Official e/γ ID efficiency scale factors

The final medium and tight identification efficiency scale factors provided by the e/γ group are a combination of Z and W tag-and-probe results⁴. A brief description of the Z tag-and-probe finalη dependent results and the combination withW tag-and-probe is presented here. For more detailed information, consult [57, 81].

4Loose identification is excluded since it is not used by any of the physics analyses.

end-cap C barrel η -2.47,-2.01 -2.01,-1.52 -1.52,-1.37 -1.37,-0.8 -0.8,0

D-H probes 400 581 200 900 1390

D-H signal 366 ± 22 540 ±26 181 ±15 818 ±33 1277 ±40

I probes 510 660 209 1116 1886

I signal 449 ± 25 598 ±28 181 ±16 1006 ± 37 1730 ±47

barrel end-cap A

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

D-H probes 1529 1011 161 553 368

D-H signal 1411 ±42 925 ±34 141 ±14 507 ±25 327 ± 21

I probes 2042 1244 206 671 480

I signal 1870 ±49 1126± 38 186 ±16 609 ±28 436 ± 24 Table 5.11: The number of probes before and after background subtraction at track quality level (efficiency denominator) for period D-H and I separately, in the different η bins. In period D-H, the number of probes in the positive end-cap is significantly larger than in the negative end-cap leading to an end-cap asymmetry. This effect is greatly reduced for period I.

Z tag-and-probe variations

To assure stability of the method against for instance the choice of signal and back-ground regions for the OS-SS sideband method, several variations of the method is performed to assess the systematic uncertainty as well as the central value. Not only the signal and background bands are altered, but also the requirement on the tag, which affects the overall level of background. All possible combinations of the following variations are used, resulting in 54 combinations.

• Three variations of the signal region: [80,100] GeV (default), [75,105] GeV and [85,95] GeV.

• Three variations of the background sidebands: [60,120] GeV (default), [58,122]

GeV and [62,118] GeV.

• Six different tag requirements: tight (default), tight + isolation, medium, medium + isolation, medium + isolation + b-layer hit, medium + b-layer + E/p.

As the central value of the scale factors, the mean of the distribution from the different combinations is used. The spread of the distribution is taken as the sys-tematic uncertainty of the measurement. This method is performed for every η bin and yields systematic uncertainties of the order of 1-2% for all bins outside the crack

regions, for both medium and tight ID. It is important to stress that applying these variations to acquire the central value is only valid for the efficiency scale factors, since the efficiencies themselves have a slight dependence on the invariant mass.

Additionally, other possible sources of systematic uncertainty are considered. The effect of pile-up has been found miniscule, as was mentioned in section 5.2.1. Another uncertainty is related to the energy scale, since the energy scaling and smearing is applied in the physics analyses (see section 6.3.5) where the SFs are to be included, while this is not the case in the Z tag-and-probe methodology. It has therefore been verified that the energy corrections does not have any significant impact on the efficiency scale factors and is well within the quoted systematic uncertainties.

An additional systematic uncertainty arises from the fact that all tags and probes in the event are chosen to enter the efficiency calculation, even though there are only two electrons originating from the Z boson. An alternative method is to choose the two electrons in the event with the invariant mass closest to the mass of the Z.

This has been found not to have any large effect on the efficiencies, as is shown in Figure 5.10. The plot also includes the method of choosing the two electrons with the highestET as the tag and probe pair. This method has been found to frequently select the background electron and exclude the signal electron from the Z in the efficiency calculation. This leads to lower efficiencies and larger uncertainties as is seen in Figure 5.10 and should not be included as an input for the measurement.

η

-2 -1 0 1 2

isEM efficiency

0.6 0.7 0.8 0.9 1

medium tight

all probes best mass highest ET

Figure 5.10: Medium and tight identification efficiencies in data from Z tag-and-probe, using all tags and probes in the event in blue (default), choosing the tag and probe pair with the invariant mass closest to the Z mass in red and choosing the two highest ET electrons in green.

Finally, an additional systematic uncertainty deduced from a closure test per-formed on background and signal Monte Carlo is considered [81]. This uncertainty is added linearly to the systematic uncertainty derived from the variations of the method as described above. Since the worst case scenario from the closure test is used, this increases the total systematic uncertainty of the η dependent scale factors to about 3-4%.

Combination with W tag-and-probe

The medium and tight identification efficiency scale factors used to correct the sim-ulated data in physics analyses is a combination of Z and W tag-and-probe results.

Note that the efficiencies themselves are more difficult to combine due to slight intrin-sic differences between W → eν and Z → ee events. The resulting scale factors as a function ofη, for both methods, with their statistical and systematic uncertainties are shown in Figure 5.11. The Z tag-and-probe scale factors show a clear trend of higher values of around one percent compared to the W tag-and-probe scale factors. One reason for this could be that the W measurement includes the charge mis-identified electrons, while this is not the case for theZ due to the opposite charge requirement.

As was discussed above, the charge mis-identified electrons are of lower quality and hence yield lower identification efficiency values. This is seen in both data and MC, but leads to additional uncertainties from the possibility of having a larger amount of charge mis-identified electrons in data than in MC, resulting in lower scale factors.

(a) ^{-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 η2.5}

Scale factors for identification efficiency

0.85

ee medium 20-50 GeV Z →

Scale factors for identification efficiency

0.85

Figure 5.11: Resulting W and Z tag-and-probe efficiency data/MC scale factors for (a) medium and (b) tight identification. The scale factors are obtained with probes with 20-50 GeV transverse energy [81].

To combine the results, theW andZ scale factors are weighted by their individual uncertainties and the total uncertainty is propagated accordingly. For the η bin [1.52,2.01], the estimated combined uncertainty is not compatible with the separate

W and Z scale factors and an additional uncertainty of half the difference between the two measurements is applied. The final combined η dependent scale factors for medium and tight identification are shown in Figure 5.12, for both the typical W/Z energy range of 20 < ET < 50 GeV as well as the “plateau” energy range of 30< ET <50 GeV to be used by higher energy electron analyses.

(a) ^η

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Scale factors for identification efficiency

0.92

Scale factors for identification efficiency

0.95

Figure 5.12: Final data/MC efficiency scale factors for (a) medium and (b) tight identification. The results are acquired from combining scale factors from W and Z tag-and-probe, with probes in the transverse energy range of 20-50 GeV (red) as well as 30-50 GeV (blue) [81].