4. Measurement of the Photon Identification Efficiency 83
4.6. Uncertainties
4.6.3. Photon Conversion Misreconstruction
where with ∆εreco convID =εtruth convreco conv −εtruth unconv
reco conv and ∆εreco unconv
ID =εtruth unconv
reco unconv−εtruth convreco unconv. As one can see, the identification efficiencies for photons that are reconstructed as converted (unconverted) depend on the purities freco convtruth conv(ftruth unconv
reco unconv). These purities are modeled in MC simulation with limited accuracy. In order to assess the systematic uncertainty that results from the mismodeling of the conversion reconstruction, the purities are varied and the resulting shower-shape transformations are used to compute alternative photon identification efficiencies.
The purities are varied by increasing and decreasing the fraction of truly converted photons among the photon sample. This is done by weighting events in which the considered photon is truly converted up and down by a factor 1.1 and 0.9, respectively. The relative difference between the efficiencies from the up- and down-variations of the conversion fractions at truth level, divided by 2, is shown in Figures 4.10. This systematic uncertainty is relatively small, being less than 0.3 % in all kinematic regions.
For unconverted photons, the uncertainty tends to be largest at low pT, except in the pseudo-rapidity region of 1.81<|η|<2.37. In order to understand this, it is helpful to consider again Eq. (4.12). The identification efficiency of photons that are reconstructed as unconverted depends on the purity ftruth unconv
reco unconv as well as on the difference in efficiency ∆εreco unconv
ID =εtruth unconv reco unconv− εtruth convreco unconv of the identification selection for unconverted photons between photons that are unconverted at truth level and photons that are converted at truth level. These efficiencies as well as the purity are shown in Figure 4.11. The purity is approximately constant over the considered pTrange. Except for the|η|region above 1.81, where the purity is about 70 %, the purity is of the order 90 %. The difference in efficiency, which determines how large the effect of purity variations is, is largest for low pT and considerable for high pT. At intermediate pT values, the two efficiency curves intersect; differences between the two efficiencies are small. As a consequence, purity variations for unconverted photons tend to have the largest effects at low pT, small effects at intermediate pT, and sizable effects at highpT.
In most kinematic regions, the uncertainty is smaller for converted photons than for uncon-verted photons. In Figure 4.12, the relevant efficiencies and purities are shown. The difference in efficiency between truth-converted and truth-unconverted photons that have been reconstructed as converted is relatively small, except in the|η|region above 1.81, where the efficiency difference is large in most pTbins. However, the purity tends to be close to 100 %, with a minimum value of about 80 % at low |η|and low pT. Therefore, changes in weights depending on the
truth-50 100 150 200 250 [GeV]
pT
0 0.05 0.1 0.15 0.2 0.25 [%] ε / ε∆ 0.3
Converted Unconverted = 13 TeV
s = 80 fb-1
Lint
|<0.60 η 0.00<|
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[GeV]
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0 0.05 0.1 0.15 0.2 0.25 [%] ε / ε∆ 0.3
Converted Unconverted = 13 TeV
s = 80 fb-1
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|<1.37 η 0.60<|
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[GeV]
pT
0 0.05 0.1 0.15 0.2 0.25 [%] ε / ε∆ 0.3
Converted Unconverted = 13 TeV
s = 80 fb-1
Lint
|<1.81 η 1.52<|
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[GeV]
pT
0 0.05 0.1 0.15 0.2 0.25 [%] ε / ε∆ 0.3
Converted Unconverted = 13 TeV
s = 80 fb-1
Lint
|<2.37 η 1.81<|
Figure 4.10.|Uncertainty contribution from the modeling of conversion reconstruction as a function of pTfor converted and unconverted photons, relative to the measured efficiencies. Each plot represents a different|η|region.
50 100 150 200 250 [GeV]
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50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Unconverted
|<0.60 η 0.00<|
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[GeV]
pT
70 80 90
truth unconv [%] freco unconv100
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Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Unconverted
|<1.37 η 0.60<|
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[GeV]
pT
70 80 90
truth unconv [%] freco unconv100
50 100 150 200 250
[GeV]
pT
50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Unconverted
|<1.81 η 1.52<|
50 100 150 200 250
[GeV]
pT
70 80 90
truth unconv [%] freco unconv100
50 100 150 200 250
[GeV]
pT
50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Unconverted
|<2.37 η 1.81<|
50 100 150 200 250
[GeV]
pT
70 80 90
truth unconv [%] freco unconv100
Figure 4.11. | Identification efficiencies for photons that are reconstructed as unconverted, based on MC simulation. Efficiencies are computed for photons that are unconverted at truth level as well as for photons that are converted at truth level. Moreover, the nominal efficiency is shown, based on the nominal sample of photons that are reconstructed as unconverted, consisting predominantly of photons that are unconverted at truth level. The fraction of truly unconverted photons that are reconstructed as unconverted corresponds to the purity, ftruth unconv
reco unconv, and is shown in the lower panels.
50 100 150 200 250 [GeV]
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50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Converted
|<0.60 η 0.00<|
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[GeV]
pT
70 80 90 100 [%] truth conv reco convf
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50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Converted
|<1.37 η 0.60<|
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[GeV]
pT
70 80 90 100 [%] truth conv reco convf
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Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Converted
|<1.81 η 1.52<|
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pT
70 80 90 100 [%] truth conv reco convf
50 100 150 200 250
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pT
50 60 70 80 90 100 [%]IDε
Nominal Truth conv.
Truth unconv
= 13 TeV s Simulation Converted
|<2.37 η 1.81<|
50 100 150 200 250
[GeV]
pT
70 80 90 100 [%] truth conv reco convf
Figure 4.12. |Identification efficiencies for photons that are reconstructed as converted, based on MC simulation. Efficiencies are computed for photons that are converted at truth level as well as for photons that are converted at truth level. Moreover, the nominal efficiency is shown, based on the nominal sample of photons that are reconstructed as converted, consisting predominantly of photons that are converted at truth level. The fraction of truly converted photons that are reconstructed as converted corresponds to the purity, freco convtruth conv, and is shown in the lower panel.
conversion information have little effects on the purity, leading to relatively low uncertainties from mismodeling of photon conversion reconstruction.