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ATLAS Tau Identification in High-Multiplicity Environments
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
2009 (English)Report (Other academic)
Abstract [en]

The tau identification in two distinct high-multiplicity environments, pile-up and ttbar events, is investigated. The tau ID performance in such environments is known to be degraded. It is shown that it is necessary to adjust the tau ID quality cut in order to reach a working point in terms of tau efficiency versus jet rejection comparable to more frequently investigated and cleaner types of events like Z->tautau and W->taunu. Three different ways of recovering performance and robustness are investigated: Firstly, the usage of dedicated probability density functions as input to the tau likelihood method is tested, obtained from Z->tautau events with pile-up and ttbar events. Effects are generally small, except for high-pT tau jets in ttbar events. Secondly, it is investigated whether likelihood input variables can be removed without reducing performance (to increase robustness and reduce systematic uncertainties), and thirdly, whether adding new variables or replacing existing variables with new variables can lead to increased performance in terms of jet rejection at fixed tau efficiency. It is shown that several variables are redundant for tau identification in high-multiplicity environments. At least one variable is identified which can efficiently replace one of the existing variables in high-multiplicity events, namely the fraction of energy deposited in a wider isolation cone than used by the default tau identification.

Place, publisher, year, edition, pages
2009. , 31 p.
Keyword [en]
ATLAS Tau Identification
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-110770OAI: oai:DiVA.org:uu-110770DiVA: diva2:278220
Available from: 2009-11-24 Created: 2009-11-24 Last updated: 2012-10-10Bibliographically approved
In thesis
1. Charged Higgs Bosons at the ATLAS Experiment and Beyond
Open this publication in new window or tab >>Charged Higgs Bosons at the ATLAS Experiment and Beyond
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the ATLAS experiment at the Large Hadron Collider (LHC) at CERN, direct searches for the elusive Higgs boson will be conducted, as well as for physics beyond the Standard Model. The charged Higgs boson (H±) is interesting both as a part of the Higgs sector, and as a clear sign of new physics. This thesis focuses on H± searches, with H± production in top-antitop pair events, and in particular the  bW± bH±, H±→τhadν, W±→qq channel. Its potential was investigated  as  part of a larger study of the expected performance of the entire ATLAS experiment. Full simulation of the ATLAS detector and trigger was used, and all dominant systematics considered. It was shown to be the most promising H± discovery channel for m<mt.

As hadronic τ decays are important for H± searches, their correct identification is critical. Possibilities of improving tau-jet identification in  pile-up and top-antitop pair events were investigated. Redundant or even performance-reducing variables in the default likelihood identification were identified, as were new variables showing discriminatory power. This allows for increased rejection of QCD jets in these environments, and higher robustness of the method.

Before any physics studies, a commissioned and well-understood detector is required. The Lorentz angle of the ATLAS Semi-Conductor Tracker (SCT) barrel was measured using 2008 cosmic-ray data. It is an important observable for the performance of several detector aspects. Potential sources of systematics were investigated and evaluated. The Lorentz angle in the SCT barrel was measured as θL = 3.93 ± 0.03(stat) ± 0.10(syst) degrees, agreeing with the model prediction.

The Compact Linear Collider (CLIC) is a proposed successor to the LHC. The potential for charged and heavy neutral Higgs bosons at CLIC was investigated, in terms of both discovery and precision measurement of parameters like tanβ or the Higgs masses, up to and beyond 1 TeV, which would be challenging at the LHC

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 61 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 702
Keyword
particle physics, Standard Model, beyond the Standard Model, supersymmetry, MSSM, Higgs physics, charged Higgs boson, LHC, ATLAS, ATLAS Semi-Conductor Tracker, SCT, tau lepton, Lorentz angle, CLIC
National Category
Subatomic Physics
Research subject
High Energy Physics
Identifiers
urn:nbn:se:uu:diva-111576 (URN)978-91-554-7693-9 (ISBN)
Public defence
2010-02-12, Polhemssalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2010-01-22 Created: 2009-12-17 Last updated: 2012-10-10
2. Looking for the Charged Higgs Boson: Simulation Studies for the ATLAS Experiment
Open this publication in new window or tab >>Looking for the Charged Higgs Boson: Simulation Studies for the ATLAS Experiment
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The discovery of a charged Higgs boson (H+) would be an unambiguous sign of physics beyond the Standard Model. This thesis describes preparations for the H+ search with the ATLAS experiment at the Large Hadron Collider at CERN. The H+ discovery potential is evaluated, and tools for H+ searches are developed and refined.

The H+→τν decay mode has been known as the most promising H+ discovery channel. Within this thesis, first studies of this channel with realistic detector simulation, trigger simulation and consideration of all dominant systematic uncertainties have been performed. Although, as shown by these studies, the discovery sensitivity is significantly degraded compared to studies using a parametrized detector simulation, this channel remains the most powerful ATLAS H+ discovery mode. Future searches will rely on multivariate analysis techniques like the Iterative Discriminant Analysis (IDA) method. First studies indicate that a significant sensitivity increase can be achieved compared to studies based on sequential cuts. The largest uncertainty in H+ searches is the expected $t\bar{t}$ background contribution. It is shown that numbers obtained from simulated events could be off by a factor of two, decreasing the discovery sensitivity dramatically. In this thesis, the Embedding Method for data-driven background estimation is presented. By replacing the muon signature in $t\bar{t}$ events with a simulated τ, events which allow an estimation of the background contribution at the 10% level are obtained.

The ATLAS τ identification focuses on comparably clean environments like Z and W decays. To optimize the performance in high-multiplicity events like H+→τν, tau leptons are studied in $t\bar{t}$ and pile-up events. Variables which do not show discrimination power in high-multiplicity events are identified, and in some cases similar, more powerful variables are found. This allows to recover some of the performance loss and to increase the robustness of the τ identification.

For the analysis of large amounts of data produced by the ATLAS detector, seamless interoperability of the various Grid flavors is required. This thesis introduces translators to overcome differences in the information system between a number of Grid projects,and highlights important areas for future standardization.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. VI+50 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 696
Keyword
charged Higgs, tau, ATLAS, HEP, CERN, LHC, Particle Physics, Grid
National Category
Physical Sciences
Research subject
High Energy Physics
Identifiers
urn:nbn:se:uu:diva-110882 (URN)978-91-554-7676-2 (ISBN)
Public defence
2010-01-15, Polhemsalen, Ångström Laboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2009-12-22 Created: 2009-11-30 Last updated: 2009-12-22Bibliographically approved

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