Swimming against the tide: explaining the Higgs

"Never before in the field of science journalism have so few journalists understood what so many physicists were telling them!" tweeted the UK Channel 4’s Tom Clarke from last December’s Higgs seminar. As a consequence, most coverage focused on debates over the use of the label “god particle” and the level of excitement of the physicists (high), whilst glossing over what this excitement was actually all about.

So what is the Higgs? Something fundamental. Something to do with mass. If your interest in physics is more than simply passing, you may find that rooms full of chattering politicians or the use of different footwear when walking through snow just don’t do the job in convincing you why the Higgs is so important. And if images of fish make you feel like a fish out of water - or at least one swimming against a strong current - then perhaps you would appreciate a different approach.

The need for the Higgs
Whilst gauge theories and their dense mathematical formulae are only within the reach of a few, the motivations that led to the development of Higgs theory have a more familiar start. Indeed, at the root of the problem are the masses of particles very well known to CERN: the W and the Z. Not only were they discovered here, but most of the LEP programme was devoted to making precision measurements of their properties. Whilst photons, the carriers of the electromagnetic force, have no mass at all and are free to zip along at the speed of light, W and Z particles, carriers of the weak force, are massive and travel more slowly. This single fact posed an enormous problem to physics.

Theoretical descriptions of forces are governed by fundamental symmetries which dictate that the particles carrying the forces must have no mass. The fact that the W and Z are massive breaks this symmetry and, without correction, leads to nonsensical predictions - for example of interactions with probabilities greater than 100%. Nature therefore must have a way of correcting the inconsistency. And by far the most promising candidate is the Higgs field¹⁾.

How the Higgs works
The Higgs field fills all space and it is through their interaction with this field that the W and Z acquire their mass. Other force-carrying particles – the photon for the electromagnetic force and the gluon for the strong force – do not feel any interaction with the Higgs field and remain massless. The existence of such a field preserves the underlying symmetry of the theory, whilst explaining the broken symmetry we observe in our experiments. As such, it underpins the entire Standard Model, the rulebook governing all particles and their interactions.

The Higgs mechanism, through which force carriers obtain different masses, also has as a direct consequence the different reaches of forces – very short for the weak force, infinite for the electromagnetic.  With the presence of the Higgs field, these forces can cohabit in one unified electroweak theory.  

Interactions with the Higgs field are not just reserved for force-carrying particles. The theory can also explain how all other fundamental particles acquire their rest mass. But don’t make the mistake of thinking the Higgs field is responsible for all mass. Interaction with the field actually contributes less than 1 kg to the mass of an average person²⁾. Your remaining mass comes from the energy of the various forces holding your bodies together – mainly the strong force binding quarks inside nucleons, with a tiny contribution from the electromagnetic force that reigns over the atomic scale.

That’s for the Higgs field, but what about the Higgs boson? Well, it’s just the detectable manifestation of the field. Just as the electromagnetic field is communicated via photons, the Higgs field also has its boson. Using energy to stir up a field and produce a boson will be covered in a future Bulletin article. Suffice it to say here that evidence for the Higgs boson in the LHC experiments would prove the existence of the field.

The end is just the start
Reading some articles, it can seem like the Higgs solves nearly all problems in physics. The boson has certainly survived 40 or so years at the top of physicists’ most-wanted list. However, in its most basic form, incorporation of the Higgs field into the Standard Model is not completely satisfying. It does the job of explaining how the symmetry between electromagnetic and weak force carriers is broken and it accounts for how particles acquire their mass. But it does not predict or explain the degree of interaction with the field and hence the relative masses of particles.  Moreover, it does not explain why symmetry is broken in this way. It seems we are looking at just the visible tip of an iceberg – hidden below must be a deeper, more fundamental theory that gives reason to what we see on the surface.

Be it some form of a Higgs field, or another mechanism altogether, the theoretical problems posed by the symmetry breaking need to be solved. And once we know the secret Nature employs, the story doesn’t end there. On the contrary, this is just the start. Exploration of the Higgs field will commence with the discovery of its boson. A new chapter in physics is only just beginning.

¹⁾ What is today known as the Higgs field was independently proposed in 1964 by Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, Carl Richard Hagen, and Tom Kibble.

²⁾ A Zeptospace Odyssey, A journey into the physics of the LHC, by Gian Giudice.

by Emma Sanders