Evidence of off-shell Higgs boson production from $ZZ$ leptonic decay channels and constraints on its total width with the ATLAS detector

This Letter reports on a search for off-shell production of the Higgs bosonusing 139 $\textrm{fb}^{-1}$ of $pp$ collision data at $\sqrt{s}=$ 13 TeVcollected by the ATLAS detector at the Large Hadron Collider. The signature isa pair of $Z$ bosons, with contributions from both the production andsubsequent decay of a virtual Higgs boson and the interference of that processwith other processes. The two observable final states are $ZZ\rightarrow 4\ell$and $ZZ\rightarrow 2\ell2\nu$ with $\ell = e$ or $\mu$. In the $ZZ\rightarrow4\ell$ final state, a dense Neural Network is used to enhance analysissensitivity. The background-only hypothesis is rejected with an observed(expected) significance of 3.3 (2.2) standard deviations, representingexperimental evidence for off-shell Higgs boson production. Assuming that nonew particles enter the production of the virtual Higgs boson, its total widthcan 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 theobserved (expected) upper limit on the total width is found to be 10.5 (10.9)MeV at 95% confidence level.