Efficiency estimates from Monte Carlo events

The simulated data used for the Z tag-and-probe study in release 16 is a 5 million event Z → ee egamma D3PD Pythia sample. Whether to apply truth matching and with what method, has a significant impact on the electron efficiencies derived from MC. If no truth matching is applied, hadrons from combinatorial background in the signal MC can fake electron probes, in particular at the denominator level.

These probes are much less likely to pass the identification cuts and will therefore contribute to a lower efficiency estimate.

Truth matching is applied through the MCTruthClassifier tool, which defines the match on a hit-by-hit basis from the track in the Inner Detector. A majority logic chooses the best matched generated track for any reconstructed track in the event.

From this information, one can classify the reconstructed electron tracks in simulated data in two ways:

• Direct truth matchingsimply includes electrons directly matched to the true primary electrons from the Z boson.

• Loose truth matchingis an extension of the direct truth matching through in-cluding electrons corresponding to electron tracks generated by bremsstrahlung photons or final-state-radiation (FSR) photons originating from the primary electron.

An example of a loosely truth matched event is when the electron undergoes an early hard bremsstrahlung and the photon converts in the first silicon layers in the ID, producing two additional tracks. This electron is of worse quality and has a lower probability of passing the identification cuts. The loose truth matching will therefore result in lower efficiencies than if one only includes directly truth matched electrons.

The same statement is valid for relaxing the opposite sign requirement, which rejects the charge mis-identified events in the efficiency calculations. In the event described above, the electron with charge opposite to that of the primary electron may be chosen as the best match to the EM calorimeter cluster. The reconstructed electron charge will then be mis-measured and have the same sign as the second electron from the Z boson.

Table 5.5 shows the identification efficiencies in MC without any truth matching and with loose and direct truth matching applied, with respect to container level as well as track quality cuts. The difference between applying direct and loose truth matching is in fact larger than the difference between loose and no truth matching.

The effect increases for tighter identification and is of course much reduced if the initial track quality cuts are applied at the denominator level.

Which truth matching should be applied in order to obtain the true electron identification efficiencies and subsequently the data/MC scale factors, is determined

Efficiency with respect no loose direct to container [%] truth match truth match truth match Loose 97.68 ±0.01 98.18 ± 0.01 98.19 ± 0.01 Medium 92.57 ±0.02 93.20 ± 0.02 94.83 ± 0.02 Tight 74.76 ±0.03 75.28 ± 0.03 77.05 ± 0.03

Efficiency with respect no loose direct

to track quality [%] truth match truth match truth match Loose 97.74 ±0.01 98.19 ± 0.01 98.19 ± 0.01 Medium 95.68 ±0.01 96.14 ± 0.01 98.18 ± 0.01 Tight 77.27 ±0.03 77.65 ± 0.03 77.15 ± 0.03

Table 5.5: Identification efficiencies in MC for loose, medium and tight with respect to container (top) and track quality (bottom), comparing no, loose and direct truth matching.

by whether the events rejected by the truth match are subtracted in data analysis as background. Applying loose truth matching is thus motivated if the signal events with an electron that has showered is considered as signal in data. If this is the case, it would then be incorrect to compare the data-driven efficiencies with the directly truth matched MC efficiencies.

To determine whether loose truth matching should be applied to the MC, the frac-tion of non-truth matched electrons and their origin was investigated. At numerator level less than 0.1% of all electrons at medium level are excluded in the loose truth match. Whether or not to apply truth matching at the numerator level therefore has no impact and the study is more interesting at the container level where the effect is larger. The fraction of probes at the container level that are not included in the loose truth match is shown in Figure 5.5 as function of electron E_T, after requiring opposite sign and 80< Mee<100 GeV. The overall effect is only 0.7±0.01%, but the fraction increases rapidly towards lowerET. If the opposite sign and 80< Mee<100 GeV requirements are relaxed, the effect increases from 0.7% to about 2%.

Investigating the origin of the probes at container level that are not included in the loose truth match shows that 59% are originating from hadrons, 24% are unknown, implying that the truth information was not kept in the truth tree, and 17% come from an electromagnetic process or conversion in the material of the detector that is not included in the loose truth matching.

A simple closure test is performed on MC in order to investigate whether the 0.7%

of the probes that are not included in the loose truth matching would be subtracted as background on data. The OS-SS sideband background subtraction method described above is thus applied to the signal MC at container level. The method finds back 0.8±0.01% as background, which is compatible with non truth matched fraction.

[GeV]

ET

20 30 40 50 60 70 80

Non truth matched fraction [%]

0 1 2 3 4 5

ee MC probes Z→

Figure 5.5: The fraction of non truth matched probes in signal MC for Z → ee tag-and-probe container level as a function of probe ET.

Applying loose truth matching to the MC in order to extract electron efficiencies is therefore fully justified.

Table 5.6 and 5.7 display the loose, medium and tight identification efficiencies with respect to track quality cuts in bins of η and ET respectively. Loose truth matching has been applied and the probes are within 20 < ET < 50 GeV. These values are then to be compared with the data-driven efficiencies in order to produce data/MC scale factors in the equivalent binning.

η -2.47,-2.01 -2.01,-1.52 -1.52,-1.37 -1.37,-0.8 -0.8,0 Loose 98.39 ± 0.04 98.85 ±0.03 94.94 ± 0.10 98.36 ± 0.03 98.11 ± 0.02 Medium 97.02 ± 0.05 96.99 ±0.04 94.36 ± 0.10 96.85 ± 0.03 94.90 ± 0.04 Tight 73.89 ± 0.13 73.23 ±0.11 62.27 ± 0.22 75.76 ± 0.09 82.72 ± 0.06

η 0,0.8 0.8,1.37 1.37,1.52 1.52,2.01 2.01,2.47

Loose 98.14 ± 0.02 98.47 ±0.02 94.77 ± 0.10 98.88 ± 0.03 98.51 ± 0.04 Medium 95.69 ± 0.03 96.94 ±0.03 94.28 ± 0.11 97.04 ± 0.04 97.14 ± 0.05 Tight 83.73 ± 0.06 76.03 ±0.08 62.22 ± 0.22 73.33 ± 0.11 73.61 ± 0.14 Table 5.6: Identification efficiencies from MC with respect to track quality as a func-tion ofηmeasured for probes that are loosely truth matched and within 20< ET <50 GeV.

E_T 20-25 GeV 25-30 GeV 30-35 GeV 35-40 GeV 40-45 GeV Loose 96.56 ±0.05 97.48 ± 0.04 97.99 ± 0.03 98.40 ± 0.02 98.86 ± 0.01 Medium 94.38 ±0.07 95.54 ± 0.05 95.85 ± 0.04 96.41 ± 0.03 96.56 ± 0.03 Tight 71.94 ±0.13 74.54 ± 0.10 77.01 ± 0.08 78.63 ± 0.06 80.25 ± 0.06 E_T 45-50 GeV 50-80 GeV

Loose 99.11 ±0.02 99.11 ± 0.02 Medium 96.98 ±0.03 97.13 ± 0.03 Tight 91.72 ±0.07 82.93 ± 0.08

Table 5.7: Identification efficiencies from MC with respect to track quality as a func-tion of ET measured for probes that are loosely truth matched. The crack region in

|η| is excluded.