## What is the Higgs boson? In quantum field theory, particles can be described as waves in a field (Image: Piotr Traczyk/CERN)

To answer this question needs an exploration into the quantum world and how particles interact…

The particle that we now call the Higgs boson first appeared in a scientific paper written by Peter Higgs in 1964. At that time, physicists were working on describing the weak force – one of the four fundamental forces of Nature – using a framework called quantum field theory.

### Particle, wave or both?

Quantum field theory describes the microscopic world of particles very differently to everyday life. Fundamental “quantum fields” fill the universe and dictate what nature can and cannot do. In this description, every particle can be represented by a wave in a “field”, similar to a ripple on the surface of a vast ocean. One example is the photon, the particle of light, which is a wave in the electromagnetic field.

### Force carriers

When particles interact with one another, they exchange “force carriers”. These force carriers are particles and can also be described as waves in their respective fields. For example, when two electrons interact, they do so by exchanging photons – photons are the force carriers of the electromagnetic interaction.

### Symmetry

Another important component of this picture is symmetry. Just like a shape can be called symmetrical if it doesn’t change when rotated or flipped, similar requirements are placed on the laws of Nature.

For example, the electrical force between particles with an electrical charge of one will always be the same, irrespective of whether the particle is an electron, muon or proton. Such symmetries form the basis and define the structure of the theory.

## The Brout-Englert-Higgs mechanism

Quantum field theory had already formed the basis of quantum electromagnetism, a very successful description of the electromagnetic interaction. Applying a similar approach to the weak interaction was however not possible due to a fundamental issue: the theory didn’t allow for particles to have mass.

Specifically, the weak force carriers known as the W and Z bosons had to be massless, otherwise a fundamental symmetry of the theory would be broken and the theory would not work. This posed a major problem since the weak force carriers had to be massive to be consistent with the very short range of the weak interaction.

The solution to this problem was found with the Brout-Englert-Higgs mechanism. This mechanism has two main components: an entirely new quantum field and a special trick. The new field is what we now call the Higgs field, and the trick is spontaneous symmetry breaking.

A spontaneously broken symmetry is one that is present in the equations of a theory but broken in the physical system. Imagine a pencil standing on its tip at the centre of a table. A perfectly symmetrical situation, but only for a moment: the pencil would immediately fall, breaking the rotational symmetry by selecting a single direction in which the pencil would be pointing. The laws of Nature however would remain unchanged, without a predefined direction written into them. So, the lack of symmetry was essentially “tricked” into the picture, without upsetting the symmetry of physics. The particle in the “Mexican hat” shape of the Higgs field (left) and the pencil standing on its tip (right) both show spontaneously symmetry breaking – symmetry is present, but only for a moment. (Image: Ana Tovar/CERN)

The way this works for particle masses is as follows: when the universe was born, it was filled with the Higgs field in an unstable – but symmetrical – state. A fraction of a second after the Big Bang, the field found a stable configuration, but one that breaks the initial symmetry. In this configuration, the equations remain symmetrical, but the broken symmetry of the Higgs field gives rise to the masses of the W and Z bosons.

As it later turned out, other elementary particles also acquire masses by interacting with the Higgs field, giving rise to the particle properties we observe today.

## The Higgs boson At CERN on 4 July 2012, the ATLAS and CMS collaborations present evidence in the LHC data for a particle consistent with a Higgs boson, the particle linked to the mechanism proposed in the 1960s to give mass to the W, Z and other particles. (Image: Maximilien Brice/Laurent Egli/CERN)

So what is the Higgs boson then? Since every particle can be represented as a wave in a quantum field, introducing a new field into the theory means that a particle associated with this field should also exist.

Most properties of this particle are predicted by the theory, so if a particle matching the description would be found, it provides strong evidence for the BEH mechanism – otherwise we have no means of probing for the existence of the Higgs field.

The Higgs boson is that particle, and its discovery in 2012 confirmed the BEH mechanism and the Higgs field, allowing researchers to probe ever further in their understanding of matter.

Measuring the properties of the Higgs boson in detail is crucial to exploring many outstanding mysteries in particle physics and cosmology, from the wild variation of masses of elementary particles to the fate of the universe.