Evidence of the off-shell Higgs and constraints on Higgs decay width at ATLAS

These slides report on a search for off-shell production of the Higgs boson using 139 $\text{fb}^{-1}$ of $pp$ collision data at $\sqrt{s}=$ 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The signature is a pair of $Z$ bosons, with contributions both from 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 $4\ell$ and $2\ell2\nu$ with $\ell = e$ or $\mu$. The background-only hypothesis is rejected with an observed (expected) significance of 3.2 (2.4) $\sigma$, representing experimental evidence for off-shell Higgs production. The observed (expected) upper limit on the signal strength, defined as the event yield normalised to the Standard Model prediction is 2.3 (2.4) at a 95% confidence level, which restricts the total width of the Higgs boson to be less than 9.7 (10.2) MeV at the same confidence level. Under the assumption 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 $4.6^{+2.6}_{-2.5}$ MeV, and the observed (expected) upper limit on the total width is found to be 9.7 (10.2) MeV at a 95% confidence level in the asymptotic approximation.