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

The standard model of particle physics has been tested by many experiments since its formulation and, after accounting for the neutrino masses, no discrepancies between experimental observations and its predictions have been established so far. A central feature of the standard model is the existence of a spinless quantum field that permeates the Universe and gives mass to massive elementary particles. Testing the existence and properties of this field and its associated particle, the Higgs boson, has been one of the main goals of particle physics for several decades. In the standard model, the strength of the interaction, or ‘coupling’, between the Higgs boson and a given particle is fully defined by the particle’s mass and type. There is no direct coupling to the massless standard model force mediators, the photons and gluons, whereas there are three types of couplings to massive particles in the theory. The first is the ‘gauge’ coupling of the Higgs boson to the mediators of the weak force, the W and Z vector bosons. Demonstrating the existence of gauge couplings is an essential test of the spontaneous electroweak symmetry-breaking mechanism. The second type of coupling involves another fundamental interaction, the Yukawa interaction, between the Higgs boson and matter particles, or fermions. The third type of coupling is the ‘self-coupling’ of the Higgs boson to itself. A central prediction of the theory is that the couplings scale with the particle masses and they are all precisely predicted once all the particle masses are known. The experimental determination of the couplings of the Higgs boson to each individual particle therefore provides important and independent tests of the standard model. It also provides stringent constraints on theories beyond the standard model, which generally predict different patterns of coupling values.

In 2012, the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN announced the discovery of a new particle with properties consistent with those predicted for the Higgs boson of the standard model. More precise measurements that used all of the proton–proton collision data taken during the first data-taking period from 2011 to 2012 at the LHC (Run 1) showed evidence that, in contrast to all other known fundamental particles, the properties of the discovered particle were consistent with the hypothesis that it has no spin. Alternate spin-1 and spin-2 hypotheses were also tested and were excluded at a high level of confidence. Investigations of the charge conjugation and parity (CP) properties of the new particle were also performed, demonstrating consistency with the CP-even quantum state predicted by the standard model, while still allowing for small admixtures of non-standard model CP-even or CP-odd states. Limits on the particle’s lifetime were obtained through indirect measurements of its natural width. In addition, more precise measurements of the new particle’s interactions with other elementary particles were achieved. The results of all these investigations demonstrated that its properties were compatible with those of the standard model Higgs boson. However, the statistical uncertainties associated with these early measurements allowed considerable room for possible interpretations of the data in terms of new phenomena beyond the standard model and left many predictions of the standard model untested.

The characterization of the Higgs boson continued during the Run 2 data-taking period between 2015 and 2018. About 9 million Higgs bosons are predicted to have been produced in the ATLAS detector during this period, of which only about 0.3% are experimentally accessible. This is 30 times more events than at the time of its discovery, owing to the higher rate of collisions and the increase of the collision energy from 8 teraelectronvolts (TeV) to 13 TeV, which raises the production rate. In this Article, the full Run 2 dataset, corresponding to an integrated luminosity of 139 inverse femtobarns (fb⁻¹), is used for the measurements of Higgs boson production and decay rates, which are used to study the couplings between the Higgs boson and the particles involved. This improves on the previous measurements obtained with partial Run 2 datasets. The corresponding predictions depend on the value of the Higgs boson mass, which has now been measured by the ATLAS and CMS experiments with an uncertainty of approximately 0.1%. The predictions employed in this article use the combined central value of 125.09 GeV.

The dominant production process at the LHC, which accounts for about 87% of Higgs boson production, is the heavy-quark loop-mediated gluon–gluon fusion process (ggF). The second most copious process is vector boson fusion (VBF), in which two weak bosons, either Z or W bosons, fuse to produce a Higgs boson (7%). Next in rate is production of a Higgs boson in association with a weak (V = W, Z) boson (4%). Production of a Higgs boson in association with a pair of top quarks (tt¯H) or bottom quarks (bb¯H) each account for about 1% of the total rate. The contribution of other qq¯H processes is much smaller and experimentally not accessible. Only about 0.05% of Higgs bosons are produced in association with a single top quark (tH). Representative Feynman diagrams of these processes are shown in Fig. 1a–e. After it is produced, the Higgs boson is predicted to decay almost instantly, with a lifetime of 1.6 × 10⁻²² seconds. More than 90% of these decays are via eight decay modes (Fig. 1f–i): decays into gauge boson pairs, that is, W bosons with a probability, or branching fraction, of 22%, Z bosons 3%, photons (γ) 0.2%, Z boson and photon 0.2%, as well as decays into fermion pairs, that is, b quarks 58%, c quarks 3%, τ leptons 6%, and muons (μ) 0.02%. There may also be decays of the Higgs boson into invisible particles, above the standard model prediction of 0.1%, which are also searched for. Such decays are possible in theories beyond the standard model, postulating, for example, the existence of dark matter particles that do not interact with the detector.

In this Article, the mutually exclusive measurements of Higgs boson production and decays probing all processes listed above are combined, taking into account the correlations among their uncertainties. In a single measurement, different couplings generally contribute in the production and decay. The combination of all measurements is therefore necessary to constrain these couplings individually. This enables key tests of the Higgs sector of the standard model to be performed, including the determination of the coupling strengths of the Higgs boson to various fundamental particles and a comprehensive study of the kinematic properties of Higgs boson production. The latter could reveal new phenomena beyond the standard model that are not observable through measurements of the coupling strengths.