Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector

Doctoral Dissertation uoadl:2920660 7 Read counter

Department of Physics
Library of the School of Science
Deposit date:
Bellos Panagiotis
Dissertation committee:
Θεόδωρος Αλεξόπουλος, Καθηγητής, ΕΜΠ
Κωνσταντίνος Βελλίδης, Αναπ. Καθηγητής, ΕΚΠΑ
Θεόδωρος Γέραλης, Ερευνητής Α', ΕΚΕΦΕ Δημόκριτος
Χριστίνα Κουρκουμέλη, Ομότιμη Καθηγήτρια, ΕΚΠΑ
Κωνσταντίνος Νικολόπουλος, Καθηγητής, Πανεπιστήμιο Birmingham
Βασίλειος Σπανός, Αναπ. Καθηγητής, ΕΚΠΑ
Δημήτριος Φασουλιώτης, Αναπ. Καθηγητης, ΕΚΠΑ
Original Title:
Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector
Translated title:
Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector
The discovery of Higgs boson in 2012 by the ATLAS and CMS experiments marked a key milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model (SM), the theory that describes our present understanding of elementary particles and their interactions. The Higgs boson was the last particle of the SM which was observed. After its observation the research in in the ATLAS experiment focused on the measurement of its properties and also on the effort to observe particles beyond the SM. During Run II (2015-2018) it collected data corresponding to about 140 /fb, having a key role in these studies. This thesis presents a study for search of new high mass Higgs-like bosons as well a study for the measurement of Higgs boson decay width. It presents also the data driven estimation of the reducible background for these studies.

The first chapter (1) is a short theoretical introduction to the SM. The particles and the way that their interactions emerge are described. The Higgs mechanism is introduced as a way to retain the invariance of a Lagrangian with massive bosons. The mechanism can be extended to give mass to the fermions too. Afterwards the Higgs boson production and decay modes are described. Except the SM Higgs boson, additional Higgs-like bosons are predicted by beyond the SM models. Lastly an innovative way to measure Higgs boson decay width is presented.

In the second chapter (2) the ATLAS experiment is described, after a short introduction about about CERN and LHC. The ATLAS experiment consists of 3 main sub-detector systems, the Inner Detector (ID), the Calorimeters and the Muon Spectrometer (MS). The ID goal is to reconstruct charge particle tracks close to the interaction point. The Calorimeters measure the energy which is deposit in them by electrons/photons and hadrons. Lastly the MS reconstructs muon tracks. A trigger and a data acquisition system are used in order to choose and store the interesting event collisions. The ATLAS detector will be undergone two major upgrades. Soon the inner wheel of the end-cap MS will be replaced by the New Small Wheel. Also during Upgrade phase II the inner detector will be replaced by the New Inner Tracker (ITk).

In the third chapter (3) a study for electron identification in the forward region is described. The new ITk will extend pseudorapidity coverage up to |η| = 4. An identification method for the electrons in the region 2.5 < |η| < 4.0 was developed. It uses track-cluster matching criteria and a Artificial Neural Network to separate the true electrons from the fakes, which come either from pile-up or jets. Lastly the potential of improving identification by using an Isolation variable is studied.

In the forth chapter (4) the event selection and the background estimation in the 4-lepton channel are described. The description focuses on the Data Driven Estimation of the llμμ reducible background. The estimation is performed in 4 Control Regions (CRs). Each of them is enriched in a specific background component. The 4 CRs are linked via a fifth region, called Relaxed Region (RR). A simultaneous likelihood fit is performed in the 4 CRs and the results are extrapolated to the RR via Fractions. The results from the RR are subsequently extrapolated to the Signal Region via Transfer Factors (TFs). The TFs are estimated and controlled using a sample containing Z+μ events.

In the fifth chapter (5) the search for a high mass ZZ resonance is described. In this analysis a cut based selection is used to categorized the events. The signal and the ZZ backgrounds are modeled with analytic functions, while the reducible background is data driven estimated. The methodology of the Likelihood fit and the extraction of CLs limits is described in detail. Lastly the final upper limits for the production of a higgs-like particle are presented.

In the sixth chapter (6) the measurement of the Higgs boson decay width is described. For this measurement, a measurement of the Higgs boson off-shell cross section is required. For this study the MC samples are corrected with the latest k-factors. Then in additional to the nominal event selection a Matrix Element based discriminant is used to suppress the ZZ background. The reducible background is estimated in a Data Driven way. Finally the results are combined with these of the llμμ channel to set limits on the decay width.
Main subject category:
Higgs boson, ATLAS experiment, electron identification, decay width, data driven estimation, search for heavy resonances
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