Sometimes referred to as the 'holy grail of particle physics' or the 'God particle' the Higgs boson is a hypothetical particle, which if it exists has so far evaded detection. It was dreamt up over 20 years ago by the British scientist Peter Higgs, to explain why other particles possess mass and became an essential part of the standard model of particle physics. The discovery of the Higgs Boson¹ would be the final experimental proof of the standard model and would bring great fame to the team behind such a discovery.

W and Z

In the mid 1980s, scientists working at CERN - the European Laboratory for Particle Physics in Geneva, were rather pleased with themselves. In 1983, under the leadership of the Italian physicist Carlo Rubia, they had discovered the W and Z particles. In doing so they had provided the experimental evidence needed to prove the 'electroweak' theory - a theory which explains the electromagnetic force and weak nuclear force as two manifestations of a single electroweak force. More importantly, they had beaten the Americans to a prestigious discovery.

In 1985 Carlo Rubia and Simon Van der Meer² were awarded the Nobel prize for physics³, and CERN scientists turned their attention to new projects. The next particle on their list of big game was the Higgs Boson, the discovery of which would confirm the 'Higgs mechanism'. Detecting the Higgs would require a higher energy machine, bigger than any built before. This would cost a lot of money. However, having shown what they could do, they persuaded European politicians to support them.

The Higgs Mechanism

To understand the Higgs mechanism, you first have to realise that fundamental particles do not behave like particles at all. On a microscopic scale, their behaviour is governed by quantum field theory, which treats all particles as waves in a particle field. For example, photons can be regarded as waves in the electromagnetic field.

In the Higgs mechanism there is a Higgs particle and therefore a Higgs field. However the Higgs field has the interesting property of having a non zero value in its lowest energy state (in empty space)⁴. Any particle moving through empty space will interact with the Higgs field and acquire mass. The strength of this interaction determines the mass of the particle. The field can be thought of as a collection of zero energy Higgs particles which cluster around the moving particle.

That, at least is Peter Higgs's theory. However as the Higgs boson has yet to be discovered, we don't know if he was on the right track. There are also a great many variations on this theory and many unanswered questions. For example, some theories say there is a single Higgs boson. Others predict the existence of several bosons, each with a different electrical charge.

Waldegrave Higgs Challenge

In 1993 the British science minister William Waldegrave, aware that British taxpayers were paying a lot of money for something very few of them understood, threw a challenge to UK particle physicists: to explain, in a simple manner on one piece of paper, 'What is the Higgs Boson, and why do we want to find it?'

Many physicists responded to Waldegrave's challenge and five official winners were chosen⁵. Of these, Professor David Miller's elaborate metaphor is probably the most quoted explanation of the Higgs Boson. He proposed the following picture:

Imagine a room full of political party workers quietly chatting with one another. This represents the Higgs field in space.

A former prime minister enters the room. All the workers she passes are strongly attracted to her. As she moves through the room, the cluster of admirers around her create resistance to her movement. This is how a particle moves through the Higgs field. The field clusters around a particle, resisting its motion and giving it mass.

If a rumour crosses the room, it creates the same clustering. The workers gather together to hear the details, the cluster can move across the room as the workers pass on the details to their neighbours. This cluster is the Higgs particle or Higgs Boson.

High Energy Physics

Particle physicists hunt for new particles using high energy particle accelerators or colliders. These machines use high electric fields and strong magnets to accelerate protons, electrons, antiprotons or positrons to close to the speed of light. The particles are then allowed to smash together and an extensive array of detectors looks for signs of new particles in the resulting debris. The heavier the particle you want to produce, the more energy required to accelerate the initial particles. The Higgs boson is heavy.

In the mid 1980s work started at CERN constructing their new machine, the Large Electron Positron collider (LEP). This was built in a 27km underground tunnel crossing the Swiss-French border. LEP ran from 1989 until 2000, initially producing W and Z particles in order to study electroweak theory and the standard model. Then from 1995 its energy was cranked up to maximum level to search for the Higgs boson, and anything else that might lie at high energies.

It was always known that LEP may not be able to achieve a sufficiently high energy to find the Higgs. Electrons and positrons are relatively light particles, and cannot achieve the energy of a proton or antiproton collider of a similar size. Therefore in 1994 CERN officially approved the Large Hadron Collider (LHC) to replace LEP. This is a proton-proton collider currently being built inside the LEP tunnel, scheduled (at the time of writing) to start operation in 2005.

Meanwhile, in the USA, American physicists were drawing up their own plans to find the Higgs, determined not to let the Europeans take it unchallenged. The USA had a large particle accelerator centre at Fermilab near Chicago. However, to search for the Higgs they too would need a higher energy machine. In 1983 the US Department of Energy recommended the construction of a high energy proton-proton collider.

The proposed Superconducting Super Collider (SSC) would have been the biggest machine ever built with a circumference of 87 kilometres. If the Higgs existed in the expected mass range such a machine could have found it easily. To some physicists at the time, the SSC sounded too good to be true. Unfortunately it turned out it was. Construction started in Ellis County, Texas at around the same time as LEP⁶. However, after several years of rising costs President Clinton cancelled the project in 1993⁷.

This decision put many physicists out of work. Some left for Europe or retrained as financial analysts. However, many moved to other US laboratories. Fermilab upgraded their existing proton antiproton accelerator, the Tevatron, which is currently the world's highest energy accelerator. There was still lots of research they could do, but it seemed almost certain the Higgs discovery would go to CERN.

The Events of September to November 2000

6 September 2000

After many years of negotiating with politicians, building accelerators and running computer models of theories, in September 2000 researchers at CERN finally observed what appeared to be the first sign of the Higgs. After running the LEP machine at its highest energy, one of the four detectors around the ring detected three events which appeared to be a sign of the elusive particle. However, in order to check this it would be necessary to run the machine for several months in order to collect more data. This was a problem as the accelerator was due to be shut down at the end of the month.

14 September 2000

After much negotiation it was decided to keep LEP running for another month in order to collect more statistics. This was an exciting time in the world of high energy physics. Emails flew thick and fast between laboratories, physicists found it difficult to talk about anything other than the Higgs - to the great frustration of their partners and children.

2 November 2000

By the start of November the atmosphere at CERN was ecstatic. The data collected over the previous month from all four detectors seemed to confirm they were on the trail of the Higgs. However, it would still take another few months to collect enough results to announce a discovery. With the machine now scheduled to close on 2 November, researchers held many last minute discussions and drew up a proposal to keep the collider running for another year.

8 November 2000

After much discussion a decision was taken not to continue running the experiment. The reasons given were that funding was not assured for another year, and it was important not to let construction of the LHC be delayed. However, these reasons failed to satisfy many European physicists. For months afterwards many went around muttering about secret deals between the CERN management and the Americans. They felt they had missed out on a big discovery. It would be five years before they could start Higgs hunting with the LHC, and they had effectively shown the Americans exactly where to look for it. They gloomily predicted that the Fermilab would now discover the Higgs. Although it was far from certain that the Tevatron could achieve a sufficient energy and collect enough data to do this.

Epilogue

Several months after the drama surrounding the final months of LEP, rumours began to circulate around the particle physics community that the results announced at CERN were not exactly all that had been originally claimed. It emerged that in their hurry to analyse the data before the closedown date, experimenters had messed up some of the calculations and underestimated the level of 'background' events from other particles. A more careful analysis could not find any evidence for the Higgs.

6 December 2001

Finally a year later when the full results had been published, researchers had to conclude that they had searched more than half of the possible mass range of the Higgs and found no sign of the particle. Newspaper stories followed with headlines like 'God Particle Disappears Down 6 bn Drain' to the horror of physicists.

Although no evidence for the Higgs has been found, it is still far more likely than not that it does exist; its mass is simply at the top end of the allowed range. The most likely future scenario now is that it will be discovered when the LHC is turned on in a few years time. However, a discovery at the Tevatron is not impossible.

The Higgs boson is such an important part of theoretical particle physics that if it were not discovered then many theoretical physicists would be lost. For years countless papers have been written assuming the Higgs mechanism is correct. However, the popularity of a theory does not necessarily mean it is right, and in many ways, showing the Higgs does not exist would be an even more interesting discovery than discovering it.

Although a certain level of transatlantic rivalry will probably always exist in particle physics, there is now a much stronger movement towards increased co-operation. The SSC showed the danger of one country trying to 'go it alone' with a really large project. Whereas CERN's success has shown the benefits of international collaborations. If another, even larger collider is built (and there is still more physics to do after the Higgs...) it will almost certainly be a global project like the International Space Station.

Peter Higgs celebrated his 70th birthday in 2000.