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

Abstract

The total decay width of the Higgs boson, ${\Gamma }_H$, encodes all of its decay modes; any decay into particles beyond the Standard Model (SM) would increase ${\Gamma }_H$ above the SM prediction of 4.1 MeV. In this Letter, a direct, on-shell measurement of ${\Gamma }_H$ is reported, exploiting the interference between the resonant $H\to \gamma \gamma$ amplitude and the continuum diphoton background, which distorts the diphoton invariant mass distribution in a way that depends on ${\Gamma }_H$ 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 ${\Gamma }_H<307$ (258) MeV is set at 95% confidence level, approximately 75 (63) times the SM prediction. Unlike indirect off-shell constraints on ${\Gamma }_H$, 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.

33 pages in total, author list starting page 16, 3 figures, 0 tables, submitted to Phys. Rev. Lett. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGP-2024-12