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 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 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 $ZZ\to 4\mathrm{\ell }$ and $ZZ\to 2\mathrm{\ell }2\nu$ with $\mathrm{\ell }=e$ or $\mu$. In the $ZZ\to 4\mathrm{\ell }$ final state, a dense Neural Network is used to enhance analysis sensitivity with respect to matrix element-based discrimination. 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 $4.4_{-2.2}^{+3.0}$ MeV, and the observed (expected) upper limit on the total width is found to be 10.1 (10.8) MeV at 95% confidence level.