Impact of Detector Simulation in Particle Physics Collider Experiments

The top quark is the heaviest of the known fundamental particles in the Standard Model (SM) with a mass of 173.3±1.1GeV. It is the sixth quark in the SM and the weak isospin partner of the bottom quark. Properties such as the mass and the cross section have been measured since its discovery in 1995 with great precision at the proton-antiproton Fermilab Tevatron Collider, Chicago at a centre-of-mass energy of 1.8GeV and 1.96GeV. With the start of the proton-proton CERN Large Hadron Collider (LHC), Geneva, a new record in the centre-of-mass energy was achieved. The LHC opens a new energy frontier and probes the SM at energies of the electroweak symmetry-breaking scale and beyond. The measurement of the top-quark pair cross-section at this new energy is an important step, since it will either re-establish the SM at these energies by confirming the theoretical calculations or it can point to new phenomena that help to understand the so far not explained experimental findings in particle physics and cosmology. This thesis presents a study of the production cross-section measurement of topquark pair in the dilepton channel at the LHC with the ATLAS experiment at a centre-of-mass energy of √ s =10TeV and for a data amount of Lint.=200 pb. This study investigates the performance of the cross-section measurement with a cut-and-count analysis on Monte Carlo simulated samples. Important background processes are taken into account such as leptonically decaying Z-boson and single top-quark events. Data-driven background methods for Z-boson and fake event rates are discussed. A full list of systematic uncertainties is investigated and finally combined with the statistical uncertainties in a likelihood to extract the combined uncertainty. A “top mass”-like variable is presented that can help to distinguish dileptonic top-quark pair decays from similar signatures in new physics models. The combined sensitivity of the analysis in all dilepton decay-channels is a relative uncertainty on the cross-section measurement of: ∆σcombined σcombined [%] = +3.1 −3.1(stat) +9.6 −8.7(syst) +26.2 −17.4(lumi) Finally the study is compared to the same analyses performed at different centreof-mass energies of 14GeV and 7GeV and two dilepton tt̄-pair event candidates in 280 pb of first ATLAS data are presented.

/pdf/study-of-top-quark-pair-cross-section-measurement-in-the-3eh6vk9uh1.pdf

Study of top-quark pair cross-section measurement in the dilepton channel with the ATLAS detector at the LHC

/pdf/determination-of-z-tau-tau-e-e-4nu-background-shapes-from-2bn152cm1n.pdf

Determination of Z -> tau tau -> e+ e− + 4nu Background Shapes From Data in Context of the b−Quark Associated Higgs Boson Production with ATLAS at the LHC

Through the last three decades, accurate simulation of the interactions of particles with matter and modeling of detector geometries has proven to be of critical importance to the success of the international high-energy physics (HEP) experimental programs. For example, the detailed detector modeling and accurate physics of the Geant4-based simulation software of the CMS and ATLAS particle physics experiments at the European Center of Nuclear Research (CERN) Large Hadron Collider (LHC) was a determinant factor for these collaborations to deliver physics results of outstanding quality faster than any hadron collider experiment ever before. 
This review article highlights the impact of detector simulation on particle physics collider experiments. It presents numerous examples of the use of simulation, from detector design and optimization, through software and computing development and testing, to cases where the use of simulation samples made a difference in the precision of the physics results and publication turnaround, from data-taking to submission. It also presents estimates of the cost and economic impact of simulation in the CMS experiment. 
Future experiments will collect orders of magnitude more data with increasingly complex detectors, taxing heavily the performance of simulation and reconstruction software. Consequently, exploring solutions to speed up simulation and reconstruction software to satisfy the growing demand of computing resources in a time of flat budgets is a matter that deserves immediate attention. The article ends with a short discussion on the potential solutions that are being considered, based on leveraging core count growth in multicore machines, using new generation coprocessors, and re-engineering HEP code for concurrency and parallel computing.

The simulation program for the ATLAS experiment at CERN is currently in a full operational mode and integrated into the ATLAS common analysis framework, Athena. The OO approach, based on GEANT4, has been interfaced within Athena and to GEANT4 using the LCG dictionaries and Python scripting. The robustness of the application was proved during the test productions since 2004. The Python interface has added the flexibility, modularity and interactivity that the simulation tool requires in order to be able to provide a common implementation of different full ATLAS simulation setups, test beams and cosmic ray applications. Generation, simulation and digitization steps were exercised for performance and robustness tests. The comparison with real data has been possible in the context of the ATLAS Combined Test Beam (2004–2005) and cosmic ray studies (2006).

The ATLAS detector simulation application

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The ATLAS detector simulation application