Evidence of off-shell Higgs boson production and constraints on the total width of the Higgs boson in the $ZZ\rightarrow 4\ell$ and $ZZ\rightarrow 2\ell2\nu$ decay channels with the ATLAS detector

This note reports a search for off-shell production of the Higgs boson using 139 fb$^{-1} $of $pp$ collision data at $\sqrt{s}=$ 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The observable signature is a pair of on-shell $Z$ bosons, with contributions both from the virtual Higgs boson and interference with other processes, and the two decay final states, $ZZ\rightarrow 4\ell$ and $ZZ\rightarrow 2\ell2\nu$ with $\ell = e$ or $\mu$. The background-only hypothesis is rejected with an observed (expected) significance of 3.2 (2.4) $\sigma$, which marks the experimental evidence of off-shell Higgs production. 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 95% confidence level in the asymptotic approximation.