This thesis develops the quantitative study of quantum entanglement in systems of identical particles. Understanding this topic is essential for the construction of quantum information processing devices involving identical particles. A brief overview of necessary concepts and methods, such as the density matrix, the entanglement in pure and mixed states of distinguishable particles, and some common applications of entanglement is given in the introduction. Some competing methods of calculating the entanglement in bipartite pure states of indistinguishable particles are examined. It is shown that only the 'site entropy' measure introduced by Zanardi satisfies all the criteria for a correct entanglement measure. A teleportation protocol which utilizes all the entanglement carried (in both the spin and space degrees of freedom) in a doubly- occupied molecular bonding orbital is presented. The output from an interferometer in a thought experiment described by Omar et al. is studied as an example to see whether entanglement can be separated into space-only, spin-only, and space-spin components. A similar exercise is performed for a doubly-occupied molecular bonding orbital. The relationship between these results and the application of superselection rules (SSRs) to the quantification of useful entanglement is discussed. A numerical method for estimating the entanglement of formation of a mixed state of arbitrary dimension by a conjugate gradient algorithm is described. The results of applying an implementation of the algorithm to both random and isotropic states of 2 qutrits (i.e. two three-dimensional systems) is described. Existing work on calculating entanglement between two sites in various spin systems is outlined. New methods for calculating the entanglement between two sites in various types of degenerate quantum gas - a Fermi gas, a Bose condensate, and a BCS superconductor - are described. The results of numerical studies of the entanglement in a normal metal and a BCS superconductor are reported, both with and without the application of superselection rules for local particle number conservation.

The Large Hadron Collider, due for completion at the end of 2007, will provide an essential piece in the puzzle of electroweak symmetry breaking. Precision measure ments from LEP predict a low mass Standard Model Higgs boson close to the current experimental lower limit. In this mass region identification of the Higgs boson will be difficult, requiring more than one channel to be studied. ttH0, H0 → bb is one of three channels proposed for this purpose. In order to reduce the large backgrounds full reconstruction of the two top-quarks in the event is necessary. This thesis inves tigates a number of new techniques to improve the reconstruction procedure of this channel using the fast simulation of the ATLAS detector, Atlfast. Methods inves tigated include the introduction of hadronic W candidates created from three jets combined with a new energy rescaling method for the W candidates. A new quantity to choose the jet pairings in the event, utilising jet and lepton energy uncertainties from Atlfast, is studied. An Artificial Neural Network is trained to select signal from background events. An increase in the expected signal significance from 1.8 to 2.4 for an integrated luminosity of 30fb_1 and Higgs mass of 120GeV is observed from the techniques proposed. An important tool for any high energy physics experiment is an event display program. Visual representation is the most efficient way to transfer data from a computer to the human brain, providing the user with a powerful tool to investigate specific events. The Atlantis event display for the ATLAS experiment is presented, along with the development carried out to enable its use in the 2004 Combined Test Beam. The test beam used a full slice of the detector to provide an essential study of the performance capabilities of ATLAS.

Measurements of the production cross section of W+W7- pairs in pp collisions at 1.96 TeV and limits on trilinear gauge boson coupling (TGC) parameters are presented. The data were recorded with the CDF experiment at Tevatron during the 2001 and 2002 data taking periods in which a total integrated luminosity of 184 pb-1 was collected. The data sample was filtered for events with two leptonic W boson decays where the charged leptons can be either electrons or muons. 17 events are ob served against an expected background of 5.01q 8 events. The resulting cross- section is found to be a(pp → W+W ) = 14.5l5 (stat)lJ g(syst) 0.9(lum) pb and agrees well with the Standard Model expectation. Limits on the TGC parameters Aac and A are set under both the equal coupling scheme, that assumes the W boson couples identically to the Z and 7, and the HISZ coupling scheme, that requires the couplings to respect SU(2)L x U(l)y gauge symmetry. In both cases this is achieved by using a likelihood fit to the lepton-PT distribution of the 17 candidate events. The resulting limits are found to be: -0.4 < Aac < +0.6 (A = 0) - 0.3 < A < +0.4 (An = 0) for the EQUAL couplings and -0.7 < Ak < +0.9 (A = 0) - 0.4 < A < +0.4 (An = 0) for the HISZ couplings.

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