Particle physics includes all the experimental research aimed at advancing in the understanding of quarks (the components of protons and neutrons) and leptons (such as electrons), as well as their mutual interactions. The study of the properties of the top quark, the heaviest of the six known quarks, in particular its mass, is of uttermost importance as it may give clues on the origin of the mass di®erence between the particles and indicate the existence of physics beyond the Standard Model of particle physics. At the start of the Large Hadron Collider at Cern in 2007, the top quark will be produced with large statistics. The Atlas experiment will have the possibility to study its properties in great detail.
The top quark is an unstable particle that decays almost instantaneously and exclusively into a b quark and a W boson, which in turn form jets of stable particles. The precision in the measurement of top quark properties will be limited by the knowledge of some of the detector performances, such as the precision to which the particle energies are measured, or the e±ciency with which we can identify some of these particles. The preparation of physics analyses requires the best knowledge of detectors and physics simulation.
I have worked during my internship at the LPNHE Paris on the preparation of two physics analyses on simulated data. A first study has been performed, corresponding to the commisioning phase of the detector with the first data. It has been shown that in Atlas the top quark can be easily reconstructed, even using a very simple selection and without fulfilling requirements such as flavour tagging or jet energy reconstruction. A second analysis has been performed after the detector and the data were understood. The background due to jets misidentified in the detector as electrons has been studied and found to be negligible. The mass reconstruction performance has been studied for different selections of the final state of the top quark decay. The statistical uncertainty on it will become negligible after a few weeks of data taking compared to the uncertainties related to the jet energy reconstruction. These analyses are a first step for the LPNHE group in their development of physics analyses for the top quark mass measurement.
Tabla de Contenido
1. Abstract
2. The Standard Model
2.1 Particle Organization
2.2 Unanswered questions
2.3 Top quark physics2.3.1 Top quark production in colliders
2.3.2 Characteristics of tt events
2.3.3 Reducible backgrounds
3. The ATLAS experiment
3.1 The LHC at CERN
3.2 The ATLAS detector3.2.1 Coordinate system
3.2.2 Tracking system or Inner Detector
3.2.3 Calorimetry
3.2.4 Muon spectrometer
3.2.5 Magnet system
3.2.6 Trigger system
3.3 Data flow
3.3.1 Physics generation
3.3.2 Detector response simulation
3.3.3 Event selection and reconstruction
3.4 Data sample
4. Particle identification
4.1 Jet reconstruction
4.1.1 Algorithm
4.1.2 Jet definition
4.1.3 Jet energy precalibration4.2 Electron reconstruction
4.3 Muon reconstruction
4.4 Missing ET
5. Fighting against reducible background: electron / jet separation
5.1 Selection criteria
5.1.1 Trigger selection5.2 Electron identification performance
5.1.2 Offline Analysis
5.3 Jet rejection
6. Top events selection without calibration and b-tagging
6.1 Selection of events6.1.1 Preselection6.2 Top mass measurement
6.1.2 Final selection
6.1.3 Selection of the light jets
7. Analysis using b-tagging information
7.1 Selection of events
7.1.1 Preselection
7.1.2 Final selection
B.2.1. Localizaciones7.2 Hadronic W reconstruction
B.2.2. Eventos de Referencia
B.2.3. Relocalizaciones
7.2.1 In situ jet energy calibration7.3 Top reconstruction
7.2.2 W mass reconstruction
7.3.1 Choice of the b-jet associated to the hadronic W
7.3.2 Top mass measurement
7.3.3 Systematic errors
8. Conclusion