4th Year Projects in 2011

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ATLAS Experiment: Detecting and identifying soft hadronically decaying tau leptons with the ATLAS detector

Supervisor: Dr Aldo Saavedra
Contact: Dr Aldo Saavedra
Room: 366
Email: aldo.saavedra AT sydney.edu.au
Phone: 9351 5970

In 2010 the Large Hadron Collider has delivered stable colliding beams of protons at a centre of mass energy of 7 TeV. This has allowed the experiments around the ring to use these collisions to rediscover previously measured resonances and particles such as W and Z bosons, and to use these to study the performance of the detector. The amount of data collected by the ATLAS experiment by mid-August 2010 has been 1.4 inverse picobarns which has allowed the observation of the decay W- → τ- + νbarτ which posseses an effective cross section of ~900 picobarns. Studies so far have only included τ candidates with a transverse momentum greater than 20 GeV.

This project will focus on whether this restriction on transverse momentum can be lowered, thus increasing the number of "soft" taus that can be included in analyses featuring tau leptons. The study will target areas such as the energy calibration, tracking performance and ability to discriminate against fake tau candidates, in the process giving a good introduction to the ATLAS experiment and the methods of experimental high energy physics.


ATLAS Experiment: Studying gauge-mediated supersymmetric decays featuring tau leptons with the ATLAS detector

Supervisor: Dr Aldo Saavedra
Contact: Dr Aldo Saavedra
Room: 366
Email: aldo.saavedra AT sydney.edu.au
Phone: 9351 5970

Supersymmetry is a popular-beyond-the-Standard-Model theory that, if manifested in nature, could be discovered at the LHC. One of its most striking properties is the doubling of the particle spectrum by proposing a supersymmetric partner to each known particle. Since we have not detected any supersymmetric particles, the symmetry is said to be broken at low energies.

The most popular and studied supersymmetric model is the one where the symmetry breaking is mediated by gravity --- referred to as SUGRA --- with the lightest supersymmetric particle being the neutralino, a massive neutral particle which escapes detection. Two previous honours projects have studied the decays of such models and the possibility of their discovery at the LHC. In this project our attention will turn to a supersymmetric model where the symmetry breaking is mediated by a gauge boson. In this model, referred to as GMSB, the lightest supersymmetric particle (LSP) is the gravitino, a neutral particle whose mass is constrained to be very light. The properties of the next lightest supersymmetric particle (NLSP) then become important for its discovery.

In this study a GMSB model will be chosen where the lifetime of the NLSP allows it to decay within the ATLAS detector, providing a means for detection. The decays to be studied will feature two energetic tau leptons and the goal will be to determine whether the decay can be extracted from the background that comes from Standard Model processes that feature real and/or fake taus.


ATLAS Experiment: Di-electron decays of quarkonium states at ATLAS

Supervisors: Dr Bruce Yabsley and Dr Anthony Waugh
Contact: Dr Bruce Yabsley
Room: 366
Email: bruce.yabsley AT sydney.edu.au
Phone: 9351 5970

In experimental work on vector quarkonium states, and especially the low-lying J/ψ and Υ(1S), the decays to pairs of leptons are the most important modes. First results on quarkonia from ATLAS have relied on the dimuon decays μ+μ- of these states, and are limited by the performance of the detector, its readout "trigger", and the experimental reconstruction procedures for muons. The di-electron decays e+ e- have been less well-studied, and may provide complementary information. Multi-electron final states present a rich and interesting set of technical challenges such as particle tracking, bremsstrahlung recovery, trigger simulation, and electromagnetic calorimetry. In this project we will study, and work to improve, the reconstruction of J/ψ → e+ e- and Υ(1S) → e+ e- decays at ATLAS, in the growing dataset from the first two-year run of the Large Hadron Collider.


ATLAS Experiment: Reconstruction of excited quarkonium states at ATLAS

Supervisors: Dr Bruce Yabsley and Dr Anthony Waugh
Contact: Dr Bruce Yabsley
Room: 366
Email: bruce.yabsley AT sydney.edu.au
Phone: 9351 5970

The quarkonia --- a rich spectrum of c cbar and b bbar bound states --- are important tools for understanding the strong interaction and the structure of mesons, and are copiously produced at the Large Hadron Collider. Early experimental work at ATLAS has focussed on the lowest-lying vector quarkonium states, J/ψ and Υ(1S). Many heavier quarkonia decay to the J/ψ or Υ via the emission of other particles such as a photon or a pion pair π+ π-. This latter mode has also been important in the recent discovery of anomalous hidden-charm states, the so-called "XYZ mesons", which appear to have unconventional (non-qq) structure. In this project we will study π+ π- J/ψ combinations, their recovery in ATLAS data, and their use in reconstructing heavier states, with an extension to π+π- Υ if time allows.