麻豆传媒

The legendary particle that physicists thought explained why matter has mass probably does not exist. So say researchers who have spent a year analysing data from the LEP accelerator at the CERN nuclear physics lab near Geneva.

The elusive Higgs boson is so central to the standard model 鈥� the theory on which physicists base their whole understanding of matter 鈥� that it has been dubbed the 鈥淕od particle鈥�. If there is no Higgs, they will be left totally unable to explain mass.

The standard model explains the collection of fundamental particles that make up matter, including muons, electrons, neutrinos and quarks. In the 1960s, researchers successfully worked out how these particles interact and bind together via the strong and weak nuclear forces.

But that didn鈥檛 explain why the particles also have mass, until Peter Higgs at Edinburgh University suggested space is filled with a heavy, treacly substance 鈥� now called the Higgs field 鈥� which gives particles their mass by dragging on them through a mediator called the Higgs boson.

His work triggered a 30-year quest to find the Higgs. From the masses and interactions of other particles that we know exist, physicists calculated that the Higgs is most likely to have a mass (or energy) of around 80 gigaelectronvolts (GeV). If particle accelerators smash particles together at that energy or higher, it should be possible to make one.

Data sifting

This is what members of the Electroweak Working Group at CERN were doing for the 5 years until LEP (the Large Electron Positron Collider) closed down last year. Since then they鈥檝e been sifting through the data they gathered鈥揳nd found nothing. They rule out most possible masses for the Higgs, including the ones considered most likely. 鈥淚t鈥檚 more likely than not that there is no Higgs,鈥� says working group member John Swain of Northeastern University in Boston.

For many it鈥檚 a big disappointment, because last year researchers from another group at LEP claimed they had found the Higgs (麻豆传媒, 9 September 2000, p 4). Their announcement came shortly before LEP was due to close, and it won them one month鈥檚 extra time on LEP. But they later admitted to having botched their calculations in the heat of the moment.

Their mistake was to assume too low a level of background noise as the experiment鈥檚 energy was ratcheted up, so that they took scattered particles that were actually background as signs of the Higgs.

Now the calcuations have been reworked, members of the Electroweak Working Group say there was no sign of a Higgs at energies up to 115 GeV, well past the 80 GeV where it would be expected. That only leaves around 30 per cent of possibilities. It鈥檚 existence is looking 鈥渓ess and less likely鈥�, says Steve Reucroft, also of Northeastern University. 鈥淲e鈥檝e eliminated most of the hunting area,鈥� confirms Neil Calder at CERN.

Improbable energy

This isn鈥檛 the first bad news for the standard model. In February, researchers at Brookhaven National Laboratory in New York ruffled feathers by saying that the magnetic moment of the muon was different to the predicted value. But the latest blow is more serious. The non-appearance of a key particle would signal the end of large parts of the standard model.

Not everyone is too bothered, yet. Frank Wilczek, a particle physics theorist at the Massachusetts Institute of Technology, points out that you could take the LEP results as evidence that the Higgs must be sitting at an improbably high energy. He says he鈥檒l start to get uncomfortable if the Higgs doesn鈥檛 show up by about 130 GeV. 鈥淭hen I would have a good long think,鈥� he says.

Swain says he鈥檇 bet large amounts of money that the Higgs doesn鈥檛 exist. But he still thinks it鈥檚 important for CERN to push on with building its Large Hadron Collider, which is scheduled to start smashing particles at even higher energies in 2007. 鈥淚t鈥檚 not until you鈥檝e ruled out more than 99 per cent of values that everyone will be convinced,鈥� he says.

Difficult step

For example David Plane, head of LEP鈥檚 OPAL experiment, is still certain that the Higgs will eventually be found. 鈥淚t鈥檚 just at a higher energy than we鈥檙e sensitive to.鈥�

The problem for physicists is that without the Higgs particle they don鈥檛 have a viable theory of matter. 鈥淭here is nothing remotely as plausible or compelling to replace it,鈥� says Wilczek. Supersymmetry, which predicts every particle is paired with a heavier partner, is a popular idea. But LEP鈥檚 results are even worse news for this theory, as it predicts several Higgs particles. The lightest one would have turned up at even lower energies, and couldn鈥檛 exist above 130 GeV.

For physicists who have spent years trying to find the Higgs, admitting it could be fantasy is a huge and difficult step. But Swain is ready to get over the disappointment and move on. 鈥淵ou search for the truth, and the truth is whatever it is,鈥� he says.

See also: 麻豆传媒 editorial