The H ! and H ! Z Decays
In this thesis, the observation of the Higgs boson via its H ! decay and the search for the rare decay H ! Z ! « in the ATLAS experiment will be discussed. For a Higgs boson mass of 125 GeV, the cross section times branching ratio of the Higgs boson decays into is 40 (50) fb at p s = 7 TeV (8 TeV), while in the H ! Z channel it is 27 (34) fb. The expected signal rate is thus quite small. However, in these two channels, all final state particles can be reconstructed in the ATLAS detector with high efficiency and excellent energy resolution, and their invariant mass provides a powerful way to separate the signal (Higgs boson decay) from background. The reconstructed Higgs mass distribution is expected to exhibit a narrow peak with the width dominated by detector resolution, while the background distribution is relatively flat. The backgrounds can be classified into two parts: irreducible background, due to other processes producing the same final states as the Higgs boson decay (i.e. or Z continuum production), and reducible background, caused by jets misidentified as photons (i.e. +jet or Z+jet events). Irreducible backgrounds dominate in both channels. In the H ! Z analysis, the leading-order diagrams for the irreducible processes (q¯q ! « ) are shown in Fig. 1.12. In the H ! analysis, the irreducible background is given by QCD production, whose lowest order diagrams are shown in Fig. 1.13.
In the H ! channel, interference between the signal from gg ! H ! and the background from gg ! can affect the signal decay rate  , and shift the central value of the di-photon mass peak . By assuming the signal invariant mass resolution to be a Gaussian function, Fig. 1.14 shows the expected di-photon invariant mass distribution (after reconstruction) with and without the inclusion of the interference effects. The central value of the Higgs boson mass shifts to lower values by about 240 MeV.
The ATLAS Detector
ATLAS is the largest particle detector ever constructed. It is 46 m long, 25 m high, 25 m wide, weighs 7000 tons, and covers almost the whole 4 solid angle. The detector layout is shown in Fig. 2.4. ATLAS is composed of 3 subdetectors: the Inner Detector (ID), the calorimeter and the Muon Spectrometer (MS). The ID is composed of three subdetectors, the Pixel detector, the Silicon Micro strip (SCT) detector and the Transition Radiation Tracker (TRT) detector. The calorimeter, based on LAr and scintillating tile sections, has an electromagnetic component and a hadronic component. A solenoidal magnet surrounds the ID and provides a magnetic field of 2T inside its volume, while 3 toroid magnets generate the magnetic field needed for tracking inside the MS. To reduce the enormous amount of data produced by the pp collisions, ATLAS records events conditionally using an advanced “trigger” system that keeps only events that are potentially interesting for the ATLAS physics programme. The design performance goals of the ATLAS detector are summarized in Table 2.2. More details on the sub-detectors and their performance are given in the following sections. ATLAS uses the following right-handed Cartesian coordinate system. The origin of the coordinate system corresponds to the nominal beam interaction point, located at the center of the detector. The z-axis is given by the beam direction and the x − y plane is orthogonal to the beam direction. The x-axis points from the interaction point towards the center of the LHC ring. The y-axis points upwards. The azimuthal angle in the x-y plane is referred to as , while is the polar angle with respect to the z-axis. The pseudorapidity is defined as = −ln[tan(/2)].
The Muon Spectrometer
The MS, shown in Fig. 2.8, surrounds the hadronic calorimeter. It provides, for charged particles passing beyond the HCAL, precision momentum measurement for || < 2.7 and trigger capability for || < 2.4. Muon momenta down to 3 GeV can be measured by the MS alone. The muon spectrometer can also provide adequate momentum resolution (about 10%) and excellent charge identification at very high pT , up to 3 TeV. In the barrel region, precision-tracking chambers are located between and on the eight coils of the superconducting barrel toroid magnet. The chambers are arranged in three concentric cylindrical shells around the beam axis at radii of approximately 5 m, 7.5 m and 10 m. In the end-cap region, the muon chambers are installed in front and behind the end-cap toroid magnets. They are located at distances of |z| 7.4 m, 10.8 m, 14 m and 21.5 m from the interaction point.
Photons in ATLAS are reconstructed through their interactions with the ECAL or by detecting their conversions to e+e− in the material upstream of the calorimeter. The reconstruction of photon conversions increases the reconstruction efficiency of particles decaying to photon final states, e.g. Higgs boson or graviton decaying to photon pairs. Mapping the photon conversion vertices also provides a precise localisation of the Inner Detector materials. Both photons and electrons deposit their energy in the EMC, forming a cluster of calorimeter cells with significant energy deposits. Electrons and photon conversions are characterized by at least one track mateched to an EM cluster. In the case of photon conversions, the track (tracks) is (are) originating from a conversion vertex candidate.
The unconverted photons are reconstructed as a cluster with no track matched to it. Photons which convert within a radial distance of 300 mm from the beam axis may be reconstructed with high efficiency from standard Si-seeds tracks (inside-out tracking), while photons which convert further from beam axis may be reconstructed from tracks formed with TRT seeds with few Si hits (outside-in tracking) or no Si hits at all (TRTstandalone).
The inside-out track reconstruction is sensitive to conversions inside Pixel detector. A segment is first formed using the hits in the silicon detectors. A Kalman fitter is then used to add successive hits to the track. Finally, a possible TRT extension is used to do a global fit. According to the fit quality with or without additional TRT extension, the reconstructed inside-out tracks are classified into three categories:
• Tracks with extensions which are used in the global fit.
• Tracks with extensions which are not used in the global fit.
• Tracks without TRT extension (|| > 2).
Photon Energy Calibration
The photon energy is reconstructed by summing the energy of all the cells of the four layer of the ECAL belonging to a a cluster of fixed size, and is corrected by applying a dedicated energy calibration afterwards. The number of cells used in the energy reconstruction depends on the photon conversion status. In the barrel region, a cluster with transverse size of × = 3×5 in units of second layer cells around the photon shower barycenter is used for unconverted photons while a cluster with size × = 3 × 7 in the same units is used for converted photons. The wider size in the direction is used to compensate for the opening between the conversion products due to the solenoidal magnetic field. In the end-cap, where the opening of the conversion electrons is smaller due to the smaller inner radius of the calorimeter, the same cluster size × = 5 × 5 is used for converted and unconverted photons.
Table of contents :
1.1 The Gauge Principle
1.2 The Electroweak Unification
1.3 Spontaneous Symmetry Breaking
1.4 The Higgs Mechanism
1.5 Constraints on the Standard Model Higgs Boson Mass
1.6 Higgs Boson Production and Decay
1.7 The H ! and H ! Z Decays
2 The Large Hadron Collider and the ATLAS detector
2.1 The Large Hadron Collider
2.2 The ATLAS Detector
2.3 Photon Reconstruction
2.4 Photon Identification
2.5 Photon Energy Calibration
2.6 Lepton Reconstruction and Identification
2.7 Jet and Missing ET Reconstruction
3 Photon performance
3.1 Photon Trigger Optimization for the 2012 data taking
3.2 Photon Trigger Efficiency Measurement
3.3 Photon Identification Efficiency Measurements
3.4 Pile-up Dependence of the Photon Identification Efficiency
4 Search for a Higgs boson in H ! Z ! «
4.2 Data and Simulation Samples
4.3 Event Selection
4.4 Discriminating Variable
4.5 Event Classification
4.6 Data-driven Background Estimation
4.7 Signal Parameterization
4.8 Background Properties
4.9 Systematic Uncertainties
4.10 Statistical Method
4.11 Exclusion Limits and p-values
4.12 Conclusions and Prospects
5 Observation of the Higgs boson inevents
5.2 Data and Simulation Samples
5.3 Event Selection and Category
5.4 Data-driven Background Estimation
5.5 Signal and Background Modelling
5.6 Systematic Uncertainties
5.8 The Higgs Boson Properties Measurements in Combined Channels
5.9 The Higgs Boson Properties Measurements in H ! Channel with the CMS Detector