SISTEMA DE CODIFICACIÓN DEL ANÁLISIS DE CONTENIDO

The efficiency of the LVL1-muon trigger, the LVL2-muon trigger and theEvent-Filter can be determined in two ways. The first method is to use a data sample which has been triggered by a muon independent trigger e.g. a jet- or an electron-trigger. In this sample it can be counted how often a reconstructed muon has also been triggered at different stages of the trigger menu. The second method is again the ’tag and probe’ technique.

86 CHAPTER 8. CROSS-SECTIONσ(PP_→Z/γ∗ _→µ+µ−) MEASUREMENT

Efficiency Statistic Uncertainty Systematic Uncertainties

|εtrue−εInsitu| |εInsitu,∆φ<2.8−εInsitu,∆φ>2.8|

0.9983 0.0003 0.0012 0.0020

Expected Background Contribution

bb¯ →µµ W± →µ±_{ν Z/γ}∗_{→ττ tt}¯_→W+_bW−_{b Overall}

¿0.002 _¿0.0004 0.0 0.0 _¿0.002

Table 8.8: Estimated Uncertainties of in-situ determined ATLAS Inner Tracker reconstruction effi- ciencies

This first approach is expected to be very useful for the low pT-regime of the muon-trigger,

since the reconstructed muons are mainly due to QCD-events and it has the additional advantage that it can be used without requiringZboson events. This has three consequences: First of all, the first method can be used in the very first phase of LHC, secondly it can be also used to determine di-muon trigger efficiency, while the ’tag and probe’ method can only be used for studying single (and therefore independent) muon triggers.

The trigger efficiency for the selected muons in this analysis depends on the properties of muon tracks, e.g. pT or isolation. Hence, the ’tag and probe’ approach is used for this

study, since it includes by construction all relevant efficiency-correlations due the kinematic and isolation-properties of the muons. The same procedure and cut-selection as described in section 8.1 is used also in this case.

The ’tag and probe’ has also the advantage, that the trigger-efficiency of all trigger stages can be determined independently from the muon reconstruction efficiency by using the same approach as described in section 8.3.3. For this study, it is only important to determine the trigger efficiency for muons, which have also been reconstructed, since two reconstructed muons are required in the analysis. It should be noted, that the background contributions can be neglected in this case, since two reconstructed muon tracks are required for the ’tag’ and ’probe’ muons. If, by chance, a muon track is used, which does not stem from aZboson, but fulfills all further cut requirements then also this muon can be used to probe the trigger efficiency.

The statistical uncertainty is driven by Equation (8.10). The difference in the predicted trigger efficiency for the two∆Φsub-samples is treated as further systematic uncertainty. Figure 8.35 shows the comparison between the efficiencies of both muon LVL1-Trigger and LVL2-Trigger determined by ’tag and probe’ and Monte Carlo Truth information8

. The inefficient regions are due to geometrical properties of the ATLAS detector such as support structures or holes, analogue to the the muon reconstruction efficiency which has been already described in section 5. No significant discrepancy can be observed. Using the ’tag and probe’ method also other correlated efficiencies, e.g. the efficiency of reconstructing a muon which has been triggered on or the efficiency of the Level 2 trigger with respect to Level 1 can be determined.

The trigger efficiency has a strong pT-dependence near its turn-on value, i.e. in this case

around 6GeV or 20GeV. It must be noted that the determination of this behavior might have a relatively large systematic uncertainty, since a lower pT-cut for the ’probe’ muon must

be used which will lead to an increase of the QCD-background contribution.

An overview of the statistical and systematic uncertainties of the determination of the trigger efficiencies is shown in Table 8.9. The systematic contributions are within the statistical

8

The LVL2-Trigger was only correctly implemented in the|η|<1.0-region in the available Monte Carlo samples.

8.3. IN-SITU DETERMINATION OF DETECTOR EFFICIENCIES 87 η -2 -1 0 1 2 Efficiency 0.6 0.7 0.8 0.9 1 1.1 η -2 -1 0 1 2 Efficiency 0.6 0.7 0.8 0.9 1 1.1 Insitu Determination Monte Carlo Truth

LVL 1 Muon Trigger φ 1 2 3 4 5 6 7 Efficiency 0.6 0.7 0.8 0.9 1 φ 1 2 3 4 5 6 7 Efficiency 0.6 0.7 0.8 0.9 1 Insitu Determination Monte Carlo Truth

LVL 1 Muon Trigger η -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 Efficiency 0.6 0.7 0.8 0.9 1 1.1 η -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 Efficiency 0.6 0.7 0.8 0.9 1 1.1 Insitu Determination Monte Carlo Truth LVL 2 Muon Trigger φ 1 2 3 4 5 6 Efficiency 0.6 0.7 0.8 0.9 1 φ 1 2 3 4 5 6 Efficiency 0.6 0.7 0.8 0.9 1 Insitu Determination Monte Carlo Truth LVL 2 Muon Trigger

Figure 8.35: Comparison of LVL1 and LVL2 muon trigger efficiencies vs. η and φ determined by ’tag and probe’ method and Monte Carlo Truth information.

Trigger Efficiency Statistical Systematic Uncertainties Uncertainty |εtrue−εInsitu| |ε∆φ<2.8−ε∆φ>2.8|

Level 1 0.8672 0.0015 0.003 0.003

Level 2 0.8422 0.0017 0.003 0.0025

Event Filter (0.77) (0.0023) (0.003) (0.0025)

Table 8.9: Estimated Uncertainties of in-situ determined Muon Spectrometer reconstruction efficien- cies.

uncertainties9

. For the study itself, only the precise knowledge of the overall trigger efficiency ε20GeV for20GeV muons with respect to the number of producedZ →µµ events is relevant,

i.e.

ε20GeV =ε20EventFilterGeV ·ε20LV LGeV2 ·ε20LV LGeV1

Hence it is sufficient to probe if a muon has passed theEventFilter, since this already implies that this muon has also passed the LVL1 and LVL2 trigger. In this way also the efficiencies of the previous trigger stages can be determined straight forward, which can be used as cross-checks for the Monte-Carlo prediction of the full ATLAS simulation.

Note that the current available Monte Carlo samples have a known problem for the Level 2 muon trigger information for the end-cap region of the Muon Spectrometer. The stated uncertainties are calculated only with respect to the barrel-region. TheEvent Filter informa- tion was not available so far at all. The efficiency distribution of the LVL1 muon trigger for

9

Note, that also the efficiency determined with Monte Carlo Truth information has a statistical uncertainty comparable to the ’tag and probe’ approach

88 CHAPTER 8. CROSS-SECTIONσ(PP_→Z/γ∗ _→µ+µ−) MEASUREMENT the η₋,φ₋plane was chosen and normalized to an overall Event Filter efficiency of 0.7710

, which is a conservative estimation. The corresponding systematic uncertainties were taken from the muon LVL2-trigger. With this assumptions, it is expected to determine the Event Filter trigger efficiencyεEV with50pb−1 to a precision of

∆εEV ≈ ±0.0023(stat)±0.003(sys).