Searches for Higgs pair production and probing the Higgs self couplings in the HH→bbττ decay channel at the ATLAS Experiment and performance studies for the High-Granularity Timing Detector for ATLAS phase-2 upgrade

This study focuses on searches for non-resonant Higgs boson pair production in the HH → b ̄bτ+τ− channel using 140 fb−1 of proton-proton collisions at a centre- of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider (LHC). The analysis strategy aims to probe anomalous values of the Higgs boson (H ) self-coupling modifier κλ and quartic H H V V (V = W, Z ) coupling modifier κ2V . However, No significant excess above the expected back- ground from Standard Model (SM) processes is observed. Observed (expected) upper limit at 95% confidence-level on the di-Higgs boson production rate is set at 5.9 (3.1) times the SM prediction. The coupling modifiers are constrained within an observed (expected) 95% confidence interval of −3.2 < κλ < 9.1 (−2.4 < κλ < 9.2) and −0.5 < κ2V < 2.7 (−0.2 < κ2V < 2.4), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The study also includes performance evaluations of Low Gain Avalanche Detectors (LGADs) with a 50 μm active thickness using testbeam data collected at CERN SPS and DESY between 2018 and 2019, focusing on the High-Granularity Timing Detector (HGTD) for the ATLAS phase-2 upgrade. The HGTD aims to enhance particle-vertex assignments by precisely measuring track time with resolutions ranging from approximately 30 ps to 50 ps, thereby mitigating pile-up effects during the High-Luminosity phase of the LHC operations.