The hunt for the Higgs boson
On the 15th June 2012 hundreds of physicists crammed into a conference room at CERN, to hear a presentation which would bring an end the search for the Higgs boson.
The Science Museum’s Collider exhibition opens with a dramatic reinterpretation of this climactic meeting. Until that morning, no-one had seen the Higgs data – it had been kept hidden (or 'blind' in the lingo) to prevent the analysts from unconsciously biasing the results.
Mingming, who was working on the Higgs analysis for her PhD thesis, was one of the first to see the moment of unblinding that brought the first convincing evidence that a new Higgs-like particle existed.
The Higgs hunt was an industrial operation, involving thousands of physicists and engineers working together on CERN’s Large Hadron Collider (LHC). The LHC’s two 'general purpose' detectors, ATLAS and CMS, independently collected and analysed the two-year’s worth of data needed to make the discovery.
Proving the existence of the Higgs is an almost impossible task. Higgs bosons are produced in only a tiny fraction of collisions at the LHC, and decay so rapidly that it’s impossible for a detector to see them directly. Instead, LHC physicists must look for the shrapnel of lighter particles produced when a Higgs boson pops fleetingly in and out of existence.
To make matters worse, there are many other processes that can give the same signature as a Higgs decay and physicists must statistically disentangle real Higgs bosons from the fakers. In the end, the smoking-gun that showed the Higgs was real was a small bump in a graph, sitting on top of a much larger sea of background noise.
From the 'God' to the 'goddamn' particle, the Higgs’ nicknames have generally reflected the fact that it was frustratingly elusive. In the decades following Peter Higgs’ 1964 prediction that such a particle ought to exist, physicists even doubted whether it would show up at all.
Peter Higgs at the University of North Carolina, 1965. © The Higgs family
Faith in a theory
So what justified directing the attention of the multi-billion-pound LHC to this bothersome boson?
The Higgs is the cornerstone of the theory that describes all the known particles of nature and the three forces that govern their behaviour. The snappily-named Standard Model, is the combined labour of many physicists throughout the latter half of the twentieth century. An intellectual triumph, the Standard Model can be justifiably claimed as the most precise scientific theory ever formulated, and is the closest physicists have to a theory of everything.
In the early sixties, the Standard Model was still under construction, and several key pieces were missing. In particular, physicists were trying to understand why the weak nuclear force (the force responsible for radioactivity and the processes that generate energy in the Sun) was, well, weak.
The Standard Model tells us that each of the three forces of nature are created by the exchange of force-carrying particles.
Take the electromagnetic force (electricity and magnetism combined) and its force carrier, photons. Because photons have no mass, they can travel infinite distances, meaning that electromagnetism is long ranged. But in contrast, the weak force only operates across distances much smaller than the size of an atom.
To explain the short range of the weak force, it was suggested that its force-carrying particles must be very massive, preventing them from reaching over huge distances like massless photons. The big problem was that having particles with mass broke the rules of the theory, leaving physicists in a bit of a pickle.
Fortunately help was at hand. An elegant and simple solution was discovered in 1964 by six theorists, p as three independent teams. Robert Brout, François Englert published first, quickly followed by Peter Higgs and later that year Gerald Guralnik, C. R. Hagen, and Tom Kibble came upon the same idea.
The Higgs theorists: (left to right) Kibble, Guralnik, Hagen, Englert, Brout © Higgs
Higgs though was the one who explicitly stated that this mechanism – an all pervading energy field that interacts with certain particles, giving them mass – could be 'poked' in such a way as to let off a 'disturbance' that would show up as a new particle, i.e. the Higgs boson.
The so-called 'Higgs' mechanism (Higgs himself dislikes the name on the grounds that it excludes the other theorists who also deserve credit) proved to be a crucial component of the Standard Model.
The handy mechanism at last explained why the weak nuclear force was weak, and also helped lay the foundations for the unification of the weak and electromagnetic forces into a single 'electroweak' force. One of the crowning achievements of the 19th century was Michael Faraday’s and James Clerk Maxwell’s fusing of electricity and magnetism, so this was a step more fundamental still.
In 2012 the discovery of the Higgs boson proved that this otherwise invisible energy field actually exists, and completes the Standard Model. But what is next?
Over the horizon
You might think that with the discovery of the Higgs boson, our picture of the subatomic world is basically complete. Far from it. Rather embarrassingly, the Standard Model only describes about 95% of the stuff that makes up the universe. It simply has nothing to say about dark matter, dark energy, or even the familiar force of gravity.
In 2015 the LHC will reopen after an upgrade and operate at almost double its previous energy. This will allow physicists to explore a whole new region of the quantum world, in hope of discovering some new particle or force that could explain the mysterious dark universe. A new chapter in our understanding of nature is about to begin. As for what it will reveal, we can only wait and see.
Written for the Science Museum's Collider exhibition, 2013. © Creative Commons - CC BY