Final PhD seminar: Data-driven background estimation and search results for Higgs boson pair production in the (b)(bbar)(tau+)(tau-) final state from ATLAS using Run 2 and Run 3 data

ATLAS has recently published a new search for non-resonant $pp\to HH$ production, using Run~3 data from 2022 and 2023, combined with the full Run~2 dataset. This analysis targets the $b\bar{b}\tau^{+}\tau^{-}$ final state. The observed Higgs boson pair production signal strength is $2.6^{+1.4}_{-1.0}$, which corresponds to an observed significance of 2.6 $\sigma$, compared to an expected significance of 1.2 $\sigma$ under the standard model (SM) hypothesis. Also, 95\% confidence intervals for the modifiers $\kappa_\lambda$ of the Higgs self-coupling and $\kappa_{2V}$ of the $HHVV$ coupling $(V=W,Z$) are derived: $\kappa_\lambda \in [-3.14, 1.6] \cup [5.5, 10.1]$ and $\kappa_{2V} \in [-0.2,2.4]$. The analysis is validated through auxiliary measurements of the $ZZ / ZH \to b\bar{b}\tau^{+}\tau^{-}$ processes. The measured signal strengths are consistent with the SM expectations, with an observed significance of 3.5 $\sigma$, and thereby the first evidence, for $ZH \to b\bar{b}\tau^{+}\tau^{-}$. The backgrounds are modeled using simulation and data-driven estimates. This talk presents the analysis strategy, the data-driven Fake Factor method used to model multijet backgrounds, as well as the results of the search.