Observation of four top quarks production and EFT interpretations in the multi-lepton final state with the ATLAS Experiment

by Sreelakshmi Sindhu
Doctoral thesis
Date of Examination:2024-10-29
Date of issue:2025-03-27
Advisor:Prof. Dr. Arnulf Quadt
Referee:Dr. Frederic Deliot
Persistent Address: http://dx.doi.org/10.53846/goediss-11120

 

 

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Abstract

English

The Standard Model has been remarkably successful in describing fundamental particles and their interactions. However its incompleteness makes the search for physics beyond the Standard Model one of the most important challenges in particle physics. One promising avenue for uncovering potential deviations from the Standard Model is through the study of rare processes in multi-lepton final states, which offer a rare and distinct signal with manageable backgrounds from Standard Model processes. This thesis presents a comprehensive study of top-quark production and interactions in proton-proton collisions, focusing on the observation of four-top-quark ($t\bar{t}t\bar{t}$) production and interpretations using the framework of Effective Field Theory. The analyses are performed using data corresponding to an integrated luminosity of 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV, recorded by the ATLAS experiment at the Large Hadron Collider. Events with two same-sign leptons or at least three leptons (electrons or muons) are selected. Through the use of multivariate techniques and control regions to constrain backgrounds, the $t\bar{t}t\bar{t}$ production cross section is measured to be $22.5^{+6.6}_{-5.5}$ fb, consistent with the Standard Model prediction within 1.8 standard deviations. This thesis also explores the Standard Model Effective Field Theory (SMEFT) interpretation of the measurement of top quark production in association with a boson using the same data and final states. The study is based on the recent $t\bar{t}W$ cross-section measurement by ATLAS which confirms an excess in $t\bar{t}W$ like events consistent with several previous results. Limits are extracted on the Wilson Coefficients of 21 SMEFT operators using detector-level profile likelihood fits.
Keywords: particle physics; top quark; four tops; EFT; ATLAS; Large Hadron Collider
 
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