On October 8 this year, Francois Englert and Peter W Higgs were jointly awarded the Nobel Prize in Physics “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS (A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid) experiments at CERN’s Large Hadron Collider”.
This was not exactly a surprise for many of us working in the field; in fact it was almost expected.
Last year, on July 4th, scientists working at the Large Hadron Collider announced the discovery of an elusive, long sought after particle; the Higgs boson. Why is this particle so important and why does this discovery carry so much significance?
Our best theoretical description of the fundamental particles that are the building blocks of the Universe and the ways in which they interact with each other is encapsulated in a theory we call the Standard Model. The development of the Standard Model over the latter half of the 20th century has been one of the most remarkable accomplishments in the history of physics. The theory has been extremely successful in correctly predicting the outcome of scores of experiments with remarkable accuracy.
We’ve tried and tested it, putting it through the wringer of dozens of experiments and it has come out largely unscathed. We know that the Standard Model is not the whole story, it describes only three of the four fundamental forces in nature, saying nothing about gravity and it also does not give any explanation for what might constitute 80 per cent of the matter in the Universe, which we call ‘dark matter’. However, it is still the best description we have so far and, until last year, it was missing one crucial piece, without which the theory would fall apart.
In the 1970s, Sheldon Glashow, Abdus Salam and Steven Weinberg realised that there were deep connections between two of the four fundamental forces, the weak force which is responsible for radioactivity and the reactions in the Sun, and the electromagnetic force, which holds electrons and protons together inside the atom. They independently came up with a mathematical framework that described these forces as manifestations of a single underlying force.Particles interact with each other via these forces by exchanging force-carrying particles known as bosons, analogous to two people on roller skates throwing and catching a ball. It turned out that in unifying these forces, the mass of all the force-carrying particles, known as the photon and the W and Z bosons, come out to be zero.
This is okay for the photon because we know it is massless, but not true for the W and Z bosons which we know have mass. Fortunately, in 1964, Peter Higgs, Francois Englert and Robert Brout (along with others) had postulated a mechanism by which the W and Z bosons and other subatomic particles like the electron could acquire mass and the photon remain massless. They proposed that a field, much like the electric field, pervades the entire Universe, except that whereas the electric field on average is zero, this new field (known as the Higgs field) is non-zero. It is this non-zero value of the Higgs field that gives our subatomic particles mass. If it were zero, particles like the electron would be massless, atoms would not have formed and the world as we know today would not have existed!
The analogy that is oft-used to explain how particles acquire mass through the Higgs field is as follows: Imagine a group of people more or less evenly distributed through a room. Then, a famous person enters the room. People in the room are attracted to the famous person, and gather around him/her. This makes the famous person harder to stop; we say they have acquired mass. Someone non-famous could just pass through the room unnoticed, representing a particle that does not gain mass, for instance the photon. The group of people in the room represent the Higgs field. We don’t know much about this field, we just know that it ought to exist in order to explain how our elementary particles acquire mass. One of the ways in which we can look for experimental evidence of the field is to search for the particle that is associated with it, the Higgs boson.
The Higgs boson is an excitation or a wiggle in the Higgs field, like a ripple on the surface of a lake or the vibrations of a string. If enough energy is available, we can produce the Higgs boson and this is where the Large Hadron Collider comes in.
The Large Hadron Collider is a 17-mile particle racetrack situated about 100 metres below the border between France and Switzerland. It accelerates protons to very close to the speed of light and then collides them head-on at the centre of huge detectors which act like giant cameras and take detailed, high resolution pictures of the collisions. Our task is to sift through millions of these pictures and try to identify which particles are produced in the collisions. Most of these collisions show nothing interesting, producing particles via mechanisms we know and understand well, but occasionally you get a picture which shows that some new and interesting particle, like the Higgs boson, was produced. It’s via analysing millions of these collisions in exceptional detail that scientists working on the ATLAS and CMS experiments were able to discover the Higgs boson which has eluded us for so long.
What’s next? Well, there’s still much more we’d like to know about the Higgs boson, and through it, the Higgs field. We don’t know whether the Higgs field is a single field or if there are several of them, meaning that there could be more Higgs bosons. Some well motivated theories predict as many as five Higgs bosons! We also don’t know whether the Higgs boson is an elementary particle or a composite of several particles. The Large Hadron Collider is currently offline for planned upgrades but when it comes back online in 2014, it will collide protons at much higher energy and we hope will shed more light on the Higgs as well as giving us an insight into what lies beyond our Standard Model.
The writer is a particle physicist. She completed her graduate degree in physics from Oxford University, followed by a PhD in particle physics from University College, London. She works on the world’s most powerful particle collider, the Large Hadron Collider at CERN.