In this section the event selection criteria are reviewed. The final state of thet¯tγ process in the single lepton channel is characterised by a high-pT lepton (electron or muon), missing transverse momentum, jets with one or more b-jets and a photon. The lepton and missing transverse mo-mentum are originated from the leptonic decay of the W-boson, the b-jets from the top-quarks decay, the other jets from the hadronic decay of theW-boson and additional jets, and the photon from the radiative emission in either thet¯tproduction or decay processes.

Events with data integrity errors in the ECAL calorimeter, and events in a time-window around around identified noise bursts, are rejected. The description of the selection criteria imposed follows. Cuts are listed in the order they are applied to data and to simulations.

• Events are separated into electron channel and muon channel, based upon the trigger fired.

• Events are required to contain a reconstructed primary vertex with at least five associated tracks.

• Reconstructed objects are ordered in sets using the definitions detailed in Sec. 3.5. Over-lapping definitions are avoided applying the following criteria. The jet closest to an electron candidate is rejected if ∆R(e, j) <0.2 [109]. In addition, any jet within a cone ∆R = 0.1 with respect to the reconstructed photon is also discarded to avoid double-counting photons being also reconstructed as jets.

• The event must contain at least one electron (muon) withET(e)>25 GeV (pT(µ)>20 GeV) matched to the appropriate trigger depending on the run period (see table 3.1). Electrons and muons are defined as explained in Sec. 3.5.2 and Sec. 3.5.3, respectively. They are labelled from here on as “good” leptons (`=e, µ).

• The event is rejected if any other good lepton is reconstructed.

• In the electron channel a minimumETmiss >30 GeV cut is imposed, while in muon channel events are required to haveETmiss >20 GeV [114].

• A W-transverse mass mT(W) = q

2pT(`)×ETmiss(1−cosϕ0) > 35 GeV, where ϕ0 is the azimuthal angle between the lepton direction and the missing transverse momentum, is required in the electron channel. In the muon channel the requirement isETmiss+mT(W)>60 GeV.

• In both channels, at least four good jets withpT(j)>25GeV and|PJVF|>0.75are required.

• In order to reduce the acceptance ofW+jets production, at least one jet should be originating from ab-quark. A jet is associated to ab-quark (b-tagged) using an algorithm as described briefly in the following.

Due to its larger lifetieme the b quark decays at a distance from the primary vertex. This produces a displaced vertex which can be reconstructed and identified. The algorithms use as inputs the coordinates of the displaced vertex and the impact parameter of the track associated to the jet with respect to the primary vertex. Systematic biases are reduced by imposing weights on the secondary vertex reconstruction and on the impact parameter determination deduced from the fit. Typically, a jet originated from a b-quark will have a large impact parameter with positive sign. The sign is determined with respect to the jet track direction.

The b-tagging algorithm used in this analysis is theMV1 at a 70% b-jet identification effi-ciency working point [115, 116]. This algorithm relies on a neural network association of jet flavours and it uses inputs from other algorithms used by the Collaboration6. The output confidence level on each jet flavour is expressed in form of a weight for each jet. TheMV1 working point corresponds to a cut on the output weight greater than'0.60.

• Events are required to contain at least one good photon,i.e.fulfilling thetightidentification menu, withET(γ)>20 GeV and|η(γ)|<2.37.

• Events in which at least one jet is found within a cone ofR= 0.5around the photon direction are discarded.

• In the electron channel, the invariant mass of the electron and photon candidates is required to be outside a 5 GeV window around theZ-mass in order to suppress Z+jets events with one electron misidentified as a photon.

• In order to reduce photon radiation off leptons a∆R(γ, `)>0.7 requirement is imposed.

The final selection yields a total of 140 and 222 events in the electron and muon channel respectively. Distributions in data for the Njets, ETmiss, the lepton (photon) ET (pT), and the photonη and ϕ under the full event selection criteria are in excellent agreement when compared to simulations, see Fig. 3.14 and Fig. 3.15. Additional data-to-simulation comparisons can be found in App. B.

6These are the IP3D, SV1 and JetFitterCombNN algorithms [117].

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104 Electron channel

Data

104 Muon channel

Data

Figure 3.14: Distributions for the jet multiplicity (Njets), the missing transverse energy (ETmiss), the electron energy in the traverse plane (ET(e)) and the muon transverse momentum (pT(µ)). Data (points) are compared to the expectation from simulations after full event selection. Distributions are shown separately for the electron (left) and muon (right) channels. The band labelled “Un-certainty” includes both, simulation based, statistical and systematic uncertainties (see Chap. 7).

The entry “Other bck” includes the contributions fromZ+jets, single top and dibosons. The last bin contains any overflow.

0 20 40 60 80 100 120 140

Events / 0.75 rad

0

Events / 0.75 rad

0

Figure 3.15: Photon kinematic variables. Data (points) are compared to the expectation from simulations after full event selection. Distributions are shown separately for the electron (left) and muon (right) channels. The band labelled “Uncertainty” includes both, simulation based, statistical and systematic uncertainties (see Chap. 7). The entry “Other bck” includes the contributions from Z+jets, single top and dibosons. The last bin contains any overflow.

However, for large jet multiplicities, typically (Njet ≥5) data-to-simulation comparisons show some differences, see Fig. 3.14 top. This is a known miss-modelling of high jet multiplicities in

Physics objects definition 64

MC@NLO[118]. As the estimation of processes other thant¯tγ production is derived from data, this miss-modelling does not affect the cross section measurement.

Although this selection cuts are primarily meant to reject background processes to t¯t produc-tion, they have a close correspondence with the cuts enhancing the t¯tγ production cross section, as explained in Sec. 1.1.2.

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 66-70)