Search for Resonant Production from $HH\rightarrow \bar{b}b\bar{b}b$ Final States in the ATLAS Detector Using Boosted and Superboosted Analyses

This thesis presents a search for resonant Higgs boson pair production in the $HH \rightarrow b\bar{b}b\bar{b}$ final state via gluon-gluon fusion (ggF), using 139~fb$^{-1}$ of ATLAS Run-2 data. The analysis is performed using two reconstruction strategies: the Boosted and Superboosted approaches. The primary objective is to search for new resonances ($X_R$) predicted by Beyond the Standard Model (BSM) theories, decaying into pairs of Higgs bosons. Two benchmark BSM theories are considered: excited massive Kaluza–Klein gravitons from the Randall–Sundrum model and heavy neutral scalar bosons from the Two-Higgs-Doublet Model (2HDM), both of which offer potential solutions to the hierarchy problem in the Standard Model (SM).

The resonant $HH$ searches in the $HH \rightarrow b\bar{b}b\bar{b}$ channel are organized into three reconstruction strategies: Resolved, Boosted, and Superboosted. The Resolved strategy targets Higgs boson pairs with mild to intermediate Lorentz boosts, covering resonance masses in the range 251--1600~GeV. The Boosted strategy focuses on intermediate to high-boost Higgs boson pairs, probing masses between 900--5000~GeV. The Superboosted strategy, newly developed for this work, is designed for extremely high-boost Higgs boson pairs, with sensitivity to resonance masses in the 1600--5000~GeV range.

The sensitivity to the process $\sigma(pp \rightarrow X_R \rightarrow HH \rightarrow b\bar{b}b\bar{b})$ is evaluated using the Boosted and Superboosted strategies with 139~fb$^{-1}$ of data. Results are expressed in terms of 95$\%$ confidence level (CL) upper limits on the production cross-section as a function of resonance mass. No significant excess is observed, and 95$\%$ CL upper limits are established. The combination of the Resolved and Boosted strategies reveals no significant deviation from SM predictions.