Higgs10 Bulletin article - 7
(Image: CERN)


Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today. But what have we physicists learned about the particle in the past ten years? 

A scalar particle exists in nature

During the early hours of 4 July 2012, the foyer outside the main CERN lecture hall looked more like the lead-up to a rock concert than the main building of the world’s leading particle physics lab. Dozens of groggy-eyed students slowly rolled up their sleeping bags, stretching out after a long night on the hard floor. A line hundreds long snaked through the foyer, around the restaurant and out the door. The excitement in the line was pulsating – even though the odds of making it into the auditorium were small, just to be there was a thrill. We had found it. A scalar particle existed in nature and 4 July 2012 was its debut.

It’s heavy and short-lived

The first measurements of the new scalar particle, H(125), relied on two experimental channels: 4-lepton decays and 2-photon decays. Although these are not the most abundant decay channels, they are the best in determining the scalar particle’s mass. The measured mass of about 125 GeV is maximally interesting: it is much heavier than was expected for popular models of supersymmetry, it puts the universe in a precarious position between being stable and metastable, and it has a rich phenomenology. In contrast to its heavy mass, the particle’s lifetime is short; it is gone in 10-22 of a second.

It has no electric charge and no spin

The discovery of the H(125) via its decay to two photons immediately established that the new particle had no electric charge and strongly disfavoured it to have spin of 1. The exact spin of the new particle can be probed by examining the angular distributions of the final-state products in decays to two protons, two W bosons and two Z bosons. The spin 0 hypothesis has held up against a myriad of other possible assignments.

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Measurements of the interaction strength between the H(125) and some of the Standard Model particles. The red line represents the Standard Model expectation. Recent progress has increased the reach to second generations fermions, like the muon, and first results concerning charm quarks. (Image: ATLAS)

It interacts with other bosons

How the new boson interacts with other particles can be probed in both how it decays and how it is produced. With its discovery via decays to two photons and two Z bosons, it was readily concluded that the H(125) particle couples to bosons (in the case of photons, indirectly). This was further reaffirmed with measurements of decays to two W bosons. Furthermore, the production of the H(125) through couplings to bosons is measured when two vector bosons (force carriers such as W and Z bosons) fuse to produce the scalar or when the scalar radiates from a heavy boson (so-called V+H production).

It interacts with fermions

The Standard Model (SM) predicts that the strength of the coupling between the H(125) and other particles is proportional to their masses. Studying fermions tests these couplings over three fermion generations spanning three orders of magnitude of masses. For the heaviest fermions, all couplings have been measured – to top quarks (via measurements of ttH production), to beauty quarks and to tau leptons. Now, the experimental challenge lies in reaching the second generation, whose coupling with the Higgs boson is weaker. First evidence of decays to muons are emerging and both the ATLAS and CMS experiments are homing in on decays to charm quarks. 

It could be a portal for dark matter

If dark matter consists of elementary particle(s), the SM simply does not predict any of them. If the H(125) and dark matter particles interact in nature, one possible signature is that of “invisible” Higgs boson decays. Such searches limit these decays to be lower than 15% and, consequently, set limits on interactions between this Higgs boson and possible dark matter particles and on the models that predict them. The SM predicts only a diminutive branching fraction of 0.1% – to four neutrinos. 

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Limits on Higgs boson pair production, a process that is sensitive to the Higgs boson self-interaction and the shape of the Higgs potential. Results are presented as a function of time along with projections for the full HL-LHC dataset that should provide enough sensitivity to challenge the SM prediction (red horizontal line). (Image: CMS)

It may touch the structure of the universe

The inclusion of the Brout-Englert-Higgs mechanism in the SM leads to precise predictions of how the universe evolved during one of its earliest stages, the electroweak epoch. A scalar field can influence several aspects of cosmology and even play a role in the observed matter–antimatter asymmetry in the universe. Depending on the shape of the vacuum potential, the universe could be metastable and decay, and one way to probe this shape is to measure the different ways in which the H(125) interacts with itself. One of the signatures that can be used to access this self-interaction is the production of Higgs boson pairs. While existing analyses of LHC data have already started to exclude some non-SM alternatives, more data and future accelerators – like Higgs factories – will allow us to explore this critical area.

It seems to be a lone child

The SM is minimalistic as far as scalars are concerned: it predicts one single elementary scalar particle, with distinct types of interactions. In straightforward extensions to the minimal SM, more than one Higgs boson is predicted, resulting in different sets of interactions. Therefore, a vigorous programme of searches for other Higgs bosons – lighter and heavier, neutral and charged (and doubly charged) – has been undertaken. With other possibilities being strongly reduced, H(125) is presently the only scalar we know of in nature.

It’s a new player in the team pushing past the SM

This Higgs boson is the newest player joining the team of particles that we use to understand the nature of the universe. Matter–antimatter asymmetry, dark matter, unification of all forces; these are some of the questions where a coherent and precise exploration of the properties of particles like the Z and W bosons, the beauty and top quarks and now the H(125), probe energy regimes far beyond those directly accessible at colliders. One possibility is to extend the SM with generic interactions that represent the effect of particles and interactions beyond the direct reach of present colliders. Making use of all the information from H(125) and its team members in a consistent fashion may point us in the direction of the next standard model.

It’s just the beginning

While we have established several properties and interactions of the H(125), much remains to be learned about this Higgs boson. Far from just being the last prediction from the SM, the discovery of the H(125) and its singular scalar quality provides an important instrument to further our understanding of nature at its deepest. Is there really only one Higgs boson in nature? Do its properties differ from the SM predictions? Can it show us what is beyond the electroweak scale? Might it interact with dark matter particles? Will we be able to use it to measure the shape of the vacuum potential of the universe?

Ten years ago, before the discovery of this formidable tool, these questions were beyond our reach. The H(125) has opened new doors, inviting us to walk through.