6.2 W plus jets production in association with a photon

6.2.1 Phase-space definition

(6.7) withNSRW γ,MCandNSRW γ,MCbeing the number ofW γ+jets events in the the SR and CR respec-tively, as estimated from simulation.

The validity of this method relies on the fact that the same Monte Carlo (MC) generator is used to simulate ensembles of events for the SR and the CR; these two sets are statistically independent. The model dependency is removed through the ratioNSRW γ,MC/NCRW γ,MCwhich cancels out the simulated Matrix-Element (ME) dependency and leaves only an acceptance dependency.

The shape of the photon’s track-isolation remains simulation independent as it is defined via the templates (see Sec. 5.3).

6.2.1 Phase-space definition

In this section the phase-space and selection requirements which define the CR are detailed. The same object definition used for the t¯tγ selection criteria is used here and only a few cuts are reversed. Specifically, each event must fulfil the following criteria.

• The number of good jets requirement is inverted from at least four (for thet¯tγ selection) to at most three.

• The same b-tagging algorithm (MV1), used in the t¯tγ selection, is used here to request a b-tagging veto in the event.

• The event must, of course, contain at least a photon matching the tight definition criteria and being separated from the other objects as discussed in Sec. 3.6.

• In the electron channel, the event inside the rangemZ−15 GeV≤m(e, γ)≤mZ+ 15 GeV is excluded, in order to suppress radiation fromZ decay products, wheremZ = 91 GeV.

e/γ backgrounds 120

W γ+jets events are simulated with both theSHERPAandALPGENMC generators, see Sec. 3.4.

Totals of2910and 6181candidate events are selected for the electron and muon channels respec-tively, includingW γ+jets events and background contributions. Out of these numbers, simulations predict1027and3090events to be fromW γ+jets in the electron and muon channels respectively.

Reconstruction level observables for simulations and data are in good agreement for the photon’s kinematic distributions (see Fig. 6.7) as well as for other reconstructed objects (see Fig. 6.8).

) / GeVγ(N

104 Muon Channel

Datatγ

Figure 6.7: Data-to-simulation comparisons for the photonpT(γ) in theW γ+jets control region for the electron (lef) nad the muon (right) channels. The filled band corresponds to the quadrature sum of statistical uncertainties of the simulated samples and of the multijet background. The last bin contains any overflow.

An overall normalisation correction is applied on simulations for W γ +jets. It is motivated by the fact that theW +jets production is charge asymmetric inpp colliders, i.e. the production ofW++jets is significantly higher than that of W+jets, as theu-quark density in the proton is larger than the d-quark one [144]. The correction exploits the fact that the theory predicts with higher precision the ratio

RW = σW++jets

σW+jets

of theW++jets toW+jets production cross sections (RW = 1.429±0.013, next-to-next-to-next-leading order calculation [24, 145]) than the total W +jets cross section [145, 146]. Data in the CR are split into two subsets based on the reconstructed lepton’s charge each containingNCRW γ,Data+ and NCRW γ,Data− events for positive and negative charged leptons respectively 1. Therefore, the normalisation correction factor for the W γ+jets simulation samples (εcW γ) can be obtained as:

εcW γ = RW + 1

RW −1×NCRW γ,Data+−NCRW γ,Data−

NCRW γ,Data++NCRW γ,Data− (6.8)

The correction holds as long as the charge asymmetry for background processes to W +jets production is negligible, which is the case for the processes considered here.

Both the CR and the SR use b-tagging algorithms in their definition. The selection efficiency on the simulated samples is dependent upon the heavy flavour content.

1The rate of the wrong lepton charge assignment is negligible [112, 147].

) / GeVe(N

103 Electron channel

Datatγ

103 Electron channel

Datatγ

103 Muon Channel

Datatγ

Figure 6.8: Data-to-simulation comparisons for the transverse energy of the electron (ET(e)), the transverse momentum of the muon (pT(µ)), the jet’s trasnverse momentum (pT(j)) and the magnitude of the missing energy in the transverse plane (ETmiss). The left-hand side distributions are in the electron channel while the right-hand side distributions are in the muon channel. The filled band corresponds to the quadrature sum of statistical uncertainties of the simulated samples and of the multijet background. The last bin contains any overflow.

e/γ backgrounds 122

Therefore, a residual model dependency upon the extrapolation from CR to SR persists, see Tab. 6.4.

The relative flavour content forW+c(c),W+b¯bandW+light quarks have been determined from data and a correction factor has been applied onW γ+jets simulations [148, 149].

Due to the different jet multiplicities for the SR (N(j) ≥4)and for the CR (N(j) <4), the independence of the extrapolation on the number of jets has to be ensured. Equation 6.7 has been computed before extracting of the number of events with prompt-photons. The results of the extrapolation, shown in Tab. 6.4, are independent when considering the systematic uncertainties of the method.

Electron channel[events] Muon channel [events]

Jet multiplicity SHERPA ALPGEN SHERPA ALPGEN N(j) = 1 13.55 ±0.31 7.02±0.16 9.6±0.15 7.17±0.11 N(j) = 2 21.17 ±0.78 8.17±0.30 11.00±0.29 7.49±0.20 N(j) = 3 16.67 ±1.12 10.90±0.73 11.42±0.60 8.88±0.47

N(j)<4 15.13 7.48 10.01 7.33

Table 6.4: Simulation-based extrapolations of W +jets candidates to the signal region as a func-tion of the jet multiplicity before extracfunc-tion of the number of events with prompt-photons. The estimates have been obtained by applying the εcW γ correction factor of Eq. 6.8. Numbers are shown before correcting for the relative flavour content in simulations (SHERPA or ALPGEN), thus enhancing the differences betweenSHERPA and ALPGEN. The uncertainties are statistical only and are determined from the size of data in the Control Region. The small dependency on the jet multiplicity is negligible when considering the systematic uncertainties of the method, which are about 27% for the electron channel and 23% for the muon channel.

Dans le document Enlightened Top Quark: measurements of the ttγ cross section and of its spectrum in transverse energy of the photon in the single lepton channel at √s = 7 TeV in 4.59 fb−1 of pp collision data collected with the ATLAS detector (Page 125-128)