Olympia Dartsi – Phd’s defense – oct. 3, 2019

The physics subject of my thesis work, is the measurement of the cross-section of the diboson Zgamma production in association with a high mass dijetsystem. Proton-proton collision data from the ATLAS experiment at theCERN Large Hadron Collider at a center-of-mass energy of 13 TeV,collected in 2015 and 2016 and corresponding to an integrated luminosity of36.1 fb^{-1} are analyzed. The electroweak production of Zgammajj events, provides a direct access to the nature of the electroweak symmetry breaking mechanism, by probing the quantum gauge boson couplings WWZgamma, ZZZgamma, ZZgammagamma and Zgammagammagamma. The last three are forbidden at the lowest order in the Standard Model. Any deviation from the Standard Model predictions could be a hint for new physics.Until now, the electroweak production of Zgammajj, which consist in pro-cesses with fourth-order electroweak coupling and include vector-boson scat-tering processes, has never been observed. The same Zgammajj final state canbe produced by a strongly-mediated process, with second order electroweakcoupling and second order strong coupling, resulting in a cross-section threeorder of magnitude larger than the electroweak cross-section. The Zgammajj pro-duction via electroweak and strong mechanism interfere since the initial andfinal states are the same. The understanding of this effect is an importantaspect of the analysis.

In my thesis, I have studied this effect and its impact on the observationof the process and on the fiducial cross-section measurements. For this inves-tigation, after having performed the computation of the three contributions(electroweak, strong and interference) to the total cross-section using MonteCarlo simulations I have performed an optimization of the selection cuts, in order to reduce the impact of the interference, defining also a procedure onhow to account for the interference in the data analysis. The experimentalfocus of my work is the measurement of the differential cross section of theZgammajj final state as function of the invariant mass of the Zgamma system, the transverse momentum of the photon, the jet multiplicity and the invariantmass of the dijet system. Distributions sensitive to new physics are unfoldedand compared with MC expectations.All physics results rely on the reconstruction and selection of final statesconsisting of jets, electrons, muon and photons. Electrons and their identi-fication therefore play an important role in the Zgammajj analysis. This thesisreport documents also the work that I have done on the electron identification efficiency, including the implementation of an additional data drivenmethod. The results I have obtained, are used in all ATLAS analyses whosesignature contain electrons.