published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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Orientador: Paulo Leal Ferreira / Doutor

Orientador: Sergio Ferraz Novaes / Banca: Oscar José Pinto Éboli / Banca: Dominique Marie Spelaler / Mestre

Orientador: Paulo Leal Ferreira / Mestre

Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 113-122).

No description available.

We compute, to next-to-leading order in perturbation theory, the rate of energy loss suffered by a heavy quark moving nonrelativistically in the quark-gluon plasma. This quantity is essentially a measure of the scattering rates of this quark against plasma constituents, and of their efficiency in slowing it down. The next-to-leading order correction is sensitive to the physics of overlapping scatterings, as well as to the self-interactions of gauge fields with small momenta (of order the electric screening scale). We find the next-to-leading order correction to be remarkably large, suggesting that the perturbative series is unreliable unless the coupling constant of the theory assumes unrealistically small values.

This thesis concerns the properties of two particles, the top quark and the tau lepton, the heaviest of the quarks and leptons, respectively, in the standard model of elementary particle physics. After a chapter introducing the standard model, the second concerns the tau lepton. Amongst all the quarks and leptons, the tau has the unique property that its polarisation can readily be measured at high energy colliders. We propose and develop new methods of using this unique property to search for new physics, such as Higgs bosons, and also for making an accurate measurement of the polarisation of taus produced via Z decay leading to a precision determination of the electroweak coupling. The third chapter of this thesis concerns the top quark. We perform the complete one- loop radiative corrections to the helicity amplitudes within the standard model. Even though the radiative corrections to top quark production and decay have already been calculated separately, our work makes it possible to combine the sequence of events by keeping full information on the correlations among final state particles.