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

Physics analyses typically focus on particular production and decay processes and measure the number of Higgs boson candidates observed after accounting for non-Higgs background processes. To determine the strengths of the interactions of the Higgs boson, simultaneous fits with different physically motivated assumptions are performed on a combined set of complementary measurements. The relative weights of the input measurements in the combination depend on the analysis selection efficiencies, on the signal rates associated with the Higgs processes studied by the analysis, on the signal-to-background ratios, and on the associated systematic uncertainties.

For each decay mode entering the combination, the production process is assessed via event classification based on the properties of particles produced in association with the Higgs boson, mostly via dedicated machine-learning approaches. Unless stated otherwise, studies of each decay mode consider all individual or combined contributions from six production processes: ggF, VBF, WH, ZH, tt¯H and tH. Higgs boson interactions are further explored via additional event classification of each production process based on the kinematic properties of the produced Higgs boson and the associated particles.

The input to the combined measurement includes the latest results from the decay modes that initially led to the Higgs boson discovery: H → ZZ → ℓ+ℓ−ℓ+ℓ− decays with two Z bosons that subsequently decay into a pair of oppositely charged electrons or muons; H → W ±W∓ → ℓ±νℓℓ∓νℓ decays targeting separately the ggF and VBF, and WH and ZH production processes; and H → γγ decays with two high-energy photons. The latter is the only measurement used to discriminate between the tt¯H and tH processes. These diboson decay modes are for the first time complemented by a search for the rare H → Zγ → ℓ+ℓ−γ decay. The decays of Higgs bosons to fermions are also extensively explored. The measurement of the dominant H→bb¯ decay mode is particularly challenging owing to a very large multi-jet background, which can be suppressed by requiring the presence of additional particles characteristic of the WH or ZH, VBF and tt¯H production processes. As a new input, the fully hadronic H→bb¯ signal events with large Higgs boson transverse momentum are also considered, providing for the first time sensitivity to the ggF production process in this decay mode. The sensitivity of the latest measurement in the H → τ+τ− decay mode is now extended to the VH and combined tt¯H and tH production processes. In addition to the tt¯H measurements obtained in the γγ, τ+τ− and ZZ decay modes, a complementary analysis that is sensitive to τ+τ−, W±W∓ and ZZ decays is performed using events with multiple leptons in the final state. The considerably more challenging measurements of Higgs boson couplings to second-generation fermions are explored via searches for the H → μ+μ− decay and, included in the combination for the first time, H→cc¯ decay. Owing to the large multi-jet background, the latter decay mode is currently accessed only via WH and ZH production. Finally, the inputs to the combination are complemented by the latest direct searches in the VBF and ZH production processes for Higgs boson decays into invisible particles that escape the detector. A summary of these input measurements used in the combination is available in Extended Data Table 1.

All input measurements are performed with the full set of Run 2 data, except for the measurements of previous works, which use a partial Run 2 dataset collected during 2015 and 2016. The direct searches for invisible Higgs boson decays and the H→cc¯ measurements are employed only for measurements of the relevant Higgs boson coupling strengths, and the H→bb¯ measurements at high Higgs boson transverse momenta are considered only when probing the kinematic properties of Higgs boson production. All other inputs are used for the measurements of production cross-sections, branching fractions and coupling strengths. The measurement of kinematic properties of Higgs boson production excludes input measurements from previous works, owing to their limited sensitivity.

Analyses performed with the Run 2 data introduce a number of improvements, often resulting in up to 50% better signal sensitivities compared to those expected from just the increase in the analysed amount of data. These improvements include better particle reconstruction (optimized to cope with an increased number of proton interactions per bunch crossing), dedicated reconstruction of highly Lorentz-boosted H→bb¯ decays, a greater number of simulated events, higher granularity of the kinematic regions that are probed in each production process, and improved signal and background theory predictions.

The standard model is tested by comparing the observed signal rates to theory predictions that require state-of-the-art calculations of Higgs boson production cross-sections and branching fractions. All signal reconstruction efficiencies and most background rates are predicted from the simulation. The simulation is complemented by the use of dedicated signal-depleted control data for measurements of selected background processes and to constrain signal-selection efficiencies. A common set of event generators were used in all analyses to describe the gluon and quark interactions in the proton–proton collisions. The generated particles were passed through a detailed simulation of the ATLAS detector response prior to their reconstruction and identification.

The statistical analysis of the data is described in more detail in Methods. It relies on a likelihood formalism, where the product of the likelihood functions describing each of the input measurements is calculated in order to obtain a combined likelihood. The effects of experimental and theoretical systematic uncertainties on the predicted signal and background yields are implemented by including nuisance parameters in the likelihood function. The values of those additional parameters are either fully determined by the included data, or constrained by Gaussian terms that multiply the likelihood. The effects of uncertainties that affect multiple measurements are propagated coherently through the fit by using common nuisance parameters.

The statistical test of a given signal hypothesis, used for the measurement of the parameters of interest, is performed with a test statistic that is based on the profile likelihood ratio. The confidence intervals of the measured parameters and the p value used to test the compatibility of the results and the standard model predictions are constructed from the test statistic distribution, which is obtained using asymptotic formulae.

The total uncertainty in the measurement of a given parameter of interest can be decomposed into different components. The statistical uncertainty is obtained from a fit with all externally constrained nuisance parameters set to their best-fit values. The systematic uncertainty, the squared value of which is evaluated as the difference between the squares of the total uncertainty and the statistical uncertainty, can be decomposed into categories by setting all relevant subsets of nuisance parameters to their best-fit values.