Higgs10: Ten things we've learned about the Higgs boson in the last ten years

Since its discovery in 2012, the Higgs boson has become one of the most powerful tools for investigating our understanding of nature and, with it, examining some of the biggest open questions in physics today. But what have we physicists learned about particles in the last ten years?

Scalar particles exist in nature

In the early hours of July 4, 2012, the foyer outside CERN’s main lecture hall looked more like a venue for a rock concert than the main building of the world’s leading particle physics laboratory. Dozens of students with groggy eyes slowly rolled up their sleeping bags, stretching after a long night on the hard floor. A long line of hundreds weaved through the foyer, around the restaurant and outside the door. The excitement in the line was throbbing – although the chances of getting into the auditorium were slim, just to be there was already thrilling. We’ve found it. A scalar particle exists in nature and July 4, 2012 is 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 decay and 2-photon decay. Although these are not the most abundant decay channels, they are the best at determining the mass of a scalar particle. The measured mass of about 125 GeV is very interesting: much heavier than would be expected for popular supersymmetry models, it places the universe in a precarious position between stable and metastable, and has a rich phenomenology. In contrast to its heavy mass, the lifetime of the particle is short; it’s gone in 1/10^22 of a second.

Has no electric charge and does not rotate

The discovery of H(125) by its decay into two photons immediately established that the new particle has no electric charge and is very averse to having spin 1. The exact spin of the new particle can be investigated by examining the angular distribution of the final state product of decaying into two protons, two a W boson and two Z bosons. The spin 0 hypothesis has defended against a myriad of other possible assignments.

Measurement of the interaction strength between H(125) and some Standard Model particles. The red line represents the expectations of the Standard Model. Recent advances have increased the range to second-generation fermions, such as muons, and the first results on charm quarks. (Image: ATLAS)

It interacts with other bosons

How the new boson interacts with other particles can be investigated both how it decays and how it is produced. With his discovery through decay into two photons and two Z bosons, it can be concluded that the H(125) particle pairs up with the boson (in the case of photons, indirectly). This was further reaffirmed by measuring the decay to the two W bosons. Furthermore, the production of H(125) via coupling to the boson was measured when two boson vectors (force carriers such as the W and Z bosons) combine to produce a scalar or when a scalar radiates from a heavy boson (called V+ production H).

It interacts with fermions

The Standard Model (SM) predicts that the coupling strength between H(125) and other particles is proportional to their mass. Studying fermions tested this coupling over three generations of fermions spanning three orders of mass magnitude. For the heaviest fermions, all couplings have been measured – to the top quark (via ttH production measurements), to the beauty quark and tau lepton. Now, the experimental challenge lies in achieving the second generation, whose coupling with the Higgs boson is weaker. The first evidence of muon decay emerged and ATLAS and CMS experiments led to the decay to charm quarks.

It can be a portal for dark matter

If dark matter consists of elementary particles, SM predicts none of them. If H(125) particles and dark matter interact in nature, one possible sign is the “invisible” decay of the Higgs boson. The search limits this decay to lower than 15% and, as a result, sets the boundaries for interactions between this Higgs boson and possible dark matter particles and on the models that predict them. SM predicts only a small branching fraction from 0.1% – to four neutrinos.

The constraint on the pair production of the Higgs boson, a process sensitive to the interaction of the Higgs boson self and the Higgs potential form. Results are presented as a function of time along with projections for the complete HL-LHC data set which should provide sufficient sensitivity to challenge SM predictions (red horizontal line). (Image: CMS)

It might touch the structure of the universe

The inclusion of the Brout-Englert-Higgs mechanism in BC led to precise predictions of how the universe evolved during one of its earliest stages, the electroweak age. Scalar fields 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 become metastable and decay, and one way to investigate this shape is to measure the different ways in which H(125) interacts with itself. One of the signatures that can be used to access this self-interaction is the production of the Higgs boson pair. While analysis of existing LHC data has already begun to exclude some non-SM alternatives, more data and future accelerators – such as the Higgs plant – will allow us to explore this critical area.

Seems to be an only child

SM is minimalist as far as scalars are concerned: it predicts a single elementary scalar particle, with different types of interactions. In a direct extension to minimal SM, more than one Higgs boson is predicted, resulting in a different set of interactions. Therefore, a vigorous search program for other Higgs bosons – lighter and heavier, neutral and charged (and double charged) – has been carried out. With other possibilities greatly diminished, H(125) is currently the only scalar we know of in nature.

This is a new player in the team that passed SM

This Higgs boson is the latest player to join the particle team we use to understand the nature of the universe. Matter-antimatter asymmetry, dark matter, the union of all forces; these are some of the questions in which a coherent and precise exploration of the properties of particles such as the Z and W bosons, beauty and top quarks, and now H(125), investigates energy regimes far beyond those accessible directly in the collider. One possibility is to extend SM with generalized interactions that represent both particle effects and interactions beyond the immediate reach of the current impactor. Using all the information from H(125) and his team members consistently can lead us to the next standard model.

This is just the beginning

While we have established some of the properties and interactions of H(125), there is still much to be learned about this Higgs boson. Far from being the ultimate prediction of BC, the discovery of H(125) and its singular scalar qualities provide an important instrument for advancing our understanding of nature at its deepest. Is there really only one Higgs boson in nature? Is its nature different from SM’s predictions? Can it show us what is beyond the electroweak scale? Could it interact with dark matter particles? Will we be able to use it to measure the shape of the universe’s vacuum potential?

Ten years ago, before the invention of this great tool, these questions were beyond our reach. H(125) has opened a new door, inviting us to walk through it.

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