Cutting edge: Peter Ratoff

May 25, 2001

The race is on to find the Higgs boson particle, the key to the origin of mass.

The discovery of the most elusive particle in the universe is the target for two international teams of particle physicists working at Fermilab in the United States. Unfortunately, my colleagues and I at Lancaster and six other participating British universities may have to wait six years before finally tracking down the Higgs boson. Nevertheless, the importance of the discovery and the excitement of the chase will make the effort worthwhile.

The Higgs boson is the brainchild of Edinburgh University theoretical physicist Peter Higgs, a consequence of the rather subtle way he proposed in 1964 to resolve some esoteric problems in theories of elementary particle physics. His idea could explain why particles acquire mass through interaction with an invisible, energy-carrying field that pervades the entire universe. The Higgs boson is an inevitable byproduct of the theory and hence its discovery is key to understanding the origin of mass.

Paradoxically, although the Higgs mechanism can account for the observed masses of other elementary particles, it gives very little information about the mass of the Higgs boson itself. This makes it hard to know how to observe it. The search, which began in the late 1970s, has been long and fruitless. Many possible sightings have been claimed, but none has stood the test of time.

The most tantalising near miss occurred last year during the final, frantic months of Cern's Large Electron-Positron collider in Geneva. Combined data from four LEP teams showed a handful of events consistent with the production and decay of Higgs bosons. However, the results were not deemed statistically significant enough to claim a discovery. No more data have been gathered because the Cern management judged continuing LEP operation could jeopardise the construction of the Large Hadron Collider (LHC), a machine with a much better chance of producing the Higgs boson.

This decision has given me and my colleagues at Fermilab's Tevatron machine a free run until the LHC is switched on in 2006. The Tevatron is the world's highest-energy particle accelerator. Counter-circulating beams of protons and anti-protons collide at two points around the 3.5 mile circumference ring. The products flung out from the resulting collisions are recorded in two large multipurpose particle detectors called CDF and D-Zero. My group, with teams from Imperial College, London, and Manchester University, works with the D-Zero collaboration.

The two detectors are able to reconstruct and identify most of the different elementary particles into which a Higgs boson is likely to decay. While broadly similar in design, they are differently optimised. Both collaborations gathered data during the early to mid-1990s before the British joined, an effort that culminated in the discovery of the top quark, one of the basic building blocks of matter.

Tevatron's second run began on March 1. Over the next five or six years, we aim to collect sufficient data for a statistically significant observation of the Higgs boson. To achieve this goal, the CDF and D-Zero collaborations will pool their data, an approach pioneered with the LEP. Without this strategy, each of the Tevatron collaborations would need to collect twice as much data and, in all probability, the LHC would get there first.

If the clues provided by LEP are correct, we should be celebrating the discovery of the Higgs boson at the Tevatron at about the same time the LHC is switched on. Nevertheless, the LHC and other future machines will be essential to enable us to study the detailed properties of the Higgs boson and achieve a deeper understanding of the origin of mass.

Peter Ratoff is head of the particle physics research group at Lancaster University.

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