PhD defence - Babar Ali

Feb. 28th, 2020 - Feb. 28th, 2020

On February 28, 2020, Ph.D. defense of our colleague, Babar Ali, took place at the Faculty of Nuclear Sciences and Physical Engineering at the Czech Technical University in Prague. The topic of the thesis is "Search for the Standard Model Higgs boson produced in association with a top quark pair in multilepton final states at √s = 13 TeV with the ATLAS detector".

This work was carried out within the ATLAS Collaboration in supervision of Ivan Štekl (Director of IEAP) and André Sopczak.



This thesis presents the results of a search for the Standard Model Higgs boson produced in association with a pair of top quarks, tt̅H, in multilepton final states. The search is performed using a dataset corresponding to an integrated luminosity of 36.1 fb-1 of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy of √s = 13 TeV at the Large Hadron Collider in 2015 and 2016. The search targets Higgs boson to WW*, tt, and ZZ* decay modes. Seven final states, categorized by the number and flavor of the charged leptons, are analyzed in the search for tt̅H production.

The thesis focuses on the analysis of the final state with exactly two same-charge light leptons and one hadronically decaying tau lepton, 2ℓSS1thad. The multivariate technique boosted decision tree is used to discriminate the tt̅H signal from backgrounds, the latter being dominated by tt̅ production with leptons from heavy-flavor hadron decays. A data-driven method is used to estimate the reducible background events. A profile likelihood fit is performed to extract the ratio of the measured tt̅H signal cross-section to the Standard Model prediction. The observed best-fit value of tt̅H is 3.5−1.3+1.7, with an observed (expected) significance of 3.4 (1.1) standard deviations. A cross-check analysis using a simple cut-and-count categorization is also reported.

The combination of all seven tt̅H multilepton final states gives the best-fit value of the tt̅H signal strength of tt̅H = 1.6−0.4+0.5. An excess of events over the expected Standard Model background is found with an observed (expected) significance of 4.1 (2.8) standard deviations. The cross section for tt̅H production corresponding to the best-fit value of the signal strength is 790−210+230 fb, as compared to the Standard Model prediction of 507−50+35 fb.

Furthermore, the performance of the ATLAS tau trigger is presented based on the 2015 proton-proton collision data recorded by the ATLAS experiment at √s = 13 TeV. A study of the Medipix-2 silicon pixel devices used as a luminosity monitoring system is also presented for the data taken in 2012 at √s = 8 TeV.


Keywords: Higgs Boson, Top Quark, Yukawa Coupling, Multilepton, MPX, Luminosity, Tau Trigger, ATLAS, LHC CERN.

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