4. Object definitions and preselection 43
4.6. Event Preselection
or new weakly-interacting particles, will be translated in a vector momentum imbalance in the transverse plane. The missing transverse energy (ETmiss) is therefore defined as the momentum imbalance in the transverse plane with respect to the beam axis and is obtained from the negative vectorial sum of the momenta of all particles detected from the pp collision. In the tt¯and t¯tV final states with two opposite-sign charged leptons, real ETmiss is expected from the neutrinos originating either from the leptonic W-boson decay, or from the invisibleZ-boson decay, for a small fraction of thet¯tZ selected events.
The important requirements for the reliable measurement of theETmiss are maximum detector coverage and small detector resolution effects. The presence of dead regions and various sources of noise, as well as cosmic-ray and beam-halo muons crossing the detector, can create fake ETmiss [140].
The ETmiss reconstruction [141] takes into account energy deposits in the calorimeter and muons reconstructed in the muon spectrometer. Track information is added to recover the contribution from low-pT particles which are missed in the calorimeter.
The ETmiss calculation uses reconstructed and calibrated objects. The calorimeter energy deposits associated with a reconstructed and identified high-pT parent object are considered in the following order: electrons, jets and muons. Hadronically-decaying taus and photons are not considered in this analysis. Deposits not associated with any such objects are also considered in the ETmiss calculation as the Cell-Out term. The ETmiss can be then expressed as the sum of the following terms:
Ex(y)miss =Ex(y)miss,el+Ex(y)miss,jet+Ex(y)miss,muon+Emiss,sof t−jet
x(y) +Emiss,cell−out
x(y) , (4.5)
ETmiss = q
(Exmiss)2+ (Eymiss)2, (4.6) where the additional term Emiss,sof t−jet
x(y) includes the contribution from jets with pT < 20 GeV 10.
The reconstructed and calibrated jets and muons correspond to the same collection used in the main analysis of this thesis, while for electrons the cut-basedtight++working point is used for theETmiss calculation, instead of the likelihood-basedmedium. Studies have shown that the impact of using the specified electron identification method in the ETmiss calculation instead of the one used in the analysis is negligible.
Given that theETmissmeasurement is significantly affected by pile-up effects (especially during 2012 data-taking), methods were developed to suppress such contributions. They are based on tracks, to suppress pile-up in the ETmiss jet term, and on both tracks and the jet area method, to reduce pile-up effects in theETmiss soft-jet/cell-out terms [140].
4.6. Event Preselection
Once the objects are defined, a set of selection criteria is required to be fulfilled by the events:
Good Run Lists
The Good Run Lists (GRLs) define a set of data-taking runs, each divided in “luminosity blocks”, for which data fulfils good quality criteria in order to be used for analyses. Only data belonging to these GRLs are therefore used.
10In 2012 data,Emiss,sof t−jet
x(y) andEmiss,cell−out
x(y) are calculated together.
4. Object definitions and preselection
Lepton Trigger
Only events collected using a single electron or muon trigger (described in Sections 4.2.2 and 4.3.2) are accepted.
Non-collision Background Rejection
After the event has been accepted by the trigger, it is required to have at least one recon-structed vertex with at least four associated tracks with pT > 400 MeV, consistent with the beam collision region in the x−y plane, in order to reduce pile-up. If more than one vertex is found, the primary vertex is taken to be the one which has the largest sum of the squared momenta of its associated tracks. Furthermore, in order to reject cosmic events, the event is dis-carded if it contains two muons of opposite sign that have a|d0|>0.5 mm, and are back-to-back, i.e. have a ∆φ >3.10.
“Bad Jets” Removal
Events are rejected if a “bad jet” (defined in Section 4.4.1) with pT >20 GeV and |η|<4.5, without any requirements on the JVF, is found.
Opposite-Sign Dilepton Preselection
The dilepton selection used in the main analysis of this thesis additionally requires the event to have two reconstructed leptons (electron or muon) with opposite-sign charges (OS), resulting in the dilepton channels: e+e−, µ+µ−, and e±µ∓. At least one selected lepton with pT > 25 GeV is required to match a lepton reconstructed by the high-level trigger with a ∆R < 0.15.
The pT requirement on the subleading lepton has been lowered topT >15 GeV in the context of the t¯tV analysis in order to increase the signal efficiency. Furthermore, events are rejected if a selected electron and muon share an inner detector track (∆θ < 0.005 and ∆φ < 0.005).
Reconstructed leptons are also required to match the leptons in the truth record of the MC simulation, in order to avoid double counting with the fake lepton estimation.
The scalar sum of the transverse momentum of all selected leptons and jets (HT) is required to be above 130 GeV in the eµ channel, in order to suppress background contributions from Z/γ∗+jets production (with theZ-boson decaying into leptonically decaying τ leptons).
A cut at low values of the invariant mass of the two selected leptons, mll, is applied in events with e+e− or µ+µ−, requiring mll ≥ 15 GeV in order to remove contributions from the weak decay of low-mass hadronic resonances, such as the J/Ψ and Υ, into same-flavour leptons.
Furthermore, a cut on the mll within a 10 GeV window around theZ-boson invariant mass is used to define two analysis categories in the opposite-sign dileptont¯tV measurement presented in this thesis. Further requirements on the numbers of jets and the number of b-tagged jets will also define different regions within the two above mentioned categories. This additional selection, together with further cuts on theETmiss, will be described in Section 7, where thet¯tV analysis strategy is introduced.
58
CHAPTER 5
Modelling of Physics Processes
5.1. Event Simulation
In Section 4, the experimental aspects of object reconstruction and calibration were intro-duced. However, the majority of measurements or searches for new physics rely at some level on the simulation of the physics processes of interest (signal and background), i.e. on the capabil-ity to reproduce the evolution of the hard process to the final object observed in the detector.
Stimulated by the study of more complex final states over the past several years, an increasing demand for higher precision descriptions of the physics processes has motivated the rethinking of calculation and simulation paradigms, leading ultimately to new generator tools.
Figure 5.1 is a sketch of an event produced at a hadron collider, which is expected to be described by an event generator. Fortunately, nature allows for the factorisation of such events into different well-defined stages, each corresponding to a different kinematic regime:
• Hard Scattering: in general, the key part of the event simulation, which can be calculated at fixed order perturbation theory in the coupling constants. It relies on computations based on matrix elements (ME). This part is process dependent and valid for well-separated hard partons.
• Parton Showers (PS): QCD evolution that links the hard scale of coloured parton creation (high-energy regime) with the hadronisation scale, where colourless hadrons are formed (low-energy regime). It is process independent and valid when partons are collinear and/or soft. The most well-known parton shower MC event generators are Pythia [142], Her-wig[143] and Sherpa [144].
• Hadronisation: QCD partons are transformed into primary hadrons at the hadronisation scale ΛQCD 1 by applying phenomenological fragmentation models, such as the string hadronisation implemented inPythiaor thecluster hadronisation implemented in Her-wig. These primary hadrons finally decay into particles that will be observed in the detector. Similar to the parton shower, this stage is process independent.
1Scale at which QCD is no longer perturbative.