Read more: “Instant Expert 35: The Higgs boson”
The standard model is our most successful theory of the nature of reality. It describes how particles of matter – fermions – feel forces and interact through the exchange of other particles – bosons.
Until recently, one crucial component was notable by its absence: the Higgs boson. This particle is thought to play an essential part both in giving all other particles mass and explaining why nature’s forces take the form they do. The hunt for it has lasted 50 years
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The mystery of mass
What happens if you break matter up into smaller and smaller pieces?
Eventually, you get the component molecules or atoms. But these can be further broken down into electrons and atomic nuclei. The nuclei can then be torn apart to reveal their constituent protons and neutrons. And inside these there are quarks.
At this point, you have reached the level we regard as fundamental within the standard model, our current theory of particle physics. Whatever material you start with, at some point you end up with a bunch of quarks and a bunch of particles like electrons.
There are in fact six types of quark: the lighter up quarks and down quarks that make up protons and neutrons, and the heavier strange, charm, bottom and top quarks. The electron belongs to a different family of six particles, the leptons, together with its two meatier cousins, the muon and the tau, and three near-massless neutrinos that partner each of these. All 12 matter particles, collectively known as fermions, have an anti-particle partner that is identical, except that it has the opposite charge. And that’s it. Matter gets no smaller than this.
This neat particle pattern fits the experimental facts, but hides a perplexing problem. All matter particles have a property called mass – a resistance to being moved around. This mass varies by over 11 orders of magnitude, from the lowly electron neutrino to the relatively humongous top quark (see diagram). Where do these masses come from – and why are they so different?
Broken symmetries
Within the standard model, the fermions that make up matter interact through forces transmitted by particles known as bosons. In the case of the electromagnetic force, which holds atoms together and drives the currents in our electronic devices, these are photons. The interaction of photons with matter depends on the magnitude of a fermion’s electric charge: electrons (charge -1) feel the electromagnetic force more strongly than do quarks (charge –1⁄3 or +2⁄3). Chargeless neutrinos don’t feel it at all.
Quarks also have a separate “colour” charge onto which particles known as gluons latch to produce the strong nuclear force. This force is indeed much stronger than the electromagnetic force, but peculiarly, gluons themselves carry colour charge and so stick to each other. Consequently quarks and gluons are never seen roaming freely, but only ever bound inside particles such as protons and neutrons – and the strong force never breaks out beyond subatomic scales.
As for the third of the standard model’s forces, the weak nuclear force, it is weak, but without it the radioactive decay that powers the sun and other stars would not occur. Its weakness comes about because its carriers, the W and Z bosons, have very large masses – almost 100 times the mass of the proton. Creating such particles takes a lot of energy. Under normal conditions, matter particles prefer to interact by swapping massless photons, if they can.
At very high energies – in the first split-second of the universe, for example, or in collisions in powerful particle accelerators – this difference melts away. The electromagnetic and weak forces, so hugely different in our everyday experience, become one unified “electroweak” force.
The process by which the electroweak force split into the electromagnetic and weak forces is known as electroweak symmetry breaking, and must have happened some time in the universe’s early moments. Whatever caused it is clearly connected to the mystery of mass. After all, it is the mechanism by which the W and Z bosons acquired mass. The Higgs boson was initially postulated to explain just how this symmetry came to be broken.
The birth of an idea
Broken symmetries are not restricted to exotic forces. An everyday example is seen when a liquid cools into a solid crystal. Here a broadly symmetrical state – everything looks the same in all directions in a liquid – is replaced by a state in which things look distinctly different along different axes.
In the 1960s, particle theorists began to wonder whether tools developed to describe this symmetry breaking could be applied to the cooling cosmos. This was no easy task. Molecular interactions in a solid or liquid can be defined by reference to a fixed set of coordinates, but thanks to the warpings of Einstein’s general relativity there is no such standard frame of reference for the universe.
In 1964, the Belgian theorists Robert Brout and François Englert devised the equations of a quantum field that would pervade the cosmos and break electroweak symmetry while being consistent with relativity. The British physicist Peter Higgs made the same proposal, and pointed out that ripples in this field would take the form of a new particle. Later that same year, Gerald Guralnik, Carl Hagen and combined these ideas into a more realistic theory that was a precursor to the standard model.
The central point about what came to be known as the Higgs field is that even the lowest-energy state of space is not empty. Particles travelling through space interact with the field to different degrees, and this creates a “sticky” quality to their movement: mass. The W and Z bosons acquire their mass by one kind of interaction with this field, while fermions do so by another. Because the Higgs field has no net electric or colour charge, photons and gluons do not interact with it at all, and so remain massless.
This was a neat trick. To find out if it was anything more, we needed to expose the field by making it wobble, those wobbles being observed as Higgs bosons. Theoretical and experimental developments gave us a good idea of the energy required: the Higgs boson’s mass had to be between about 100 and 400 gigaelectronvolts. We would need a truly huge machine.
This article appeared in print under the headline “Why do we need the Higgs?”
