The Large Hadron Collider and the ATLAS detector

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ATLAS Structure and Physics Requirements

Introduction

Inside the LHC, bunches of 1011 protons collide 40 million times per second to provide 14 TeV proton-proton collisions at a design luminosity of 1034cm 2 s 1 [37]. To meet the requirements of high precision measurement under high interaction rate, radiation dose, particle multiplicity and energy, the particle detectors have to be designed with series of stringent standard. The ATLAS detector has been built for p-p collisions as one of the two general purpose detectors. Physicists from more than 175 institutions in 38 countries are involved in the experiment. ATLAS is 46 metres long, 25 metres in diameter, and weighs about 7,000 tonnes, with 3000 km of cable inside. The structure of ATLAS is shown on Fig.2.4. The detector are composed by the following parts from the inside to the outside:
– Inner detector,
– Calorimeter,
– Muon spectrometer,
– Magnet system,
– Forward detectors.
The ATLAS detector was designed using requirement linked to several typical physics processes covering most of the expected new physics objects at the TeV scale. The high collision energy and luminosity of the LHC boost the cross-sections of major process, in-cluding multi-jets, electroweak interactions and heavy flavour physics . The production of top quark at LHC is at a rate of a few tens Hz, which is high enough for the couplings and spin study. Also, one of the most important goals of ATLAS is the search for the missing piece of SM, the Higgs boson. And this search has become a benchmark for the perfor-mance of the ATLAS subsystems. At the time of the design, the Higgs boson had a wide mass range up to 1 TeV depending of its production and decay mechanism. At low Higgs masses, less than 2 Z boson mass, the natural width would only be a few MeV, therefore the resolution would be quite important for the observed width. For high Higgs masses around 600GeV, the channel diboson decaying to forward jets would be promising. So, the tagging of forward jets would be quite important. Also, for beyond Standard Model Higgs search, good understanding of b-tagging and lepton performance are required. On July 2012, ATLAS has reported the discovery of a higgs-like boson at 125GeV, together with CMS [1, 2, 3, 4, 5, 6]. Specifically, for supersymmetric search, the decay of squarks and gluinos would always end up with a lightest stable supersymmetric particle(LSP). This LSP would have little interaction with the detector, which forms missing transverse energy (ETmiss). The rest of the decay products are including multi-jets and multi-leptons. Therefore, the detec-tor should have good performance for ETmiss, as well as jets and leptons. Further more, for SUSY searches with leptons, the hadronic dacaying modes of are contaminated with QCD background. In this case, high resolution of the EM calorimeter would be helpful for the discriminating of leptons and jets.
Generally speaking, the physics goals of ATLAS are requiring the detectors to have the following properties: fast, radiation-hard electronics and sensor elements, as well as high detector granular-ity, which is essential to handle the particle fluxes and reduce the overlapping events impact.
Large acceptance in pseudorapidity ( ) with almost full azimuthal angle coverage.
Good charged-particle momentum resolution, as well as reconstruction e ciency in the inner tracker.
Very good electromagnetic (EM) calorimetry for electron and photon identification and measurement, full-coverage hadronic calorimetry for accurate jet and ETmiss mea-surements.
Good muon identification and momentum resolution over a wide range of momenta and the ability to determine unambiguously the charge of high pT muons.
Highly e cient triggering on low transverse-momentum objects with su cient back-ground rejection.
The details of all the subsystem of the ATLAS is introduced in the following sections.
A brief summary of the ATLAS Coordinate System are introduced here since they are repeatedly used in the following chapters. The ATLAS Coordinate System is a right-handed system with the x-axis pointing to the centre of the LHC ring, the y-axis pointing upwards, and the z-axis following the beam line. The side-A of the detector is defined as that with positive z and side-C is that with negative z. The azimuthal angle is measured in the xy-plane from the positive x-axis, increasing towards positive y-axis. The polar angle is the angle from the beam axis. However, the polar angle is usually specified as is used, where E is the energy of the particle and pz is the momentum along the z-axis. The transverse momentum pT , the transverse energy ET , and the missing transverse energy ETmiss are defined in the x-y plane unless stated otherwise. The distance R in the pseudorapidity-azimuthal angle space is defined as:
The impact parameter d0 is the closest distance from the track to the interaction point in the transverse plane, while z0 is the closest distance from the track to the interaction point in the longitudinal plane.

Inner detector

The layout of the inner detector of ATLAS is shown on Fig. 2.5 and Fig. 2.6. Its first part is located a few centimeters away from the proton beam axis, and extends to a radius of 1.2 metres. The length of the inner detector is 6.2 m along the beam pipe. The inner detector consists of 4 sub-detectors:
Silicon pixel layers, Semi-Conductor Tracker (SCT), Transition Radiation Tracker (TRT). Insertable BLayer (IBL).
In average, a track of a charged particle going through the inner detector would have 36 hits among all the 3 sub-detectors, which is enough to provide continuous tracking. The pattern recognition is thus enhanced and the momentum resolution within j j < 2:0 is much improved.
Pixel detector As the innermost part of the ATLAS detector, the Pixel detector is sit-uated at 5 10cm distance of the interaction point, The Pixel detector is mainly used to measure the momentum and impact parameter of charged particles, as well as providing information for vertices position and identification [38]. It provides a high resolution of 3D space point measurements. It has 1,744 modules made of silicon in all the three concen-tric layers each with 3 disks on either of the 2 end-caps. Each of the modules contains 16 readout chips, and the basic unit of a chip is a pixel which is around 100 micrometres. The proximity to the interaction point makes the pixel detector exposed to a high radiation rate, therefore its radiation hardness is an important characteristic for the materials that compose the detector.
The average noise for each active module is shown in Fig. 2.7(left). The pixel occu-pancy for the active modules in one of the end-cap disks is shown in Fig. 2.7(right). The occupancy is around 10 7 – 10 8 for Bunch-Crossing IDentification (BCID) value 4, 5 or 6, and 10 9 – 10 10 for other BCID values.

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SCT

The Semi-Conductor Tracker (SCT) locates in the middle level of the inner detector. In the barrel region, the SCT has 4 cylindrical layers, while in the end cap, it has 9 disk layers each side. The basic working principle and function of the SCT is similar to the Pixel detector, which can be seen from Fig. 2.6. However, the SCT is using strips instead of pixels, which allow to cover a larger area. Moreover, the SCT is covering more area in the radial direction than the Pixel detector, making it a critical sub-detector of the Inner Detector. The SCT is thus providing more information for tracking.
The SCT consists of 4088 modules, covering a surface of 63m2 of silicon, of which 2112 modules are in the barrel region [39]. These barrel region modules 80 mm pitch micro-strip sensors, connected to signal readout chips. The spatial resolution of a single SCT modules is around 16 mm in R .

TRT

The TRT is the short term of Transition Radiation Tracker, which is the outermost level of the inner detector. It contains 72 layers of straws interleaved with fibres in the barrel, and, 160 straw planes interleaved with foils in the end-cap. The foil composed by polypropylene allow to produce transition radiation induced by relativistic particles going through it. Each straw is filled with gas, mixture of Xe (70%), CO2 (27%) and O2 (3%), which will be ionized when a charged particle passes through.
In the TRT, all charged tracks with pT > 0:5GeV and j j < 2:0 will at least pass through 36 straws, except those with 0:8 < j j < 1:0 (at least 22 crossed straws). In this way, the TRT provides important transition radiation information for electron identification. Moreover, relativistic light particles (such as electrons) would have a higher speed than heavier ones (such as pions) at a given energy, therefore they would produce a higher amount of transition radiation allowing their separation. For electrons with E > 2 GeV, 7 10 high-threshold hits from transition radiation are expected.
The track position resolution of TRT is larger than the SCT and Pixel detector ( 200 mm). At the end of Run-1, the number of nonoperational TRT channels was about 2.5%. These channels were due to mechanical problems or electrical problems.
IBL The update during the first long shutdown of LHC machine in 2013-2014 consists in the construction of a new innermost Pixel Detector layer, also called Insertable BLaye (IBL). The IBL is installed together with a new beam pipe to maintain an excellent vertex detector performance and compensate possible ine ciencies of the current Pixel Detector.
Being the fourth layer added to the present Pixel Detector, the IBL contains 14 tilted ( = 14 ) staves which is 64 cm long and 2 cm wide. It locates between a new beam pipe and the current inner Pixel Detector layer (B-layer). The front-end chip foreseen for the IBL is called FE-I4[40]. The FE-I4 chip was designed in 130 nm CMOS technology and consists of 26880 pixel cells. These cells are organized in a matrix of 80 columns by 336 rows[41]. The FE-I4 keeps tracks of the firing time of each discriminator as the time over threshold (ToT) with 4-bit resolution. The basic unit of the IBL is a module that consists of two or one front-end chips bump bonded to one sensor. For single-chip (two-chip) assemblies the nominal active coverage for particles normal to the beam is 98.8% (97.4%).

Table of contents :

1 The Standard Model and Beyond 
1.1 The Standard Model
1.1.1 Gauge invariance and interactions
1.1.2 Quantum Chromodynamics
1.1.3 Higgs physics and Symmetry breaking
1.1.4 Fermions and Glashow-Weinberg-Salam (GWS) theory
1.1.5 Elementary particles
1.2 Beyond Standard Model (BSM)
1.2.1 Introduction
1.2.2 Minimal Supersymmetric Standard Model (MSSM)
1.2.3 Super particles
1.2.4 Cross-sections and phase space of SUSY models
2 The Large Hadron Collider and the ATLAS detector 
2.1 Introduction
2.2 The LHC and Beam
2.3 ATLAS Structure and Physics Requirements
2.3.1 Introduction
2.3.2 Inner detector
2.3.3 Calorimeter
2.3.4 Muon detector
2.3.5 Magnetic system
2.3.6 Forward detector
2.4 Trigger system
3 Object Reconstruction 
3.1 Reconstruction of Electrons
3.1.1 Calorimeter-seeded reconstruction and identification
3.1.2 Electron identification within largeregion
3.2 Reconstruction of Muons
3.2.1 Categories of reconstructed muons
3.2.2 Standalone muon performance
3.2.3 Combined muon performance
3.2.4 Tagging muon performance
3.3 Reconstruction of Taus
3.3.1 Tracking and vertexing
3.3.2 Calorimeter-based algorithm for oine reconstruction
3.4 Reconstruction of Jets
3.4.1 Jet reconstruction procedure
3.4.2 Anti-kT algorithm
3.4.3 Jet cleaning and Calibration
3.4.4 Performance of jet reconstruction
3.5 Reconstruction of Missing Transverse Energy
3.5.1 The algorithms for Emiss
3.6 Conclusions
4 Track counting uncertainty for hadronic tau decays 
4.1 Introduction
4.2 Data/MC samples and comparison
4.2.1 Data/MC samples
4.2.2 Discrepancy check
4.3 Average number of tracks
4.4 Uncertainty of track counting
4.5 Pileup density
5 Search for supersymmetry in final states with two opposite-sign taus 
5.1 Introduction
5.2 Signal Monte Carlo Samples
5.2.1 The Simplified Models
5.2.2 The pMSSM model for direct electro-weakino production
5.2.3 Direct Stau Production
5.3 Object selection
5.3.1 Jets
5.3.2 Taus
5.3.3 Electrons
5.3.4 Muons
5.3.5 MET
5.4 Cut-based Analysis
5.4.1 Signal regions optimization and definition
5.4.2 Background Estimation
5.4.3 Results for cut-based analysis
5.4.4 Summary for cut-based analysis
5.5 MVA for the Direct Stau Production
5.5.1 MVA
5.5.2 Discriminating variable and training
5.5.3 BDT response and SR definition
5.5.4 Results and uncertainty
6 Search for supersymmetry in final states with jets and two same-sign leptons or three leptons 
6.1 Introduction
6.2 SS2l/3l SUSY scenarios
6.2.1 Gluino pair production with stop-mediated decay
6.2.2 GG2step
6.2.3 Direct sbottom
6.3 Object selection
6.3.1 Jets
6.3.2 Electrons
6.3.3 Muons
6.3.4 Missing transverse energy
6.4 Event selection
6.5 Signal regions optimization and definition
6.6 Background Estimation
6.6.1 Background Estimation – fake leptons
6.6.2 Background with prompt leptons
6.6.3 Background with charge-flipped electrons
6.7 Validation regions
6.8 Results and Interpretation
7 Conclusion

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