Higgs: From King's to CERN
by Professor Neil Lambert
On 4 July 2012 the ATLAS and CMS experimental groups at CERN announced that they had discovered the ‘Higgs Particle’. This was a milestone achievement for Particle Physics, in a variety of ways, and was celebrated throughout the physics community and beyond. So why all the fuss about this short-lived elusive particle, long predicted by the so-called Standard Model of Particle Physics? What is the Higgs mechanism anyway and what has it to do with King’s?
Peter Higgs started his career as a physics undergraduate at King’s. In 1998 he was made a Fellow of the College and in 2009 he was awarded an honorary PhD. The mechanism that is named after him (it should be mentioned that other people, such as R. Brout, F. Englert and T. Kibble, are also credited with discovering the mechanism) is a corner stone of the Standard Model and plays a central role throughout modern physics. It is used on a daily basis in the research of theoretical physics groups around the world including here in the Department of Mathematics at King’s.
What is the Higgs mechanism? The Standard Model is constructed using advanced concepts in modern mathematics and it is without doubt the most accurate and successful scientific theory ever devised. It contains some two dozen particles and a similar number of parameters that have been determined by experiment. Given these particles and parameters the Standard Model describes everything we know about matter and its interactions as deep inside the atom as we have been able to test, up to fourteen decimal place accuracy. Wow.
It is difficult to explain here why a ‘mechanism’ is required to do something so simple as to give masses to particles, which might otherwise be thought of as just more parameters in the model. The up-shot is that that the mathematical consistency of the Standard Model forbids simple mass terms but this can be evaded by thinking of the mass parameters themselves as a being dynamical and this gives rise to the Higgs particle. Other particles then get their masses as a side effect of interacting with the Higgs particle.
Mathematically the case for the existence of the Higgs has been more or less clear for 40 years, ever since the birth of the Standard Model. However it has eluded detection for almost as long. Indeed the Higgs is the last of the particles predicted by the Standard Model to be observed. Thus its discovery is, presumably, the final chapter in the story of the Standard Model. The next job of the experimenters at CERN is to further study the Higgs particle to ensure that it behaves exactly as expected. What comes after this is, quite simply, unknown. Thus we have entered a new phase of Particle Physics where we expect that the Standard Model will eventually break-down, but we don’t know when or how. Indeed the Higgs particle may hold the clue to what comes next. Any deviations of the Higgs from the predictions of the Standard Model will be of great interest.
Finally it is worth noting that even though the Higgs particle provides the origin of mass for the elementary particles, such as quarks and electrons, it is not the main source of mass in practice. Indeed most of the mass that makes up matter is due to the binding energy of quarks into protons and neutrons (these are roughly 100 times more massive than the combined masses of the three quarks that they’re made of). On the other hand, the fact that the Higgs particle gives all the mass to electrons is crucial, even though an electron has only 1/2000 the mass of a proton. In particular elementary quantum mechanics tells us that the size of an atom, the so-called Bohr radius, is inversely proportional to the mass of the electron. Therefore massless electrons would mean that there were no stable atoms. So no Higgs means no you, me or King’s College London, and that’s certainly worth celebrating.