3rd Year Special Projects in 2012

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Here you will find a list of the 3rd year special projects that were offered in Semester 2, 2012.

Particle Theory/Cosmology: Cosmological Zitterbewegung

Supervisor: Dr Archil Kobakhidze
Contact: Dr Archil Kobakhidze
Email: archil.kobakhidze AT coepp.org.au
Phone: (03) 8344 5439

The phenomenon of zitterbewegung ("trembling motion") of a relativistic electron proposed by E. Schrodinger in 1930, is currently understood as an unphysical, unobservable effect, since it is based on the incorrect treatment of positive and negative energy solutions to the Dirac equation. Recently, however, we have pointed out that zitterbewegung can be observed by an observer moving with a constant acceleration due to the inherent mixing of positive and negative energy modes in such non-inertial reference frames. In this project we analyse the phenomenon of zitterbewegung in cosmological spacetimes (i.e. the expanding Universe) where positive-negative energy mode mixing also happens. The project represents an interesting pedagogical illustration of many subtle points of relativistic quantum physics in non-inertial reference frames. Moreover, it may lay down a foundation for interesting research related to possible observational manifestations of cosmological zitterbewegung, e.g., in the Cosmic Microwave Background Radiation.


Particle Theory: Klein "paradox" with an accelerated potential barrier

Supervisor: Dr Archil Kobakhidze
Contact: Dr Archil Kobakhidze
Email: archil.kobakhidze AT coepp.org.au
Phone: (03) 8344 5439

The famous Klein paradox of relativistic quantum mechanics finds a natural resolution within quantum field theory due to particle production from the vacuum by the potential barrier. We will study this phenomenon for the barrier moving with a constant acceleration. Besides the pedagogical significance of gaining an understanding of advanced topics in relativistic quantum mechanics, this investigation will contribute to uncovering previously unaccounted for effects in the dynamics of the electroweak phase transition in relation to the generation of the observed dominance of matter over anti-matter in our Universe.


Particle Theory: Non-standard implementation of the Higgs boson

Supervisor: Dr Archil Kobakhidze
Contact: Dr Archil Kobakhidze
Email: archil.kobakhidze AT coepp.org.au
Phone: (03) 8344 5439

Recent experiments at the Large Hadron Collider provide a tentative evidence for the existence of a Higgs boson-like particle with mass ~125 GeV. Currently available data, although statistically compatible with the Standard Model predictions, somewhat favour a non-standard scenario with enhanced h -> γ γ decays and suppressed h -> WW, ZZ decays. In this project we will study such a non-standard Higgs boson within an alternative theoretical framework where the Higgs particle is a singlet of the non-linearly realised electroweak symmetry.


ATLAS Experiment: Supersymmetry and the new measured limits

Supervisors: Dr Aldo Saavedra & Dr Geng-Yuan Jeng
Contact: Dr Aldo Saavedra
Room: 366
Email: a.saavedra AT physics.usyd.edu.au
Phone: 9351 5970

So far, the searches for physics beyond the Standard Model performed by the Large Hadron Collider (LHC) experiments have come up empty handed. Instead they have placed limits on the parameters that govern popular theoretical models such as gravity mediated supersymmetry. In 2012, helping to improve the sensitivity to new physics, the amount of data collected by the LHC experiments could be four or five times higher than in 2011, and an increase in the centre of mass energy of the proton collisions has been achieved. A study of theoretical models that include all of the new measured limits have been shown to be kinematically favourable to the production of tau leptons. In this project we will explore how they can be distinguished from Standard Model processes using a number of experimental techniques.


ATLAS Experiment: Searching for New Physics in the Third Generation

Supervisors: Dr Aldo Saavedra and Dr Geng-Yuan Jeng
Contact: Dr Aldo Saavedra
Room: 366
Email: a.saavedra AT physics.usyd.edu.au
Phone: 9351 5970

A common feature of many supersymmetric (SUSY) models is that third generation sparticles are lighter than those in the second and first generation. If they exist, they would have a higher production rate than the other sparticles in the proton collisions at the LHC. These sparticles decay into their respective third generation Standard Model particles such as b quarks and tau leptons which are then detected and reconstructed by the ATLAS detector. In this project we incorporate both signatures to select SUSY events and study the different Standard Model processes possessing the same signature or alternatively capable of faking it. As in the previous project a number of techniques will be studied to discriminate between the fake and real signatures from Standard Model processes. The performance of the analysis will be evaluated using simulated and real LHC data.


ATLAS Experiment: Improving the ATLAS Tau Trigger for higher energy running

Supervisors: Dr Aldo Saavedra and Dr Geng-Yuan Jeng
Contact: Dr Aldo Saavedra
Room: 366
Email: a.saavedra AT physics.usyd.edu.au
Phone: 9351 5970

One of the major challenges for the ATLAS experiment, or any Large Hadron Collider (LHC) experiment, is which collisions to save for analysis. At the moment the LHC is colliding protons every 50ns, so decisions need to be snappy. The decision mainly rests on the composition of the event and the properties of the reconstructed particles found within the event. The trigger aims to keep the purity of the collected sample high and to be as efficient as possible. For the tau trigger, which specialises in selecting events where a tau lepton has decayed hadronically, the challenge is to reject bunches of pions that originate from strong interactions known as jets, which can fake the tau decay.

As the centre of mass energy of the LHC increases, (7 TeV in 2011, 8 TeV 2012 and 14 TeV in 2015), this becomes a major problem for collecting events with which to search for the Higgs boson or probe for new physics such as Supersymmetry. In this project a strategy to cope with the expected increase in jet production will be explored. This may include selecting events with a particular topology and/or improving the identification of hadronically decaying taus. The project will involve looking at simulated and real data from 2011 and 2012 runs to test the new algorithms.


Belle Experiment: Semileptonic decays of B mesons to neutron-antineutron pairs

Supervisors: A/Prof. Kevin Varvell and Dr Alexei Sibidanov
Contact: A/Prof. Kevin Varvell
Room: 344
Email: k.varvell AT physics.usyd.edu.au
Phone: 9351 2539

B mesons (bound states of a b quark and a lighter quark) only decay rarely to pairs of baryons. In the case of the decay B- -> p pbar l- ν, where l is an electron or muon, it has recently been suggested that the decay is maybe not as rare as first thought, and it is currently being searched for in the data of the Belle experiment at KEK in Japan. In this project we consider the related decay B- -> n nbar l- ν, which has not been searched for, and ask the question as to whether it has any observable consequences in the context of the Belle experiment.