Searching for New Physics Using di-Higgs Final States with the ATLAS Detector

Searches for new physics using di-Higgs final states are presented. These states are investigated in two contexts. The first analysis targets pairs of Higgs bosons decaying to $b\bar{b}b\bar{b}$, produced with missing energy from the decays of supersymmetric higgsinos. This follows up on an excess at 250 GeV observed using 24.3 fb$^{-1}$ of data and searches for higgsinos in the well-motivated low-mass regime. The second analysis targets pairs of Higgs bosons produced via the trilinear Higgs self-coupling and considers multiple final states, with an emphasis on the $b\bar{b}b\bar{b}$ channel. The searches are performed using 126 fb$^{-1}$ and up to 139 fb$^{-1}$ of data respectively in $pp$ collisions at $\sqrt{s}=$13 TeV with the ATLAS detector. The $b\bar{b}b\bar{b}$ backgrounds are dominated by difficult-to-model QCD multijet events and are estimated using novel data-driven approaches utilizing machine-learning techniques. No significant excess above the Standard Model is observed. For pure higgsino to Higgs boson decays, the supersymmetry analysis excludes higgsino masses up to 530 GeV at a 95\% confidence level. Interpretations in alternate scenarios as well model-independent limits are also presented. For the Higgs self-coupling analysis, the self-coupling parameter is constrained to $\lambda_{hhh}\in[-3.9\lambda_{hhh}^\text{SM},11.1\lambda_{hhh}^\text{SM}]$ at a 95\% confidence level for the $b\bar{b}b\bar{b}$ channel and $\lambda_{hhh}\in[-0.4\lambda_{hhh}^\text{SM},6.3\lambda_{hhh}^\text{SM}]$ for the combined channels.