4th Year Projects in 2010

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ATLAS Experiment: Measuring the rate of top decays into W → τ ντ

Supervisors: Dr Aldo Saavedra and Dr Kevin Varvell
Contact: Dr Aldo Saavedra
Room: 366
Email: A.Saavedra AT physics.usyd.edu.au
Phone: 9351 5970

The Large Hadron Collider, the premier particle accelerator of the European Particle Physics Laboratory (CERN), is scheduled to start colliding protons in November 2009. The starting centre of mass energy will be 7 TeV and will be increased to 10 TeV during the initial run that is expected to end late 2010. The stated energy is below its full capability of 14 TeV but higher than any previous attempts by a proton collider, the current record being 1.96 TeV which is held by the Tevatron, a proton-antiproton collider near Chicago, USA.

At 7 TeV there is enough energy for a large number of different interactions to take place during the collision. It is expected that a few thousand events should feature the production of a pair of top and anti-top quarks which is usually referred to as a ttbar event. Currently these are only produced at the Tevatron in limited numbers. The top quark was discovered in 1995 and due to its high mass (175 GeV/c2), it does not form mesons but readily decays into W bosons and b-jets. In this project the focus will be on extracting and measuring the properties of events where the W decays into a τ lepton and its associated neutrino.

The motivation of the project is two-fold: (1) To measure the rate and kinematics of the process for the first time with the ATLAS detector and to (2) to determine how to identify τ leptons with high transverse momentum. This is essential when searching for the Charged Higgs boson which appears in extensions of the Standard Model of Particle Physics and decays in a similar manner to the W boson.

The project will involve using software developed by the ATLAS collaboration to access the reconstructed data and ROOT (a the high energy physics software package) to analyse and display the results of the study.


ATLAS Experiment: Measuring the Fake Rate of the Tau Trigger in first ATLAS data

Supervisors: Dr Aldo Saavedra and Dr Kevin Varvell
Contact: Dr Aldo Saavedra
Room: 366
Email: A.Saavedra AT physics.usyd.edu.au
Phone: 9351 5970

The decay of the τ lepton (mass 1.7 GeV/c2) has two types of decay modes that are called leptonic or hadronic. The leptonic mode refers to when the τ decays into either a single muon or electron and two neutrinos. The hadronic mode involves the τ decaying into a number of mesons and an associated τ neutrino. The two most popular channels of the hadronic mode are the decay into a single charged pion (one prong) and into three charged pions (three prong). Heavier particles produced in ATLAS will readily decay into taus and decays of high transverse momentum taus will produce a striking signature within the ATLAS detector. This means that the τ will be an important probe for new physics - important enough for its own trigger to have been developed. A trigger is a means of reducing the prohibitively large event rate inside ATLAS to a manageable rate which can be recorded for later analysis.

One of the tasks to be accomplished with the first data collected by the LHC will be to determine the fake rate of the Tau Trigger. This is achieved by studying events where there are no τ leptons created but other decaying particles that can mimic its signature were. It is this sort of event the trigger needs to reject for the detector to have any chance of discovering new physics. The aim of this project is to tune the trigger parameters with first data from the ATLAS experiment rather than just relying on Monte-Carlo simulated events.

The study will focus on processes such as the production of two jets and ttbar events (see project above). A jet refers to the large number of particles created during the hadronisation of a quark produced during the proton-proton collision. These are collimated and will deposit most of the jet energy in the ATLAS Calorimeter system. The project will involve learning about the tier system of the ATLAS tau trigger and determining its performance against other physics triggers and the data that is processed afterwards. Software developed by the collaboration will be used for access the data together with ROOT, a high energy physics software package, to analyse and display the results.


ATLAS Experiment: Lepton Flavour Violation in Supersymmetry

Supervisors: Dr Aldo Saavedra and Dr Kevin Varvell
Contact: Dr Aldo Saavedra
Room: 366
Email: A.Saavedra AT physics.usyd.edu.au
Phone: 9351 5970

Supersymmetry (SUSY) is an attractive extension of the Standard Model which solves some of its shortcomings. Whilst there is no current evidence for SUSY, there are arguments that suggest that the masses of the lightest SUSY particles should lie in the TeV range and hence be accessible to the new generation of experiments at the Large Hadron Collider (LHC). In some SUSY scenarios, lepton number violation is allowed in sparticle decays. In this project, we will consider a specific scenario in which tau lepton number is violated, and examine the ability of ATLAS to detect a signal for such a process and disentangle it from Standard Model backgrounds.

The starting point of the project will be the results obtained on the two Supersymmetric scenarios that were covered in a 2009 honours project. The aim of this project will be to further refine the results obtained in 2009 and look at the Lepton Flavour violation scenario to determine the level of violation that the ATLAS experiment is sensitive to. The supersymmetric decays that the project deals with are related to the neutralino (a half-integral spin neutral supersymmetric particle), stau (the super partner of the τ lepton) and the smuon ( the super partner of the μ lepton). The neutralino, because of its weakly interacting nature, is a candidate for Dark Matter.

If time permits there is the chance of studying some of the background processes that should feature in the data collected from the first LHC run. This will enable the tuning of the selection criteria developed from the Monte-Carlo study. This project will involve learning about Supersymmetry specific software that calculates the mass and lifetimes of the Supersymmetric particle spectrum, the ATLAS sofware to access the data and the CERN ROOT package for the analysis and display of the results.


ATLAS Experiment: Quarkonium states in first data from ATLAS

Supervisor: Dr Bruce Yabsley
Contact: Dr Bruce Yabsley
Room: 366
Email: B.Yabsley AT physics.usyd.edu.au
Phone: 9351 5970

The quarkonia — bound states of a heavy quark and its antiquark — are among the most important particles in experimental work, and have played a key role in the development of the Standard Model. In particular, the lowest lying vector states, the J/ψ and the Υ(1S), have a very clean experimental signature (→ μ+ μ-), are ideal for detector calibration, and serve as important signals for other physics processes. They will be copiously produced in pp collisions at the Large Hadron Collider, and will accessible in the first data taken by the ATLAS experiment. In this project we will use data from early ATLAS running to find J/ψ → μ+ μ- and Υ(1S) → μ+ μ- decays, and use them as a tool for a range of physics studies.


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

Supervisors: Dr Bruce Yabsley and Dr Aldo Saavedra
Contact: Dr Bruce Yabsley
Room: 366
Email: B.Yabsley AT physics.usyd.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. Dimuon decays μ+μ- of quarkonium and other states are an established area of work at ATLAS (see previous project); di-electron decays e+ e- have been less well-studied. Multi-electron final states present a rich and interesting set of technical challenges such as particle tracking, bremsstrahlung recovery, and simulation of the experimental "trigger" that initiates data readout; and they have systematics largely independent of those for multi-muon states. 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, which will be plentiful in data from the early running of the Large Hadron Collider.


Belle Experiment: Full Reconstruction of B mesons in an upgraded Belle Detector

Supervisor: Dr Kevin Varvell
Contact: Dr Kevin Varvell
Room: 355
Email: K.Varvell AT physics.usyd.edu.au
Phone: 9351 2539

The Belle experiment in Japan is just completing a successful decade of data taking in which it has collected over 800 million decays of pairs of B mesons. It will then commence an upgrade of the accelerator and detector (the new detector will be called Belle II) with the idea of increasing this data set by a factor of 50. Our group has been involved in the first phase of Belle and may participate in the next phase. If we do, a number of projects could present themselves, of which this is one example.

Searching for B meson decays to a particular final state involves selecting the correct particles from the B in amongst all the particles observed in the event. The complications are that two B mesons (a particle-antiparticle pair) are produced and one does not know a priori which particles came from which B. There is also the problem of eliminating backgrounds from events in which no B mesons were produced at all. If we have found a candidate B decay, we can try to see if the remaining particles in the event are consistent with coming from a corresponding antiparticle B meson, thus fully reconstructing the event. Whilst this is likely to be possible only in a small fraction of cases, such a sample of events ought to be essentially background free. Belle has utilised this "full recon tag" sample for a number of its studies, but the quality of the algorithm used to produce it is not ideal. We will have to do better to benefit fully from the much bigger event sample that Belle II will produce in the future.

This project will investigate ways of improving the efficiency, and importantly the purity (how often it gets it right) of this technique with an eye to Belle II, testing on selected decay modes in Belle data. It will involve the study of real and simulated data and will be computer analysis oriented. Some programming skills and knowledge (or desire to gain knowledge) of the C++ programming language would be helpful.