Searching for Beyond the Standard Model Resonances in the $HH \to b\bar{b}b\bar{b}$ Final State Using the ATLAS Detector

An ATLAS analysis searching for Beyond the Standard Model resonances, $X$, that are produced and decay through the $pp\to X \to HH \to b\bar{b}b\bar{b}$ process is presented. These events are studied in the phase space region where the four $b$-quarks produce four well-separated jets. A significantly stronger upper-limit is set on the $pp \to X \to HH$ cross-section for these resonances than was set by the Early Run 2 ATLAS analysis. Above 350 GeV these limits are also stronger than those of the combination of all ATLAS di-Higgs searches with 27.5 fb$^{-1}$ to 36.1 fb$^{-1}$ of data. No statistically significant excesses are found, with the largest excess having a global significance of $1.2^{+0.4}_{-0.3}\sigma$. In particular, with an improved pairing algorithm and more sophisticated background modelling, the 2.8$\sigma$ excess found at 280 GeV by the Early Run 2 analysis is not confirmed. The results of this analysis suggest the upcoming version, which uses an evolution of the same technique on the same data to look for Standard Model di-Higgs production, will be able to set stringent limits on that process.