PhD defense by Kristian Gregersen – University of Copenhagen

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PhD defense by Kristian Gregersen

Title: Anomalous trilinear gauge coupling in ZZ production at the ATLAS experiment

The Standard Model of Particle Physics (SM) describes matter and forces at a fundamental level. Its building blocks are elementary particles which can interact according to certain patterns governed by underlying symmetry principles. Even though the SM has been extremely successful in describing phenomena in experimental particle physics in the last 50 years, most recently exemplified by the discovery of a Higgs boson in 2012, it turns out that it cannot constitute a final theory of the Universe for several reasons.

This thesis explores the opportunities for discovering physics beyond the SM through a model independent search for novel self-interactions among the neutral gauge bosons. Observed deviations from the allowed interactions suggest that the symmetries contained in the SM are incomplete and can be seen as an indication of new physics operating at a higher energy scale. The potential signal is modelled with an effective Lagrangian in which the self-interactions are parametrised with anomalous trilinear gauge couplings (aTGCs). The search is carried out in the ZZ channel where the Z bosons decays to electrons or muons.

The work include developing a signal modelling technique using event by event matrix element reweighting, an evaluation of the sensitivity to aTGCs of a vast range of observables, an extensive study on different statistical approaches to limit setting in the context of aTGCs, and finally a data analysis using the data collected during 2012 with the ATLAS experiment. No deviations from the SM expectation are seen, and limits are set on the aTGCs at a 95% confidence level.