In 2018, with up to 80 fb−1 of data at √s = 13 TeV, ATLAS observed Higgs boson production in association with a pair of top quarks (ttH) with a significance of 5.8σ by combining measurements in the γγ, bb, ZZ, and multi-lepton Higgs decay channels. The ttH production cross-section was measured to be 670 ± 90 (stat) +110 -100 (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1σ.
With 140 fb−1, a search for non-resonant Higgs pair production in the bbγγ final state was performed. No significant signal was observed and upper limits at 95% confidence level were set. The observed limit on the SM cross-section was 130fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7].
Both the ttH (H → γγ) and HH → bbγγ analyses will benefit tremendously from the increased statistics expected from the High-Luminosity LHC (HL-LHC). To ensure the continued efficiency of the detector in the harsh HL-LHC environment, ATLAS will install a new Inner Tracker (ITk) consisting of silicon pixel sensors in its innermost layer. At SLAC National Accelerator Laboratory, a variety of electrical tests are performed for the construction of a prototype integrated pixel system, in order to provide early feedback and validation of the ITk design.