PhD defense by Pavel Jez – University of Copenhagen

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PhD defense by Pavel Jez

A search for supersymmetric low mass Higgs boson at the LHC with the ATLAS detector


The origin of mass is one of the big unanswered question in Physics. The current theory – the Standard Model – explains the origin of masses with so-called Higgs mechanism, that is manifested by the existence of a special particle called Higgs boson. The search for this particle is one of the main quests of the experiments at the Large Hadron Collider in CERN. We think, however, that the Standard Model is not the final picture; that there is some theory Beyond Standard Model that can describe the elementary particles better.


One of those theoretical extensions of the Standard Model is Supersymmetry – a symmetry between elementary fermions and bosons that is characterized by the presence of many Higgs bosons. It is possible that one of those Higgs bosons will be very light (called a1) – this will be very interesting because it would provide explanation of some experimental results that do not fit into the Standard Model. Additionally, it is possible to produce this very light a1 directly in proton-proton collision at LHC with a large probability. Search for such a particle is one of the topics of the thesis. No evidence for its existence have been observed and the result of the thesis allowed to set new limitations on theories with very light Higgs boson. Setting of a new limits in previously uncharted region was possible because the decay of a1 to tau leptons were used.


Therefore also the ways of how to improve online selection of tau leptons in the ATLAS experiment were studied, with a focus on scenarios with multiple interactions every event (“pile-up”). The pileup will be very important in 2012 and the conclusions of the study from this thesis will be used to optimize the online tau lepton selection.