Doctoral Dissertations

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


University of Chicago


Department of Physics

First Advisor

Mark Oreglia, Ph.D.


Higgs boson, neural network, tau, CERN, LEP, supersymmetry

Subject Categories

Elementary Particles and Fields and String Theory | Physical Sciences and Mathematics | Physics


Searches for neutral Higgs bosons have been performed with the OPAL detector at LEP. Approximately 170 /pb of e+e- collision data at a center-of-mass energy of 189 GeV have been used to search for the SM process e+e- to HZ as well as for the processes e+e- to hZ and e+e- to hA which occur in extended Higgs theories. The searches are sensitive to final states containing quarks and tau leptons, for which an artificial neural network for the identification of tau leptons was designed. The results have been combined with OPAL searches for other final states to obtain a 95% confidence level lower limit on the SM Higgs boson mass of 91.0 GeV/c^2. In a constrained MSSM scenario, the limits mh > 74.8 GeV/c^2 and mA > 76.5 GeV/c^2 are obtained at 95% CL assuming tan beta > 1, and values of tan beta between 0.72 and 2.19 are excluded at 95% CL for the case of zero scalar top mixing. The parameter spaces of the MSSM and general Type II Two Higgs Doublet Models are explored in detailed scans, and in addition, limits on the production of Higgs-like bosons outside the context of specific models are obtained.



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