data

Using the partial 2011 ATLAS dataset with an integrated luminosity of 1.04 fb⁻¹, the top mass measurement is performed. In the exact same manner as for the ensemble tests, the dataset is fit once using the likelihood in each channel separately. The fit results for the µ+ jets channel ande+ jets channel are found in Figures 12.1 and 12.2.

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Background Fraction [%]

Figure 12.1.: Results of the fit of the top mass measurement in the µ + jets channel.

(Top): Stacked signal and background templates created using the fit pa-rameters. They are compared to the collected data. The fitted top mass corresponds to a value and statistical uncertainty of 175.54^{+ 1.13}_{− 1.12}GeV/c². (Bottom left): Marginalized probability distribution onto the top mass axis. The probability density is shown to be almost symmetric along with the 68 central percentile highlighted. (Bottom right): 2-d probability den-sity of the fitted top mass and background distribution.

12. Top Mass Measurement Results

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Background Fraction [%]

Figure 12.2.: Results of the fit of the top mass measurement in the e + jets channel.

(Top): Stacked signal and background templates created using the fit pa-rameters. They are compared to the collected data. The fitted top mass corresponds to a value and statistical uncertainty of 172.93^{+ 1.50}_{− 1.41}GeV/c². (Bottom left): Marginalized probability distribution onto the top mass axis. The probability density is shown to be almost symmetric along with the 68 central percentile highlighted. (Bottom right): 2-d probability den-sity of the fitted top mass and background distribution.

The fits to data in both channels show results similar to those expected from the en-semble tests. In the µ + jets channel, the observed statistical uncertainty corresponds identically to the expected statistical uncertainty. The observed background fraction is within 1σof expectation and the fit yields a top mass of 175.5±1.1statGeV/c², containing a symmetric top mass probability density.

In the e + jets channel, the fit on data is also very close to the expected values pre-dicted by the ensemble tests. The median observed statistical uncertainty is 1.46 GeV/c², compared to the expected 1.35±0.14 GeV/c²; within the 1 σ expectation. The data fit is however slightly on the higher statistical uncertainty side of the distribution. The expected background fraction fit is also higher than expected, highlighted in the bottom right of Figure 12.2. The observed background is slightly over 2 σ from the expected.

The top mass probability distribution is less symmetric than the µ + jets channel, with a slightly higher uncertainty in the upwards direction than in the downwards direction:

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