Evidence of the off-shell Higgs, constraints on the Higgs decay width and the EFT interpretation

Ten years after the on-shell Higgs boson discovery, we have found evidence of the off-shell Higgs for the first time at ATLAS and provided the most precise Higgs width measurement.
This thesis presents the measurement of off-shell Higgs boson production using the ${139 fb^−1}$ of pp collision data with a centre-of-mass energy of 13 TeV, collected by the ATLAS detector at the Large Hadron Collider. The Higgs bosons decay via Z boson pairs and two final states are considered: $ ZZ \to 2ℓ2\nu$ and $ZZ \to 4ℓ$, where ℓ represents a muon or an electron. In $ZZ \to 4ℓ$, the neural network method is implemented to improve the Higgs signal sensitivity. The observed (expected) significance for rejecting the hypothesis of background-only is 3.3 $(2.2)\sigma $. The total Higgs width is constrained by measuring its off-shell cross-section. The observed Higgs width is $4.5_{-2.5}^{+3.3}$ MeV, and its upper limit is 10.5 (10.9) MeV for the observed (expected) result at a 95 % confidence level.
Moreover, off-shell Higgs boson events with masses on the order of 1 TeV are sensitive to phenomena beyond the Standard Model. Additionally, inclusive on-shell measurements cannot distinguish couplings between Higgs-top quark and Higgs-gluon interactions, but off-shell events can break this de- generacy. This thesis studies effective field theory (EFT) operators affecting
these couplings and provides the associated Wilson coefficients for Higgs-top and Higgs-gluon coupling modifiers.
Furthermore, this thesis describes the excellent performance of muon isolation across a broad range, covering the transverse momenta from 3 GeV to a few hundred GeV, and the acceptance of the entire muon detector.
The performance of the SemiConductor Tracker (SCT) during the Long Shutdown-2 and early Run-3, is evaluated and presented in this thesis, particularly radiation damage.