A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery

The Standard Model of particle physics describes fundamental particles and forces, excluding gravity. A key feature is the Higgs field, pervading space and interacting with particles. Its quantum excitation, the Higgs boson (a spinless particle), was observed in 2012 by ATLAS and CMS experiments at CERN's Large Hadron Collider. The strength of the Higgs boson's interaction (coupling) with other particles is defined by the particle's mass and type; massless particles like photons and gluons have no direct coupling. Three types of couplings exist: gauge coupling to weak force mediators (W and Z bosons); Yukawa interaction with matter particles (fermions); and self-coupling of the Higgs boson. These couplings, scaling with particle masses, are precisely predicted in the Standard Model. Experimental determination of these couplings provides crucial independent tests of the Standard Model and constraints on beyond-Standard-Model physics. Early measurements, while consistent with the Higgs boson hypothesis, had large uncertainties, leaving room for interpretations involving new phenomena. Run 2 data (2015-2018) at the LHC provided a dataset approximately 30 times larger than Run 1, allowing for significantly more precise measurements. This article utilizes the full Run 2 dataset (integrated luminosity of 139 inverse femtobarns) to analyze Higgs boson production and decay rates, thereby studying Higgs boson couplings.

The Standard Model, formulated through the contributions of Weinberg, Salam, Glashow, 't Hooft, and Veltman, has withstood numerous experimental tests. The Higgs mechanism, proposed by Higgs, Englert, Brout, Guralnik, Hagen, and Kibble, explains how particles acquire mass through interaction with the Higgs field. The 2012 discovery of the Higgs boson by ATLAS and CMS experiments confirmed a key prediction of this model. Subsequent analyses, using Run 1 data, demonstrated the boson's spin-0 nature and CP-even quantum state, excluding spin-1 and spin-2 hypotheses with high confidence. However, the limited statistics of Run 1 data left many Standard Model predictions untested and allowed for considerable room for beyond-Standard-Model interpretations. Previous combined analyses from ATLAS and CMS, using Run 1 and partial Run 2 data, provided initial constraints on Higgs boson couplings, but significantly improved precision was desired with the larger Run 2 dataset.

The ATLAS experiment at the LHC is a multipurpose detector with near 4π solid angle coverage, designed to identify various particles and measure their momenta and energies. It includes inner spectrometers, calorimeters, and a muon spectrometer. A two-level trigger system selects events of interest. Extensive software is used for data simulation, reconstruction, and analysis. The analysis combines multiple Higgs boson production and decay processes. The dominant production process is gluon-gluon fusion (ggF), followed by vector boson fusion (VBF), associated production with weak bosons (VH), and associated production with top quark pairs (ttH). The Higgs boson decays predominantly into b quarks, W bosons, Z bosons, tau leptons, and photons. The analysis considers six production processes: ggF, VBF, WH, ZH, ttH, and tH. Event classification, often using machine learning, identifies different production processes based on associated particle properties. The input measurements include decays into ZZ (to 4 leptons), WW (to 2 leptons and 2 neutrinos), γγ, Zγ, bb, ττ, μμ, and cc. The H→bb decay mode is particularly challenging due to a large multi-jet background. The analysis utilizes the full Run 2 dataset, except for some previous analyses that used partial datasets. Improvements in signal sensitivity (up to 50%) compared to expectations from the increased dataset size were achieved through better particle reconstruction, dedicated reconstruction of boosted H→bb decays, increased simulated events, finer kinematic region granularity, and improved signal and background predictions. A likelihood formalism is used for the statistical analysis, incorporating nuisance parameters to handle experimental and theoretical uncertainties. A profile likelihood ratio-based test statistic is used to test hypotheses and construct confidence intervals. The uncertainty is decomposed into statistical and systematic components.

The analysis yields a global signal strength modifier μ = 1.05 ± 0.06, consistent with the Standard Model prediction of 1. The uncertainty is significantly reduced compared to previous results. Higgs boson production cross-sections were measured for each process, with the ggF and VBF processes observed with 7% and 12% precision, respectively. WH, ZH production were observed with high significance. An upper limit was set on tH production. Branching fractions were measured for γγ, ZZ, WW, ττ, bb, μμ, and Zγ decays. The bb decay mode was observed with high significance. Coupling strength modifiers (κ) were measured. A model assuming a single scale factor for vector bosons and a second for all fermions was tested, with the results compatible with the Standard Model. A model with independent coupling strength modifiers for W, Z, t, b, c, τ, and μ, assuming no invisible or undetected decays, was also tested, showing compatibility with the Standard Model's predicted mass scaling of couplings. A model allowing invisible Higgs boson decays was also considered, providing upper limits on the invisible and undetected branching fractions. A study of kinematic properties of Higgs boson production, in 36 kinematic regions, revealed consistency with the Standard Model predictions, particularly at high Higgs boson transverse momenta. The couplings to the three heaviest fermions (top, bottom quark, tau lepton) were measured with uncertainties of 7-12%, while couplings to weak bosons (Z and W) were measured with 5% uncertainty. Indications of rare Higgs boson decays into second-generation fermions and Zγ were found.

The results show excellent agreement with the Standard Model predictions across a wide range of Higgs boson production and decay modes, significantly strengthening the Standard Model's description of the Higgs sector. The reduced uncertainties compared to previous measurements provide more stringent constraints on potential deviations from the Standard Model. The observed consistency does not exclude beyond-Standard-Model physics but does constrain many such models. The inclusion of rare decay modes such as H→μμ and H→Zγ enhances the experimental sensitivity to explore the full extent of Higgs boson interactions. The detailed study of kinematic properties, especially at high transverse momenta, further probes the internal structure of Higgs boson couplings and offers a path toward discovering potential new physics.

This comprehensive study of Higgs boson production and decay rates using the full ATLAS Run 2 dataset demonstrates remarkable agreement with Standard Model predictions. Couplings to the heaviest fermions and weak bosons were measured with improved precision. Indications of rare decay modes were observed. A detailed investigation of Higgs boson production kinematics also confirms the Standard Model. While the Higgs boson's nature is currently consistent with Standard Model predictions, key properties, such as self-coupling, and rare decay modes require further investigation. Future detector upgrades, reduced systematic uncertainties, and larger datasets promise substantial progress in this area, offering opportunities for potential discoveries of physics beyond the Standard Model.

While the analysis incorporates many improvements, certain limitations remain. The H→bb decay mode remains challenging due to significant background. The sensitivity to some rarer decay modes (e.g., H→cc, H→μμ) is still limited by statistics and background. The assumptions made in the coupling parameterizations, such as the Standard Model values for first-generation fermion couplings, might influence the interpretations. The reliance on theoretical calculations for some processes introduces model-dependent uncertainties.