Systematic Uncertainties

6.5 Systematic Uncertainties

Numerous factors contribute to systematic uncertainties in the cross section measurement. This section describes the various factors, estimates of their uncertainty and the resulting uncertainty on the final measurement. Many of these uncertainties are based on numerous studies performed by various members of the DØ collaboration. We take a conservative approach by assuming all systematic uncertainties to be completely uncorrelated.

The considered systematics include: • Data quality: the data quality correction factor has been previously measured to be 0.97 ±

0.005 [51], therefore a ±0.5% uncertainty is evaluated for all MC samples. • Primary vertex selection: the uncertainty in primary vertex selection between data and

MC is taken from [64]. A value of ±3.0% is assigned as systematic error for this. • Vertex z simulation: the uncertainty due to difference in z vertex simulation from data is

taken from [64]. A value of ±2.2% is assigned as systematic error for this. • MC luminosity profile: this uncertainty arises from differences in the luminosity profile of

the zerobias overlay events used in MC compared to the data sample. The value is ±0.5% for Monte Carlo t¯ t sample and ±2.0% for other Monte Carlo samples [64].

• Muon identification: the systematic uncertainty on muon identification are estimated to

be ±0.7% in the muon certification documentation [33]. • Muon track: the systematic errors on muon track reconstruction are estimated to be ±0.7%

in the muon certification documentation [33]. • Muon isolation: we use an estimate from the single top analysis, which is ±2.0%.

• Muon trigger: the uncertainty on muon trigger efficiency is taken from the lepton+jets analysis [65], by calculating the percent systematic error listed in Table 39. The value is

found to be very close to zero, and we decided to assign a zero value. • Jet trigger: the uncertainty on jet triggers in the µ+jets triggers are taken from the

lepton+jets analysis [65]. The value is ±0.3%. • Tau reconstruction: the uncertainty in the data/MC agreement with respect to tau

reconstruction is taken from the H → ττ analysis [66]. The assigned value is ±3%. • Jet-tau fake rate: the data/MC correction factor for jets faking taus was measured to be

1.04±0.08 as described in Section 5.4.1. An 8% uncertainty is applied to the W , Z → µµ(ee), and t¯ t → ℓ + jets.

• K-factor: we assign an uncertainty of 0.1 on the K factor for both W (k = 1.6, Sec. 5.3.3) and Z (k = 1.23, Sec. 5.5.2) by comparison with data.

• HF K-factor: we use the uncertainties described in Sec. 5.2.2 scaled to the appropriate fraction of events that contain heavy flavor.

• Tag rate function: this uncertainty is evaluated by shifting the value of the tagging probability (Sec. 3.6.2) for each jet by (±1σ) from the central value of the tag rate function.

• Jet energy scale: this uncertainty is evaluated by shifting the jet energy scale in the JSSR processor by (±1σ).

• Jet energy resolution: this uncertainty is evaluated by shifting the jet energy correction by (±1σ).

• NLO t¯t cross-section: this uncertainty is evaluated by shifting the NLO t¯t cross-section by (±1σ). This uncertainty is only considered for measurement of σ (p¯ p → t¯t)·BR t¯t → µτ h b¯b

• Background Statistics: an uncorrelated combination of the statistical error on the Monte Carlo and same-sign data contributions to the background (for the individual component

statistical errors see Tables 6.7). Table 6.8 lists the various sources of systematic uncertainties and their contributions to the

uncertainty on the cross-section and σ × BR. The values and uncertainties for the cross-section and σ × BR after inclusion of systematic uncertainties are:

σ(t¯ t) = 8.0 +2.8 − 2.4 (stat) +1.8 − 1.7 (syst) ± 0.5 (lumi) pb (6.9) σ(t¯

t) × BR(t¯t → µ + τ + 2ν + 2b) = 0.18 +0.13 − 0.11 (stat) +0.09 − 0.09 (syst) ± 0.01 (lumi) pb. (6.10)