Event yields

E T

γ [GeV]

20 40 60 80 100 120 140

/ binγ

0 0.05 0.1 0.15 0.2 0.25

simulation (true photons) tγ

t

ATLASwork in progress

Figure 6.3: Distribution of the true photon ET in t¯tγ simulations. The last bin includes the overflow bin.

chosen as low as possible, because the distribution of the photon E_T int¯tγ events is dropping strongly towards high values ofE_T, as shown in Fig. 6.3.

In order to suppress Z+jets events with one of the electrons misidentified as a photon, the invariant mass of the electron and the photon in the electron channel was required to be outside a window of 5 GeV around the Z boson mass: m(eγ)∈/ [86 GeV,96 GeV]. As discussed in Sec. 5.6, the distance in η-φ-space between good photons and the closest jet had to be larger than 0.5.

Fig. 6.4 and 6.5 show the photon E_T, η and the fraction of non-converted and converted photons of thet¯tγ event candidates in the electron and muon channel, respectively. The amount of realt¯tγ events within these candidates was estimated from a template fit to thep^cone20_T distri-bution as described in Sec. 7. This estimate is data-driven, because simulations are not trusted for the description of the background rates. Consequently, no expectations from simulations are shown in these plots.

It is worth pointing out that the t¯tγ candidates contain a sizable fraction from hadrons misidentified as photons. However, the E_T spectrum of the photon candidates drops towards large transverse momenta, as expected for true photons as well as for fake photons. More photon candidates are found in the very central part of the detector than at larger values of |η|, and photon candidates are non-converted and converted to approximately equal parts.

6.3 Event yields

Tab. 6.1 shows the event yields for the preselection for data and the expectations from MC simulations as well as the estimation of the multijet background from data in the electron and muon channels, respectively. The yields before and after requiring at least one b-tagged jet in the event selection are shown.

For the uncertainties on the expectation of the different processes, only the uncertainty on the cross section calculations were taken into account for ttγ¯ ,t¯t, single top and diboson production as mentioned in Ch. 4. The procedure for the estimation of the multijet background is described in Sec. 11.2. The uncertainty on the multijet background was estimated conservatively to 100%

if at least oneb-tagged jet was required in the events, and to 50% otherwise.

Additional systematic uncertainties on the expectations were not taken into account, because this table is only intended to illustrate that the yields observed in data are well under control

6 Event selection

Figure 6.4: Photon distributions for the t¯tγ event candidates in data in the single electron channel. Shown are the photonET and η, and the fraction of non-converted and converted photons. In theE_T distribution, the last bin includes the overflow bin.

[GeV]

Figure 6.5: Photon distributions for thettγ¯ event candidates in data in the single muon chan-nel. Shown are the photon E_T and η, and the fraction of non-converted and converted photons. In theET distribution, the last bin includes the overflow bin.

for the preselection. The yields observed in data are roughly 100 events larger than expected in both lepton channels after the full preselection, which is well covered by the uncertainty on the expectation. Overall, the yields observed in data are consistent with the sum of the expectations within the considered uncertainties before and after theb-tagging requirement.

Tab. 6.2 shows the event yields for data and the expectations for signal and the different background sources for the final event selection. Thet¯tγsignal expectation from the WHIZARD MC simulation yields 22±4 and 28±6 events in the electron and muon channels, respectively.

This corresponds to a combined efficiency and acceptance of 0.97% and 1.27% with respect to the total generated signal. The efficiency of the final event selection fort¯tγ events was found to be roughly 20% with respect to the preselection (cf. Tab. 6.1 and 6.2).

The yields for t¯tγ events outside of the signal phase space, t¯tγ background as described in Sec. 4.2, are discussed in Sec. 11.1. The contributions from W+jets+γ, multijet+γ,Z+jets+γ,

6.3 Event yields

single top+γ and diboson+γ production are discussed in Sec. 11.2 – 11.4. The contributions from electrons misidentified as photons in dileptonic t¯t decays, Z+jets, single top and diboson events was estimated by measuring the misidentification rate in data (Ch. 10).

Hadrons misidentified as photons can occur in all of the background processes by the presence of additional jets. However, a prediction for the yield cannot be obtained from simulations as discussed in Sec. 4.2. In order to estimate the number of hadrons misidentified as photons in the selected data events, a template fit to the p^cone20_T distribution was performed (Ch. 7). The final result of the fit including systematic uncertainties is presented in Ch. 13.

As a cross-check for the final fit result, the amount of fake photons from hadrons was estimated from photon candidates withp^cone20_T >3 GeV. While only 2% of true photons havep^cone20_T values larger than 3 GeV, this holds true for 42% of the hadron fakes. Hence, p^cone20_T >3 GeV defines a control region (CR) largely dominated by hadron fakes.

[GeV]

Figure 6.6: Template fits to thep^cone20_T distribution for photon candidates withp^cone20_T >3 GeV in the electron (left) and the muon channel (right) with templates for true photons and for hadrons misidentified as photons. The data distribution is compatible with being only due to hadron fakes. In both plots, the last bin includes the overflow bin.

Fig. 6.6 shows template fits, as described in Ch. 7, to the p^cone20_T distribution for photon candidates in the CR in the electron (left) and the muon channel (right). Templates for true photons and for hadrons misidentified as photons were used. The data distribution is compatible with being only due to hadron fakes.

The fits yield 10±3 and 9±3 hadron fake events in the CR in the electron and muon channel, respectively, which translates into expectations of 24±8 and 22±7 events for the wholep^cone20_T spectrum. The uncertainty quoted is the statistical uncertainty from the fit only. The numbers from the final template fit (cf. Ch. 13) read 20⁺⁷₋₆(stat.)±3 (syst.) and 27⁺⁸₋₇(stat.)±4 (syst.) in the electron and muon channel, respectively. The final estimates in both channels are consistent with the cross-check in the regionp^cone20_T >3 GeV within the statistical uncertainties. However, it needs to be pointed out that the statistical uncertainties of the final and the cross-check estimates are partly correlated. The final estimate is believed to be closer to the real hadron fake contribution, because it takes into account more information from the wholep^cone20_T spectrum.

The uncertainties on the other background contributions quoted in Tab. 6.2 are the total uncertainties including statistical and systematic uncertainties as derived in Ch. 10 and Ch. 11.

In data, 52 and 70 events in the electron and muon channels were identified, respectively. The sum of the signal and background expectations yield 60±10 and 67±10. Hence, the number

6 Event selection

of observed events in data is compatible with the expectation within the uncertainties in both lepton channels.

e+jets µ+jets

beforeb-tag after b-tag before b-tag after b-tag

t¯tγ 108 ± 21 95 ± 19 144 ± 28 126 ± 25

t¯t 4710 ± 460 4160 ± 400 6840 ± 660 6040 ± 590

W+jets 5600 ± 2700 890 ± 460 10000 ± 4900 1600 ± 800

Z+jets 630 ± 300 99 ± 48 860 ± 410 122 ± 59

Single top 394 ± 17 265 ± 13 552 ± 22 360 ± 17

Diboson 79 ± 4 13 ± 1 125 ± 6 22 ± 1

Multijet 790 ± 390 150 ± 150 1600 ± 800 490 ± 490

Sum 12300 ± 2800 5670 ± 630 20200 ± 5000 8760 ± 1100

Data 11856 5761 18978 8863

Table 6.1: Event yields for data and expectations for signal and the different background con-tributions for 1.04 fb⁻¹ for the preselection in both lepton channels.

e+jets µ+jets

t¯tγ 22 ± 4 28 ± 6

Background t¯tγ 0.8 ⁺₋ ⁺¹₋₀^.1+_.8− 1.3 ⁺₋ ⁺¹₋₁^.9+_.3−

W+jets+γ 1.8 ± 0.7 3.7 ± 1.4

Z+jets+γ 1.3 ⁺₋ ⁺²₋₁^.4+_.3− 1.6 ⁺₋ ⁺²₋₁^.3+_.6−

Single top+γ 0.6 ⁺₋ ⁺⁰₋₀^.7+_.6− 0.2 ⁺₋ ⁺⁰₋₀^.3+_.2−

Diboson+γ 0.16 ⁺₋ ⁺⁰₋₀^.34+_.16− 0.04 ⁺₋ ⁺⁰₋₀^.18+_.04−

Multijet+γ 1.2 ⁺₋ ⁺¹₋₁^.6+_.2− 0.3 ⁺₋ ⁺¹₋₀^.0+_.3−

Dileptonic t¯t(e→γ) 6.8 ± 2.3 9.6 ± 2.7

Z+jets (e→γ) 1.7 ⁺₋ ⁺³₋₁^.1+_.7− 0.7 ⁺₋ ⁺¹₋₀^.8+_.7−

Single topW t-channel (e→γ) 0.22 ⁺₋ ⁺⁰₋₀^.25+_.22− -0.10 ± 0.10 Diboson (e→γ) 0.04 ⁺₋ ⁺⁰₋₀^.14+_.04− 0.00 ⁺₋ ⁺⁰₋₀^.14+_.00−

Hadrons misidentified as photons 24 ± 8 22 ± 7

Sum 60 ± 10 67 ± 10

Data 52 70

Table 6.2: Event yields for data and expectations for signal and the different background con-tributions for 1.04 fb⁻¹ for the final event selection in both lepton channels.