A step in the direction of the Higgs self-coupling – CERN Courier

A defining but unobserved property of the Higgs boson is its capacity to couple to itself. The ATLAS collaboration has now set new bounds on this interplay, by probing the uncommon manufacturing of Higgs-boson pairs. Since the self-coupling power immediately connects to the form of the Higgs potential, any departure from the Standard Model (SM) prediction would have direct implications for electroweak symmetry breaking and the early historical past of the universe. This makes its measurement some of the vital aims of contemporary particle physics.

Higgs-boson pair manufacturing is a thousand instances much less frequent than single-Higgs occasions, roughly similar to a single incidence each three trillion proton–proton collisions on the LHC. Observing such a uncommon course of calls for each huge datasets and extremely refined evaluation strategies, together with the cautious alternative of a delicate probe. Among the best is the HH → bbγγ channel, the place one Higgs boson decays right into a backside quark–antiquark pair and the opposite into two photons. This remaining state balances the statistical attain of the dominant Higgs decay to backside quarks with the exceptionally clear signature provided by photon-pair measurements. Despite the small sign branching ratio of about 0.26%, the decay to 2 photons advantages from the superb di-photon mass decision and presents the very best effectivity among the many main HH channels. This gives the HH → bbγγ channel with a superb sensitivity to variations within the trilinear self-coupling modifier κ_λ, outlined because the ratio of the measured Higgs-boson self-coupling to the SM prediction.

In its new examine, the ATLAS collaboration relied on Run 3 information collected between 2022 and 2024, and on the complete Run 2 dataset, reaching an built-in luminosity of 308 fb^–1. Events have been chosen with two high-quality photons and not less than two b-tagged jets, recognized utilizing the newest and most performant ATLAS b-tagging algorithm. To additional distinguish sign from background, dominated by non-resonant γγ+jets and single-Higgs manufacturing with H → γγ, a set of machine-learning classifiers known as “multivariate analysis discriminants” have been educated and used to filter real HH → bbγγ alerts.

The collaboration reported an HH → bbγγ sign significance of 0.84σ  below the background-only speculation, in comparison with a SM expectation of 1.01σ (see determine 1). At the 95% confidence degree, the self-coupling modifier was constrained to –1.7 < κ_λ < 6.6. These outcomes lengthen earlier Run 2 analyses and ship a considerably improved sensitivity, similar to the noticed (anticipated) significance of 0.4σ (1σ) within the mixed Run 2 outcomes throughout all channels. The enchancment is primarily as a result of adoption of superior b-tagging algorithms, refined evaluation strategies yielding higher mass decision and a bigger dataset, greater than double that of earlier research.

This consequence marks important progress within the seek for Higgs self-interactions on the LHC and highlights the potential of Run 3 information. With the complete Run 3 dataset and the High-Luminosity LHC on the horizon, ATLAS is ready to increase these measurements – enhancing our understanding of the Higgs boson and trying to find doable indicators of physics past the SM.