Search for Higgs Boson Pair Production in the Multiple Lepton Final State Using Proton-Proton Collision Data at $√s$ = 13 TeV from the ATLAS Detector

In this thesis, a search is presented for the production of Higgs boson pairs in final states containing three light leptons ($e/\mu$). The analysis utilizes data collected by the ATLAS detector at the Large Hadron Collider from proton-proton ($pp$) collisions at a center-of-mass energy of $\sqrt{s}=13\ $TeV, corresponding to an integrated luminosity of 140 $fb^{-1}$. The investigation aims to identify any significant deviations from the expected background levels defined by the Standard Model. The Higgs boson, as the cornerstone of the electroweak symmetry breaking mechanism, plays a crucial role in our understanding of particle physics. Studying the production of Higgs boson pairs is of particular interest as it provides insight into the self-interactions of this fundamental particle. After an extensive study, no significant excess above the anticipated background expectations was observed in the analyzed dataset. However, upper limits on the signal strength of the Higgs boson pair production process was established at a confidence level of $95\%$. The observed (expected) limit on the signal strength was found to be 20 (28) times the Standard Model cross section.