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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.
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.
The design specifications of the LHC are shown in table 2.1. The LHC running condi-tions for Run 1 and Run 2 period are summarised in table 2.2.
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
The experiments installed on the LHC ring are briefly described below:
ALICE (A Large Ion Collider Experiment) [20]: designed with the intention of study-ing the quark gluon plasma that results from the intense temperatures generated during the heavy ion collisions. Design considerations of ALICE have been made with the abil-ity to cover a large phase space and to detect hadrons, leptons and photons.
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.
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.
Table of contents :
Introduction
1 Theoretical introduction
1.1 Introduction to Standard Model of particle physics
1.1.1 Elementary particles and fundamental interactions
1.1.2 The Standard Model and the Brout-Englert-Higgs mechanism
1.2 The Higgs boson
1.2.1 Higgs production in hadron colliders
1.2.2 Higgs decays
1.3 Summary
2 The ATLAS experiment
2.1 The Large Hadron Collider
2.1.1 CERN
2.1.2 The LHC machine
2.2 The ATLAS Detector
2.2.1 The Inner Detector
2.2.2 The calorimeters
2.2.3 The muon spectrometer
2.3 The trigger system
2.4 Data processing
2.5 Event reconstruction
2.5.1 Charged particle tracks and primary vertex
2.5.2 Jet reconstruction
2.5.3 Muon reconstruction
2.5.4 Electron identification
2.5.5 Missing transverse energy
3 Identification of double b-hadron jets
3.1 Introduction
3.2 Identification of b-jets in ATLAS
3.2.1 b-tagging ingredients
3.2.2 b-tagging algorithms
3.3 Multi Secondary Vertex Finder algorithm
3.4 Simulated samples
3.5 Performance of the Multi Secondary Vertex Finder algorithm
3.5.1 Vertex Purity Fraction in single-b jets
3.5.2 Vertex Purity Fraction in bb-jets
3.6 Development of MultiSVbb taggers
3.6.1 Boosted decision trees
3.6.2 Multivariate analysis
3.7 Performance of the MultiSVbb1 and MultiSVbb2 taggers
3.8 Summary
4 Search for the Higgs boson in the single lepton t¯tH(H ! b¯b) channel
4.1 Status of the t¯tH(H ! b¯b) analysis
4.2 Data and simulation samples
4.2.1 Data
4.2.2 Simulated samples
4.3 Object selection
4.4 Event selection and categorisation
4.5 Multivariate analysis
4.5.1 MVA-based event reconstruction
4.5.2 Discrimination between signal and background
4.6 Background modelling
4.6.1 t¯t + jets background
4.6.2 Misidentified lepton background
4.6.3 Other backgrounds
4.7 Systematic uncertainties
4.7.1 Experimental uncertainties
4.7.2 Uncertainties on the background modelling
4.7.3 Uncertainties on the signal modelling
4.8 Statistical analysis
4.9 Results
4.9.1 Combination with the dilepton analysis
4.10 Summary
5 Conclusion
Bibliography
