Constraints on the Higgs boson total width from on-shell signal-background interference in the $H\toγγ$ decay channel with $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The total decay width of the Higgs boson, $Γ_H$, encodes all of its decay modes; any decay into particles beyond the Standard Model (SM) would increase $Γ_H$ above the SM prediction of 4.1 MeV. In this Letter, a direct, on-shell measurement of $Γ_H$ is reported, exploiting the interference between the resonant $H \to γγ$ amplitude and the continuum diphoton background, which distorts the diphoton invariant mass distribution in a way that depends on $Γ_H$ without requiring assumptions on the Higgs boson couplings. Using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded by the ATLAS detector during Run 2 of the LHC, an observed (expected) upper limit of $Γ_H < 307$ (258) MeV is set at 95% confidence level, approximately 75 (63) times the SM prediction. Unlike indirect off-shell constraints on $Γ_H$, this measurement does not rely on assumptions about the Higgs boson couplings, providing a novel complementary probe sensitive to potential Higgs boson decays into new particles.