Missing transverse energy

for jet transverse momenta below 100 GeV. The offset decreases to−1% at higherp_T (p_T &200 GeV).

MC/ResponseDataResponse

0.85

MC/ResponseDataResponse

0.85

Figure 6.7: Mean p_T balance obtained in the data and with the Pythia simulation, for anti-k_tjets withR= 0.4 calibrated with the EM+JES scheme (left) [80]. Ratio of the average jet responsehp^jet_T /p^ref_T i measured in data to that measured in MC simulations for jets within |η|<1.2 as a function of the jet transverse momentum, p^jet_T , shown separately for the three in-situ techniques, used in the combined calibration (right) [80].

In the forward rapidity region, |η_det|> 1.2, the calibration can be per-formed by exploiting the transverse momentum balance in events with two jets at high transverse momentum. A jet in the forward region can be ba-lanced against a well-calibrated jet in the central region, and therefore, the whole detector response can be equalized as a function of η^jet. In addition to this simple approach, the matrix method described in Ref. [83] is used, in which theη-intercalibration is estimated from jets in all regions (not only the central one).

6.4 Missing transverse energy

The missing transverse momentum, ~p^miss_T , is defined as the momentum im-balance in the plane transverse to the beam axis. The vector momentum imbalance in the transverse plane is obtained from the negative vector sum of the momenta of all particles detected in app collision. The~p^miss_T recons-truction includes contributions from energy deposits in the calorimeters and muons reconstructed in the muon spectrometer [84]. The two ~p^miss_T compo-nents are calculated as:

p^miss_x(y)=p^miss,calo_x(y) +p^miss,µ_x(y) . (6.11)

The magnitude of this vector is the so-called missing transverse energy, E_T^miss. The values ofE_T^miss and its azimutal coordinate φ^miss are defined as:

E_T^miss =q

(p^miss_x )²+ (p^miss_y )², φ^miss = arctan (p^miss_y /p^miss_x ).

(6.12) The reconstruction of the calorimeter term of the ~p^miss_T uses calorime-ter cells calibrated according to the reconstructed high-p_T physics object to which they are associated, in a chosen order: electrons, photons, hadroni-cally decayingτ-leptons, jets and muons. Cells not associated with any such objects are also taken into account in theE_T^miss calculation. Once the cells are associated with objects as described above, the~p^miss_T calorimeter term is calculated as follows [85] (the reason why thepmiss,calo,µ

x(y) term is in between parenthesis will become clear below):

p^miss,calo_x(y) =p^miss,e_x(y) +p^miss,γ_x(y) +p^miss,τ_x(y) +p^miss,jets_x(y) pmiss,softjets

x(y) + (pmiss,calo,µ

x(y) ) +pmiss,CellOut

x(y) . (6.13)

Each of the terms in the previous equation is computed from the negative sum of calibrated cell energies inside the corresponding object, according to the following expression:

p^miss,term_x =

N_cell^term

X

i=1

pisinθicosφi

p^miss,term_y =

N_cell^term

X

i=1

pisinθisinφi

(6.14)

where pi, θi and φi are the energy, the polar angle and the azimutal angle respectively, and all the summations are performed on cells in the range

|η|<4.5.

The different terms in Equation 6.13 are described in the following:

• p^miss,e_x(y) is reconstructed from cells in clusters associated to electrons passing the “medium” identification criteria withp_T>10 GeV.

• p^miss,γ_x(y) is also reconstructed from cells in clusters, associated with pho-tons passing the “tight” identification criteria [86] withp_T >10 GeV at the EM scale.

• p^miss,τ_x(y) is reconstructed from cluster cells associated to LCW calibrated τ-jets reconstructed with the “tight” identification criteria [87], with p_T>10 GeV.

6.4. MISSING TRANSVERSE ENERGY 91

• p^miss,jets_x(y) is computed from cells in clusters associated to LCW cali-brated jets with pT > 20 GeV, reconstructed with the anti-kt algo-rithm, and with the jet energy scale factor applied.

• pmiss,softjets

x(y) is reconstructed from cells in clusters associated to LCW calibrated jets reconstructed with the anti-k_talgorithm (withR= 0.6), with 7 GeV< p_T<20 GeV.

• pmiss,calo,µ

x(y) is the contribution originating from energy lost by muons in the calorimeter. Its calibration will be discussed below.

• pmiss,CellOut

x(y) is calculated from the cells in topoclusters with the LCW calibration and from reconstructed tracks with pT >400 MeV which are not included in the reconstructed objects. The tracks are added to recover the contribution from low-pT particles which do not reach the calorimeter or do not have enough energy to seed a topocluster. They are also used to improve the determination of the momentum in the topoclusters, since the calibration and resolution of the lowpT tracks is better compared to that of the topoclusters.

On the other hand, the ~p^miss_T muon term (see Equation 6.11) from the momentum of muon tracks reconstructed with|η|<2.7:

p^miss,µ_x(y) =− X

muons

p_x(y)^µ, (6.15)

where the summation effects all the selected muons. In the central region,

|η|<2.5, combined muons (reconstructed muons in the MS with a matched track in the ID, see Section 6.2) are considered. Instead, since the region 2.5<|η|<2.7 lays outside the fiducial volume of the ID, there is no matched track requirement and the MSp_T alone is used.

The muon term is calculated differently for isolated and non-isolated muons, where non-isolated muons are defined to be those within a distance

∆R < 0.3 of a reconstructed jet in the event. For isolated muons, the p_T is determined from the combined measurement in the ID and MS, account-ing for the energy that the muon deposits in the calorimeter. Therefore, pmiss,calo,µ

x(y) is not added to the calorimeter contribution (this is the reason why it appears in between parenthesis in Equation 6.13). Instead, for non-isolated muons, the termpmiss,calo,µ

x(y) has to be considered.

The systematic uncertainty on each individual term of the E_T^miss can be evaluated from the propagation of the uncertainties of the reconstructed ob-jects that are used to build it. Only the contribution to theE^miss_T scale and resolution uncertainties coming from the “soft terms” (softjets and CellOut terms) needs to be estimated with dedicated studies [84]. The overall sys-tematic uncertainty on the E_T^miss scale is then calculated by combining the uncertainties on each term.

As it will be discussed in Section 7.2, a slightly modified definition of the E_T^miss is used³ in the analysis presented in this Thesis. The τ-lepton term, p^miss,τ_x(y) , is omitted because in the analysis presented,τ-leptons are not identi-fied as such, but considered as jets. Furthermore, the muon terms,pmiss,calo,µ

x(y)

andp^miss,µ_x(y) , are also omitted. The reason for not considering them is related to the precise estimation of the most important irreducible background in the analysis,Z(→νν)+jets, which will be explained in Chapter 7.¯

3TheE_T^misscollection in the analysis presented is called “MET Egamma10NoTau”.

Chapter 7

The monojet analysis

The monojet analysis is described in detail in this chapter. The data and the Monte Carlo simulated samples used for the analysis are presented, together with the definition of the different physics objects and the event selection criteria. The statistical treatment of the data and the estimation of the different Standard Model (SM) background processes are discussed. The observations are then compared to the SM predictions in the different signal regions.

7.1 Data sample

The data sample considered in the analysis presented in this Thesis was collected with the ATLAS detector in proton-proton collisions at a center of mass energy of 8 TeV between April 4, 2012 and December 6, 2012. A total integrated luminosity of 20.3±0.6 fb⁻¹ was recorded after requiring tracking detectors, calorimeters, muon chambers and magnets to be fully operational during the data taking. Events are selected using the lowest unprescaled E_T^miss trigger logic called EF xe80 tclcw, that selects events with E_T^miss above 80 GeV, as computed at the final stage of the three-level trigger system of ATLAS discussed in Section 5.2.4. The details of the implementation of the E_T^miss trigger can be found in Ref. [88].