Higgs boson production with a top quark pair in the diphoton channel, double Higgs production in the bb-diphoton channel, and Inner Tracker upgrade with the ATLAS detector at the Large Hadron Collider

Since the discovery of the Higgs boson in 2012, the diphoton ($\gamma\gamma$) decay channel of the Higgs boson has been one of the most potent channels due to the contrast between the smoothly falling background and sharply peaked signal in the diphoton invariant mass. In 2018, with up to 80 fb$^{-1}$ of data at $\sqrt{s} = 13$ TeV, ATLAS observed Higgs boson production in association with a pair of top quarks ($t\bar{t}H$) with a significance of 5.8 $\sigma$ by combining measurements in the $\gamma\gamma$, $b\bar{b}$, $ZZ$, and multi-lepton Higgs decay channels. The $t\bar{t}H$ production cross-section was measured to be 670 $\pm$ 90 (stat) $^{+110}_{-100}$ (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1 $\sigma$. With 140 fb$^{-1}$, a search for non-resonant Higgs pair production in the $b\bar{b}\gamma\gamma$ 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 130 fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7]. Both the $t\bar{t}H (H \to \gamma\gamma$ and $HH \to b\bar{b}\gamma\gamma$ 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.