Luminosity measurements in ATLAS

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The ATLAS experiment

The ATLAS experiment is a general purpose particle physics experiment at the Large Hadron Collider (LHC) at CERN. It investigates a wide range of physics, from the search for the Higgs boson to new physics in proton collisions at very high energy. The CMS experiment, at the other side of the LHC ring has the same physics programme.
A brief presentation of CERN and its chain of accelerators and experiments are given in section 2.1. An overview of the sub-detectors of the ATLAS detector will be presented in section 2.2. The ATLAS trigger system and data processing are summarised in sec-tion 2.3 and section 2.4 respectively. The object reconstruction and identification are described in section 2.5.

The Large Hadron Collider

CERN

The European Organization for Nuclear Research (CERN) is located at the French-Swiss border near Geneva. The name is derived from the french acronym Conseil Europeen pour la Recherche Nucleaire and was founded in 1954 with 12 member states. There are now 21 member states mainly from European countries. CERN employs around 2500 people, scientific and technical staff and 12000 visiting researchers from more than 70 countries working with the CERN facilities. These scientists represent a large community of 120 different nationalities and over 600 universities.

The LHC machine

The LHC [19] project was approved by the CERN Council in December 1994 to replace the Large Electron–Positron (LEP) collider machine. The LHC is a hadron accelerator and collider installed in the existing 27 km long tunnel previously constructed to host the LEP ring. The tunnel lies between 45 m and 170 m below the surface on a plane inclined at 1.4 % .
The aim of the LHC and its experiments is to test the Standard Model or reveal the physics beyond the Standard Model. In order to achieve theses goals, it was decided that the LHC machine would accelerate protonsa to centre of mass collision energies of 14 TeV.
a The LHC also collides lead-ions over one month per year as part of the diverse research programme.
The number of events per second generated in the LHC collisions is given by: Nevent = Lσevent,
where σevent is the cross section of the process studied and L the luminosity which depends only on the LHC machine parameters and on the configuration of the magnets in the proximity of the experiments, mainly quadrupoles, which have to focus the beams into the point where the collisions takes place. The luminosity can be written as: where Nb is the number of particles per bunch, nb is the number of bunches per beam, frev is the revolution frequency, γr is the relativistic gamma factor, n is the normalised transverse beam emittance, β∗ is the beta function at the collision point, and F is the geometry luminosity reduction factor due to the crossing angle at the interaction point (IP): F = (1 + (θ2cσσ∗z )2)−1/2 θc is the full crossing angle at the IP, σz is the root mean square (RMS) of the bunch length distribution, and σ∗ is the RMS of the transverse beam size at the IP.

Accelerator and Energy

The CERN accelerator complex is illustrated in Figure 2.1. The first stage in the accel-eration is linear, LINAC II strips the electrons from hydrogen atoms to produce protons, which are then linearly accelerated to approximately one third of the speed of light. Then, the protons are injected into the Booster, a small synchrotron. The protons are divided into 4 bunches and circularly accelerated via a pulsing electric field. The Booster has 157 m in circumference. It accelerates the protons to 0.916c (c denotes the speed of light in vacuum). The third stage of acceleration is the Proton Synchrotron, a circular accelerator with a circumference of 628 m increases the protons to 0.999c in 1.2 seconds. The final stage of acceleration before injection into the LHC is the Super Proton Syn-chrotron with a circumference of 7 Km. At this step the proton beam is separated in two parts to be injected in a counter-rotating configuration in the LHC. The energies reached by the protons at the end of each accelerator are:
Proton LINear ACcelerator (LINAC): Up to 50 MeV
Proton Synchrotron Booster (PSB): 1.4 GeV
Proton Synchrotron (PS): 26 GeV
Super Proton Synchrotron (SPS): 450 GeV
LHC: 7 TeV
The maximum beam energy that the LHC can deliver depends strongly of the mag-netic field of the dipole magnets needed to keep the particle along the trajectory. The use of superconducting dipoles, shown in Figure 2.2, must supply a magnetic field of 8.3 T which corresponds to a beam energy of 7 TeV. The magnets need to be cooled down to a temperature of 1.9 K.
Almost the same chain of successively energetic accelerators is used to accelerate heavy lead ions P b82 to an energy of 574 TeV which corresponds to a centre of mass energy of 2.76 TeV/nucleon in P b-P b collisions.
The accelerator tunnel comprises eight straight sections and eight arcs, as shown in Fi-gure 2.3. The tunnel contains the two rings which produce two counter-rotating particle beams colliding at Points 1, 2, 5 and 8. The four main detectors are built around these points. The beam is accelerated using superconducting Radio-Frequency (RF) cavities, located in the straight section at Point 4, which provide RF energy to the beams and keep the bunches tightly bunched to ensure optimal condition at the collision point. The Points 3 and 7 contain beam collimation systems which shape and clean the beam. The straight section in Point 6 is used as the beam dump, where the beams are removed from the LHC and “dumped” into a graphite target to dissipate the beam’s energy.
The arcs are built using a total of 1232 superconducting dipole magnets which keep the beams in the (nearly) circular orbit. Additionally, there are 392 quadrapole magnets, located in the straight sections, which serve to focus the beam.

The Experiments on the LHC

ATLAS (A Toroidal LHC Apparatus) [21]: the ATLAS detector is the largest detector in operation at LHC. Its design philosophy was to create a detector with the ability to detect the full range of masses allowed for the Higgs boson while retaining the ability to detect the known SM particles such as heavy quarks and gauge bosons.
CMS (Compact Muon Solenoid) [22]: The CMS detector has the same research prospect as the ATLAS experiment. CMS is built with a strong superconducting mag-netic field of 4 T to collect the maximum energy from the particles. CMS has a very compact design of 12500 tonnes of material.
LHCb (Large Hadron Collider beauty) [23]: LHCb is a single-arm spectrometer with a forward angular coverage from approximately ±15 mrad to ±300 mrad in the bend-ing plane. In terms of pseudo-rapidity the acceptance is 1.9 < η < 4.9 . The LHCb experiment has as main purpose study the CP violation and the physics of decay in the B-meson system. The geometry is influenced by the fact that both and ¯ hadrons are b b created in the same forward (or backward) cone. LHCb has excellent particle identifi-cation and vertex resolution necessary for the study of rapidly oscillating B mesons.
LHCf (Large Hadron Collider forward experiment) [24]: It is the smallest of all the LHC experiments. Its aim is to study the particles generated in the forward region of collisions, to verify hadronic models at very high energy for the understanding of ultra-high energetic cosmic rays. It consists of two small detectors, 140 m on either side of the ATLAS intersection point.
TOTEM [25]: The TOTEM experiment measures the total pp cross-section and study elastic scattering and diffractive dissociation at the LHC. TOTEM also aims to measure the luminosity at the CMS interaction point where it is based. It covers the very forward region in the pseudo-rapidity range.

The ATLAS Detector

The ATLAS detector is a general purpose particle physics experiment. It is designed to achieve the maximum coverage in solid angle around the interaction point. This is re-alised by several layers of active detector components around the beam axis (barrel) and perpendicular to the beam axis in the forward regions (endcaps). The ATLAS detector consists of four major components, the Inner Detector which measures the momentum of the charged particles, the Calorimeter which measures the energies carried by the particles, the Muon spectrometer which identifies muons and the Magnet system that bends charged particles for momentum measurement. Figure 2.4 show an overview of the ATLAS detector, including all subdetectors and the magnet systems (one solenoid and three air-core toroids). Table 2.3 lists the design performance of the ATLAS detector.
The ATLAS Coordinate System is a right-handed system with the x-axis pointing to the centre of the LHC ring, the z-axis following the beam direction and the y-axis going upwards. The azimuthal angle φ is defined with respect the beam axis in the x-y plane.

The Inner Detector

The ATLAS Inner Detector (ID) provides charged particle tracking with high efficiency over the pseudorapidity range of |η| < 2.5. The ID consists of three independent but complementary sub-detectors. Figure 2.5 shows a cutaway view of the barrel ID. All the sub-detectors allow precision measurement of charged particle trajectories in an envi-ronment of numerous tracks: the Insertable B-Layer (IBL) and the Pixel detector mainly contribute to the accurate measurement of vertices, the silicon microstrip (SCT) mea-sures precisely the particle momentum, and the transition radiation tracker (TRT) en-hances the pattern recognition and improve the momentum resolution, with an average of 36 hits per track. The TRT contributes also to electron identification complementary to the calorimeter over a wide range of energies. Figure 2.6 illustrates in more details the sub-detector layers in the barrel and end-cap regions. The ID is immersed in a 2 T axial magnetic field generated by the central solenoid, which extends over a length of 5.3 m with a diameter of 2.5 m.

The Pixel detector

The Pixel detector is the innermost part of the ID, originally it was a three layers system. In 2014 a fourth innermost layer, the IBL described in the next section, was installed for Run 2. A Pixel sensor or module is a 16.4 × 60.8 mm wafer of silicon with 46080 individual channels called pixels of 50 × 400 micros each. A Pixel module comprises an un-packaged flip-chip assembly of 16 front-end electronics chips bump bonded to a sensor substrate. There are 1744 modules in the pixel detector with a total of more than 80 millions detection units. A cylinder of 1.4 m long and 0.5 m in diameter centred on the interaction point supports the active parts. The barrel part of the pixel detector consist of the 3 cylindrical layers with radial positions of 50.5 mm, 88.5 mm and 122.5 mm respectively. There are 22, 38 and 52 staves in each of these layers respectively. Each stave is composed of 13 pixel modules. The staves are mounted with a tilt angle of 20o to form a layer, this geometry allows overlaps between the modules.
The two pixel end-caps each have three identical disks perpendicular to the beam axis. Each of the disks consist of 8 sectors. Six pixel modules are directly mounted on each sector. The modules are rotated with a tilt angle of 7.5o to ensure overlap between modules.
The intrinsic measurement accuracies of the pixel detector in the barrel are 10 µm (R-φ) and 115 µm (z) and in the disks are 10 µm (R-φ) and 115 µm (R). The Pixel detector is designed to measure 4 hits per track in the barrel region and 5 hits per track in the endcaps. The initial Run 1 Pixel detector design allowed to measure only 3 hits per track in the barrel region. The IBL adds an additional hit.

The Insertable B-Layer

The Insertable B-Layer (IBL) is the fourth layer added to the Pixel Detector between a new beam pipe and the inner Pixel Detector (B-layer). It consists of 14 tilted staves which are 64 cm long, 2 cm wide and tilted in φ by 14o, equipped with 32 front-end chips per stave and sensors facing the beam pipe over the range of |η| < 2.5. The inner radius of IBL is 31 mm with an outer radius of 38.2 mm while the sensor are present at an average radius of 33.4 mm. The IBL sensors have 50 × 250 micron pixels adding an additional 12 million pixels to the pixel system.
The performance of the IBL is critical to the full realisation of the physics capabil-ities of the ATLAS experiment. The addition of the IBL provides improved precision for vertexing and b-tagging (identification of jets originating from bottom quarks). The improvement in the b-tagging performance due to the addition of the IBL and the algo-rithmic updates can be found in ref. [26].

The SemiConductor Tracker (SCT)

The SCT consist of four cylindrical layers in the barrel region and 9 disks at each end of the barrel (endcap). The barrel SCT consist of 2112 rectangular shape modules. A module is constructed with four rectangular planar p-in-n silicon strip sensors which have a thickness of 285 µm and 768 effective strips with pitch of 80 µm. The four sensors, two of each on the top and bottom side, are rotated with their hybrids by ± 20 mrad around the geometrical centre of the sensors. They are glued on a 380 µm thick thermal conductive mechanical support. A barrel module with its components is shown in Figure 2.7. The two 768 strip sensors on each side form a 128 mm long unit. The endcap SCT consist of 1976 trapezoidal shape modules placed on 18 endcaps disks, using 4 types of modules which were placed in three rings named as outer, middle and inner on disks. The endcaps modules were constructed in the same manner as the barrel. The strip pitch is varied from 56.9 to 90.4 µm. The intrinsic accuracies per module in the barrel are 17 µm (R-φ) and 580 µm (z), while in the endcap region they are 17 µm (R-φ) and 580 µm (z). The total number of readout channels in the SCT is approximately 6.3 millions. The SCT detector is design to measure 8 hits per track in the central region and 9 hits per track in the endcaps.
Figure 2.7.: A SCT barrel module. The thermal pyrolytic graphite (TPG) provide a high thermal conductivity path between the coolant and the sensors.

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The Transition Radiation Tracker (TRT)

The Transition Radiation Tracker (TRT) itself is subdivided into two sections, the TRT barrel (|η| < 1.0) and the TRT end-caps (1.0 < |η| < 2.0). The TRT barrel has the sensor layers running parallel to the beam axis, while the sensor layers of the end-cap TRT are radially oriented.
The TRT is based on straws, which in case of the barrel are 144 cm long. They are electrically separated into two halves at |η| = 0 and arranged in a total of 73 planes. The end-cap straws are 37 cm long, radially arranged in wheels with a total of 160 planes. The straws themselves are polyimide tubes with a diameter of 4 mm. Its wall is made of two 35 µm thick multi-layer films bonded back-to-back to forms the cathode. The straw wall is held at a potential of -1530 V. The anodes are 31 µm diameter gold-plated tungsten wires. They are directly connected to the front-end electronics and kept at ground potential. The straws are operated with a gas mixture of Xe/CO2/O2(70:27:3). To maintain straw straightness in the barrel, alignment planes made of polyimide with a matrix of holes are positioned each 25 cm along the z-direction of the module.
The TRT operates as a drift chamber: when a charged particle traverses the straw, it ionises the gas, creating about 5-6 primary ionisation clusters per mm of path length. The electron drift towards the wire and they cascade in the strong electric field very close to the wire, thus producing a detectable signal.
As mentioned above the TRT plays a central role for electron identification, cross-checking and complementing the calorimeter. TRT provides substantial discriminating power between electron and pions over the energy range between 1 and 200 GeV.
Typically, the TRT provides 36 hits per track with a precision of about 140 µm in the bending direction.

The calorimeters

The ATLAS calorimeters cover the pseudorapidity range |η| < 4.9. The design has been guided by the benchmark process of a Higgs boson decaying to two photons, H → γγ. For such a physics search the calorimeter must have excellent photon resolution, with uniform photon measurement and good γ/π discrimination across the entire calorime-ter. The overview of the ATLAS calorimeter system is illustrated in Figure 2.8. Different technologies are used across different regions in η. Surrounding the inner detector the EM calorimeter is finely segmented for precision measurements of electrons and pho-tons, while the rest of the calorimeter is segmented more coarsely, since it is mainly aimed at reconstructing jets and measuring the missing transverse momentum.
The depth of the calorimeter is important to provide good containment for electro-magnetic and hadronic showers and must also limit punch-through into the muon sys-tem. The total thickness of the EM calorimeter is > 22 radiation lengths (X0) in the barrel and > 24 X0 in the endcaps. The approximately 10 interaction lengths (λ) both in the barrel and in the end-caps are adequate to provide good resolution for high en-ergy jets. The total thickness, including the outer support, is 11 λ at η=0 and has been shown by simulation and measurements to be sufficient to reduce punch-through into the muon system well below the irreducible level of prompt or in-flight decays muons.

The electromagnetic calorimeter

The Electromagnetic Calorimeter is based on a highly granular liquid-argon technology (LAr). LAr is also used in the end-caps of the Hadron Calorimeter. Both detector ele-ments share the cryostat at the end-cap, which also accommodates a special LAr forward calorimeter. The design is an novel arrangement of the absorber plates and electrodes which are arranged with the “accordion” geometry, with a total of ∼174000 readout channels.
It comprises a barrel section, made of two identical half-barrels, together covering the central pseudorapidity range, |η| < 1.475 and two endcaps, each covering a region 1.375 < |η| < 3.2. In addition, there is a forward combined electromagnetic/hadronic liquid argon calorimeter at each end, covering the region 3.2 < |η| < 4.9.
In front of the barrel and part of the endcaps, for |η |<1.8, there is a 10 mm thick presampler to provide an estimation of energy lost in dead material in front of the calorimeter. In the barrel region, the material budget in front of the detector, associated with the solenoid and the tracker, varies from ∼2X0 at η=0 to 5–6X0 for η from 1.5 to 1.8. In the endcaps the material budget is ∼2.3X0.
The liquid argon was chosen as an active medium because it offers an intrinsically linear response which is stable over time and tolerant to high levels of radiation. The accordion geometry provides high granularity and good hermeticity. The readout is at the front and back of the calorimeter, rather than at the sides, which means that adjacent modules can be tightly packed, with full φ ttH searches at LHC.
The top yukawa coupling is the strongest coupling to the Higgs boson, given that the top quark is the heaviest fundamental particle in the SM, with a mass equal to 173.21 GeV (see Table 1.1). Its measurement is estimated using the ggF production where the Higgs is indirectly coupled to the top quark via a loop (see Figure 2.2). Its value is found to be compatible with the SM expectation (see Figure 2.12), where the top coupling modifier is estimated by t = 0:87 0:15, combining ATLAS and CMS data at Run1, assuming no BSM in the loops (see Section 2.4.4). However, new physics could be hidden in the loops mediating the Higgs production via ggF. This issue could be solved by performing the measurements in another production mode involving a direct, tree-level, Higgs-top coupling.
ATLAS only takes into account H ! focusing on single-lepton +jets (having the highest bb significance) and dileptonic final states (where one or more isolated charged leptons tt are coming from W boson decays from the top quarks). In « H ! leptons », the Higgs decays to the all possible multilepton final state (two, three or four leptons) mainly via W +W , pionneered in Ref. [51] and subsequently in Ref. [52], and ZZ weak bosons or via leptons. It provides a clean signature and low QCD activities comparing to the previous channel. Finally, in the « H ! photons », analysis where the Higgs decays to a pair of photons H ! , the high invariant mass resolution is used to separate the signal from the background.
Figure 2.13: Di erent nal states used for the direct measurements of the top-Yukawa coupling in the ttH channel. Numbers correspond to the expected sensitivity (signi cance) at the beginning of Run2, using 13.2 fb 1of luminosity, by the ATLAS experiment. The signi cance is calculated using a simple approximate formula in order to give an overview of the sensitivity in the di erent channels. These numbers are ’realistic’, meaning that the reducible background are taken from MC and rescaled to the data output t values (see Chapter 6 for more details). Boxes in red highlights the nal state channels of my analysis in Chapter 6.
The raised questions from SM discussed in Section 1.4 and the predicted mechanism of the generation of the particle masses (Higgs mechanism) suggested in Chapter 2 neces-sitate a powerful particle accelerator and detectors with high technology. The current world-wide energy frontier machine is called the Large Hadron Collider (LHC) built by the European Organization for Nuclear Research (CERN). The data collected by ATLAS and CMS at the LHC allow to probe the predictions of the SM at a prodigious energy regime at center of mass energy of 13 TeV.
In this chapter, an overview of the CERN complex accelerators ramping up the energy to the LHC is described in Section 3.1. The ATLAS detector is described in Section 3.2.

The CERN accelerator complex

The accelerator complex at CERN [55, 56] is an injector chain of hadron, proton or lead ion, linear and circular accelerator machines designed and arranged in order to increase gradually the particle beam energies on the one hand and squeeze1 the beam to get high intensity proton bunches and low emittance 2 on the other hand.
The basic idea of each accelerator is to apply a potential difference V allowing a particle with charge q to pass through an electric field to gain energy E ( E = qV ). The used technologies are mainly radio-frequency (RF) cavities with different powerful magnets (dipoles, quadrupoles, kickers). Protons are chosen to probe very wide energy spectrum. Unlike an electron-position collider that provides a single collision energy.
Figure 3.1 shows a general layout of the proton acceleration through the accelerator complex at CERN. First at the linear accelerator3 LINAC 2, bunch of protons are obtained by stripping orbiting electrons from hydrogen atoms. They are accelerated to one third of the speed of light c (an energy of 50 MeV) using RF source with small quadrupole mag-nets to control the tightness of the proton beam. Protons are then injected from LINAC 2 into a series of ring accelerators starting from the PS Booster (PSB) where the beam is divided into four packets, to maximise its intensity, and be accelerated to 91.6 % of the speed of light with an energy of 1.4 GeV. A magnetic field is applied to bend the beam of protons around the circle and squeeze them together. The four packets are then gath-ered together again and sent to the Proton Synchrotron (PS) where the proton packets 1 The squeeze reduces the beam size at the interaction point thereby increasing the collision rate. 2A low emittance particles are con ned to a small distance and have nearly the same momentum.
3A linear accelerator is a number of conducting drift tubes arranged in a line where some of them obey to a RF voltage source and others are xed to the ground. The RF allows the particle to go from one tube to another. The frequency of the voltage is set according to the time needed for a particle in one tube to arrive to the gap in order to change the direction and allow it to achieve the next tube.
achieve 99.9 % of c with 25 GeV of energy; followed by the Super Proton Synchrotron (SPS) where the beam is accelerated to 450 GeV. Finally, the beam is ready to be trans-ferred to the Large Hadron Collider (LHC), the last piece of this chain, and split out into two opposite direction beams where they are accelerated for 20 minutes to 6.5 TeV. The center of mass energy of the collision is then equal to 13 TeV. Four different detectors are installed there, namely two general-purpose detectors, ATLAS (Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid) sharing the same scientific goal (measurements of the Higgs boson properties and searches for new physics) but with some different tech-nical solutions and design; ALICE and LHCb detectors for quark-gluon plasma, produced through heavy-ion collisions, and heavy flavour physics searches respectively. Other de-tectors are also installed for multi-purposes such as TOTEM, LHCf and MoEDAL. More details about the LHC machine follow.
The Large Hadron Collider LHC [56] is a two ring proton proton (pp) collider, favored over the alternative single ring pp collider to reach higher luminosities of the order of 1034 cm 2s 1. To achieve that, the central part of the LHC is designed to be the coldest place in the galaxy thanks to the cryogenic systems, made of liquid Helium, providing a temperature less than 1.9 K (-271 C). It has the largest number of high-technology magnets ever built and the largest most complex electronic instruments. The LHC con-sists of two main systems: The beam acceleration system made of RF cavities and the circulation, beam orbits and dimensions are controled by around 9000 magnets: 1232 dipole superconducting magnets defining the orbit, 392 quadrupoles acting on the beam size and many higher order multipoles for further corrections. In each dipole the current circulates in cables of 36 twisted stands, each stand made out of 6000-9000 supercon-ducting Nb-Ti filaments 7 micro-meters thick. Each dipole provides a magnetic field of 8.3T.

Table of contents :

Introduction 
1 Standard Model 
1.1 History
1.2 Matter particles
1.3 Particles interactions
1.3.1 Quantum Electrodynamics
1.3.2 Weak interactions
1.3.3 Quantum Chromodynamics
1.3.4 Electroweak interaction
1.4 Problems/Outlook
1.5 Summary
2 Higgs Physics 
2.1 Brout-Englert-Higgs mechanism
2.2 Higgs production
2.2.1 Gluon-gluon fusion (ggF)
2.2.2 Vector boson fusion (VBF)
2.2.3 Higgs-strahlung
2.2.4 ttH production
2.3 Higgs decays
2.3.1 Fermionic tree-level decay modes
2.3.2 Bosonic tree-level decay modes
2.3.3 Loop-induced decay modes
2.4 Measurement of Higgs properties
2.4.1 Higgs mass
2.4.2 Higgs spin-parity
2.4.3 Higgs width and lifetime
2.4.4 Higgs couplings
2.5 ttH searches at LHC
3 ATLAS at LHC
3.1 The CERN accelerator complex
3.2 The ATLAS detector
3.2.1 ATLAS layout
3.2.1.1 The inner detector
3.2.1.2 Solenoid
3.2.1.3 Calorimetry
3.2.1.4 Muon spectrometer
3.2.1.5 Trigger system [1]
3.2.2 Data quality monitoring in ATLAS
3.2.3 Luminosity measurements in ATLAS
4 Object reconstruction 
4.1 Tracks and primary vertices (PV)
4.2 Electrons
4.2.1 Electron trigger
4.2.2 Electron reconstruction
4.2.3 Electron identification
4.2.4 Electron isolation
4.3 Photons
4.3.1 Photon reconstruction
4.3.2 Photon identification
4.4 Muons
4.4.1 Muon reconstruction
4.4.2 Muon identification
4.5 Taus
4.6 Jets
4.6.1 Jet reconstruction
4.6.2 b-tagged jets
4.7 Conclusion
5 Measurement of the electron reconstruction efficiency
5.1 Definition
5.2 Method
5.2.1 Tag-and-Probe method with Z ! ee events
5.2.2 Background estimation
5.2.2.1 Background for clusters with an associated track
5.2.2.2 Background for clusters with no associated track
5.2.3 Uncertainties
5.3 Dataset and simulation
5.4 Results
5.4.1 Background level at denominator and numerator
5.4.2 Efficiency
5.4.3 Scale Factors
5.4.4 Combination
5.5 Conclusion
6 ttH Multilepton analysis with 13.2 
6.1 Dataset and simulation
6.1.1 Data sample
6.1.2 Signal MC sample
6.1.3 Background MC samples
6.2 Signal region
6.2.1 Object definition
6.2.1.1 Electron
6.2.1.2 Muon
6.2.1.3 Tau
6.2.1.4 Jets and b-tagged jets
6.2.1.5 Overlap removal
6.2.2 Signal region definition
6.3 Data-driven background estimation
6.3.1 Charge flip estimation
6.3.2 Fake lepton estimation with the Matrix Method
6.3.2.1 Description of Matrix Method
6.3.2.2 Inputs to Matrix Method
6.3.2.3 Validation
6.3.2.4 Results
6.3.3 Comparison with the Fake Factor method
6.4 Validation regions for prompt leptons background
6.5 Fit results in 2`ss channel
6.5.1 Likelihood function
6.5.2 Systematics
6.5.3 Signal strength
6.6 First ttH combination with 13 TeV data
6.6.1 ttH Multilepton
6.6.2 All ttH decays
6.7 Conclusions and prospects
Conclusion
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