Evidence of the off-shell Higgs, Higgs decay width and the EFT interpretation at ATLAS

A search for off-shell production of the Higgs boson using 139 fb^{-1} of pp collision data at s√= 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The signature is a pair of Z bosons, with contributions from both the production and subsequent decay of a virtual Higgs boson and the interference of that process with other processes. The two observable final states are ZZ→4ℓ and ZZ→2ℓ2ν with ℓ=e or μ. In the ZZ→4ℓ final state, a dense Neural Network is used to enhance analysis sensitivity. The background-only hypothesis is rejected with an observed (expected) significance of 3.3 (2.2) standard deviations, representing experimental evidence for off-shell Higgs boson production. Assuming that no new particles enter the production of the virtual Higgs boson, its total width can be deduced from the measurement of its off-shell production cross-section. The measured total width of the Higgs boson is 4.5+3.3−2.5 MeV, and the observed (expected) upper limit on the total width is found to be 10.5 (10.9) MeV at 95% confidence level. $\newline$ Beside, off-shell Higgs boson events with masses of the order of 1 TeV are sensitive to phenomena beyond the Standard Model. Furthermore, the Higgs boson coupling to the top quark and its effective coupling to the gluon are degenerate in the inclusive on-shell measurements while the off-shell events can break the degeneracy. EFT operators affecting these couplings are studied and their associated Wilson coefficients are observed (expected) to be [-9, 18] ([-9, 17]) for $c_{t\varphi}$ (Higgs-top coupling modifier) and [-0.04, 0.03] ([-0.04, 0.03]) for $c_{\varphi G}$ (Higgs-gluon coupling modifier) at the 95% confidence level with new physics energy scale $\Lambda = 1$ TeV.