Orjan Dale - Master Thesis

Master Thesis Project Proposal -- Ørjan Dale

Study of top quark events with tau leptons in ATLAS

The top quark is the heaviest quark in the Standard Model of particle physics (SM) with a mass of M_t>170 GeV. It was discovered relatively recently in 1995 at the Tevatron and hence is also the least studied of all quarks. The Large Hadron Collider (LHC) is designed to collide protons at a center of mass of up to 14 TeV and thus will constitute a top factory. Even in the first year of running (2010) top events should be abundant. Events with tops will be characterized by high transverse momentum (p_T), jets with b-quarks (b-jets) and large missing transverse energy (E_T^miss), all of which are also important characteristics of beyond SM physics processes. Thus top quarks studies provide an important early measurement with implications for new physics beyond the SM.

One of the most studied extensions to the SM is SUper SYmmetry (SUSY) in which additional super-partners to the SM particles are introduced. SUSY can solve the Hierarchy problem, moreover the Lightest SUSY Particle (LSP) can provide a Dark Matter constituent particle. In SUSY models where the mass difference between of the Neutralino (LSP) and stau (next to LSP) is low co-annihilations balance the production of Dark Matter in the early universe. If such a SUSY model is realized in nature we would expect to see events with large p_T, E_T^miss, b-jets and to two tau leptons in the ATLAS experiment at LHC. The largest background to such a signal would be SM top quark events, where tau leptons arise from W-bosons created in the top decays.

Specifically the aim of this work is to analyze tau leptons from top quark events both as SM signal in early LHC data and as background to a possible SUSY signal. The general properties of these events will be examined to be able to identify them correctly, and with this a selection tool will be built. It is to be hoped that the tool can be tested and tuned on real top quarks in early ATLAS data. Finally it will be applied to reject top background in SUSY signal simulations. Broken down by semester the project plan is as follows

1. Learn relevant ATLAS tools to analyze and present data. Take first look at early ATLAS data. Train and participate in ATLAS data quality monitoring.
2. Analyze top events in early ATLAS low luminosity data and compare to Monte Carlo simulations.
3. Make a cut selection for top events in high luminosity data to discriminate between top background and possible SUSY events. Write a selection tool in the ATLAS software framework to implement the cut selection.
4. Apply the selection to early SUSY analysis. Finish and document the tool. Write thesis.

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