What is the Higgs boson?

Anthea Lacchia, Science Editor
with Glenn Moynihan, Contributor

“This really is an historical milestone,” said Prof Rolf-Dieter Heuer, Director General of CERN, speaking about the discovery of a Higgs-like boson, which was announced by CERN on July 4th. But has the Higgs boson been discovered or not? “As a layman, I say we have it. As a scientist, I must say what do we have?” said Prof Heuer, speaking at a press conference in Dublin last week. The CERN announcement heralded the discovery of a new subatomic particle, a particle that looks like the Higgs boson.

This breakthrough is important because for a long time the Higgs boson has been a vital, yet missing, ingredient in a recipe physicists call “The Standard Model”. So far this model has been incredibly successful in explaining a great deal about the fundamental nature of matter. In order to grasp what this discovery means to physicists, it’s worth giving a basic explanation of the model. The Standard Model encapsulates everything we know about three of the four fundamental forces in nature: the weak nuclear force (which is responsible for radiation); the strong nuclear force (which describes how quarks, the fundamental building blocks of matter, are held within particles and how protons stick together inside a nucleus); and, finally, electromagnetism. The only fundamental force it doesn’t include is gravity.

“This is one of the reasons why people don’t believe the Standard Model is the end of the story”, says Dr Sinéad Ryan, from Trinity’s School of Mathematics. “In some sense, it doesn’t really matter, because the Standard Model is concerned with subatomic particles and their interaction, and the effects of gravity at those very small scales is very weak. At the same time theorists don’t like it because it’s not complete. “The Higgs boson,” she goes on to explain, “tells us that there is something called the Higgs field: this field pervades the universe and was formed a trillionth of a second after the Big Bang. As fundamental particles, in particular the gauge bosons, for example W and Z bosons, move through the field, they interact with it and it is the interaction of those particles with the field that gives a notion of mass to the particles. Finding this Higgs boson means that the theory called Higgs Mechanism, which explains how fundamental particles – some of them in the Standard Model – gain mass, is correct. So we understand the mechanism at play within the Standard Model for how these fundamental particles gain mass.”

The Higgs boson is needed in the first place because, despite the success of the Standard Model, it never makes any inclusion of mass into its recipe. When it tries, it doesn’t work. But of course everyday experience tells us that all matter has mass and is in some way heavy. “The equations describe massless particles, but the Higgs mechanism explains how they gain mass. The Higgs boson is a smoking gun for this mechanism or the existence of this field. This field has the Higgs boson as a by-product. There’s no field without the boson and no boson without the field,” says Dr Ryan.

(Artist Josef Kristofoletti’s rendering of the Higgs boson
– watch the making of the mural here.)

Analogising Higgs

A good analogy is that of a snowfield: imagine you are in the middle of a flat plain of perfect snow, akin to the Higgs field, which surrounds you in all directions. You want to move through it. If you are wearing skis, you can skim across the top of the snow without being slowed down; you are not interacting with the field so you have no mass (photons have no mass and hence move at the speed of light). If you are wearing snow shoes, you’ll progress more slowly since the snow is now affecting you to a greater degree; you now have mass and are heavy, but you’ll make your way. If you are wearing boots, then you’ll sink into the snow and have to battle through: you are now a very heavy particle so the Higgs field slows you down even more. If you used your hands to “excite” the snow and make a snowball, you could make a Higgs boson. But you need to use just enough energy: press too hard and it will fall apart, press too lightly and it won’t stick together. Before you can throw the snowball it decays and falls apart, but it did exist, if even for a second.

The so-called cocktail party analogy is also instructive. Originally borne out of a competition that challenged scientists to explain what the Higgs boson is, it imagines a high-profile politician, the Higgs boson in the analogy, entering a party full of journalists. All the journalists clump around him. As he moves, more and more journalists are attracted to him, thereby slowing his progress. However, a less-important minister, a photon in the analogy, can move through the party without interacting with any of the journalists.

This all goes back to the work of Peter Higgs and five others, who put forward a way in which mass could be included into the Standard Model through the introduction of a scalar field, known as the Higgs field. But it was Peter Higgs who pointed to the implication of this idea, which is the existence of the Higgs boson. In fact, the mechanism by which the field gives the particles mass predicts the existence of the Higgs boson. The experiments at CERN aimed to look for the boson in order to verify the theory and, hence, explain the origin of mass. As Dr Cormac McGuinness, from Trinity’s School of Physics, explains: “The Higgs boson was first proposed in 1964, with associated experimental circumstantial evidence for it since the building of what is called the Standard Model in the early 1970s. The confirmation of the fundamentals and the overwhelming accuracy of that Standard Model proceeding through the 1980s, 1990s and early 2000s, have reassured us that it was probably there, but as Leon Lederman said it was a Goddamned particle that was difficult to find. It was only his book editor that changed the title of his book to God particle.”

This particle has remained undiscovered until now mainly because of the huge energies involved and also because physicists had no idea by how much they had to excite the Higgs field in order to create the Higgs boson. Another reason is that the Higgs boson is very unstable and quickly decays into other particles before it even reaches the detectors, making it a very elusive particle indeed. “It’s like looking for a needle in many haystacks, but the haystacks are also needles!” said Prof Heuer during his address at ESOF (Euroscience Open Forum) in Dublin.

(Higgs boson sonified by Domenico Vicinanza
– it has even been put to a piano.)

Boson Ramifications

“If they hadn’t found this boson, then it would be back to the drawing board and we would have had to think long and hard about what the theory behind particle physics is. There are theories out there, so-called Higgsless theories, but they never gained as much traction as the Higgs Mechanism has. They’re not so elegant or somehow so satisfactory as theories,” says Dr Ryan.

“This is a triumph of theory, experiment and engineering,” said Prof Themis Bowcock, a particle physicist working at the LHC (Large Hadron Collider), during a talk delivered at ESOF on Saturday 14th July. The discovery of the Higgs not only demonstrates the capabilities of CERN and the LHC, and indeed Peter Higgs himself, but also opens up new avenues for discovery that could greatly improve our understanding of nature. As Prof Heuer remarked during an interview with Trinity News: “The next major step will be the investigation of dark matter. Even if the particle is confirmed as the Higgs Boson, 95% of the universe is still unknown.” The first step is to study the properties of the particle, such as its spin. Given that the Higgs boson has no spin, this will be a key piece of information. “It may be that it is just one of a family of Higgs bosons”, said Prof Heuer, but more time and research are needed: “stay tuned over the next few years. Our understanding of the universe is about to change.”

One thing is certain: the announcement has generated great excitement and emotion in physics. Dr McGuinness commented: “As with many other younger and older physicists I have been watching as an informed outsider the experimental discoveries and developments in particle physics throughout my career. I learned of the Higgs as a student twenty years ago, and my older colleagues much before then. It is a delight to hear that it is real! In particular, it is a joy to see my physics colleagues near and far finally confirm the existence of the Higgs particle. Although personally involved in other areas of physics where I use x-rays to study the electronic structure of materials, I am looking forward to teaching final year Physics students about the existence of the Higgs boson during their undergraduate courses in November.”

Watch Rolf-Dieter Heuer’s keynote address, “The search for a deeper understanding of our universe at the Large Hadron Collider: the world’s largest particle accelerator”, delivered on Saturday July 14th at ESOF: http://livestre.am/41rXA.