Higgs, the measurement of a lifetime Probing the Higgs Boson lifetime via its off-shell production and decay to W Boson pairs with the ATLAS experiment at the LHC

Progress in fundamental physics relies on the continual interplay between theory and experiment, where increasingly precise measurements can expose subtle deviations from established models and point toward deeper underlying principles. This thesis follows that paradigm by probing one of the most elusive observables in particle physics: the lifetime of the Higgs boson. Although the Standard Model has proven extraordinarily successful, it is known to be incomplete, and many proposed extensions predict measurable modifications to Higgs boson properties. In particular, the Higgs boson lifetime, or equivalently its total decay width, is highly sensitive to new physics beyond the Standard Model.
The Higgs boson lives for an extremely short period, complicating a direct measurement of its lifetime with current detector technology. Instead, this work exploits recent theoretical developments that enable indirect constraints via off-shell Higgs boson production and interference effects. A measurement of the Higgs boson total width is presented, derived from a combination of on-shell and off-shell analyses of Higgs boson decays to pairs of W bosons. The analysis uses the full Run 2 proton–proton collision dataset collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 inverse femtobarns at a center-of-mass energy of 13 TeV.
In addition to the Higgs width measurement, this thesis includes contributions to analysis and reconstruction techniques within the ATLAS experiment. These include the application of transformer-based machine learning models to charged particle tracking and the development and benchmarking of parallelized statistical tools, notably within the RooFit framework and the StatAnalysis software project. Together, these efforts advance both the precision of Higgs boson measurements and the methodological toolkit of experimental high-energy physics, preparing it for the prospective High-Luminosity Large Hadron Collider.