Published online 1 August 2008 | Nature | doi:10.1038/news.2008.993

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Fermilab still playing Higgs hide-and-seek

As the Large Hadron Collider prepares for its first particles, the ageing Tevatron is unwilling to concede the race for the Higgs boson.

When the Large Hadron Collider (LHC) finally fires up its particle beams in a month or so, many physicists hope that it will find a long-awaited prize: the Higgs boson, thought to confer mass on other elementary particles.

As the most powerful particle collider in the world, the LHC will draw hundreds of physicists to its home at CERN, Europe's particle-physics facility near Geneva.

TevatronDusk falls over the Tevatron ring.FERMILAB

But for now, the Tevatron at the Fermi National Accelerator Laboratory in Batavia, Illinois, is still the champ of particle accelerators. And physicists there, desperate to eke as much as they can out of the machine before she loses her crown, are on the verge of announcing an important ‘undiscovery’. If the Higgs exists at all, they reckon that it does not have an energy of 170 giga electronvolts (GeV) — give or take 2.5 GeV, the accuracy of their experiment.

Fermilab, and other particle facilities, have previously made predictions about where particle hunters might find the Higgs. But these have all relied on indirect evidence, such as the determination of the masses of the top quark and the W boson.

Now, the Tevatron has finally given scientists enough data to specifically look for the Higgs particle itself, and that data seems to unequivocally rule out seeing the Higgs at this particular energy.

It would be the first time such an exclusionary statement has been made since the Large Electron–Positron Collider at CERN explicitly ruled out Higgs bosons below 114 GeV before it closed in 2000.

Last throw of the dice

Cockcroft-Walton generatorThe Cockcroft-Walton generator helps to get particles up to speed at the Tevatron.FERMILAB

“This is our last chance to see something in the Higgs regime,” says Darien Wood, a physicist at Northeastern University in Boston, Massachusetts, and a spokesperson for DZero — one of the two main Tevatron experiments. “Now we’re finally seeing it come to fruition.”

Scientists from DZero and the Collider Detector at Fermilab (CDF), the other main experiment, are feverishly combining their data to make a definitive statement on the 170 GeV undiscovery in time for a talk on Sunday at the International Conference on High Energy Physics in Philadelphia, Pennsylvania.

Rob Roser, a spokesperson for the CDF, says that there could be more to come, with the two experiments expected to double their data if Fermilab runs Tevatron through 2010, as is hoped. That should provide enough data, Roser says, to explicitly find or rule out the Higgs particle in a range of energies from 115 GeV to 180 GeV. The folks at the LHC might have reason to be “a little nervous”, he says. “We certainly would like to steal their thunder.”

Boson battle

The LHC — built to collide particles with seven times the energy of the Tevatron — might find the Higgs boson comparatively quickly, especially if it exists at higher energies. But with the first beam collisions expected in early September, LHC scientists will only have two or three months for experiments before a scheduled winter shutdown.

That is probably only enough time for the calibration of instruments and the rediscovery of standard-model particles that have already been found elsewhere, and not enough time to find (or exclude) the Higgs particle, says Fabiola Gianotti, a deputy spokesperson for the ATLAS experiment, one of the two general particle-detector experiments at the LHC. Next year, however, the race will be on. “There’s always a bit of competition, but it’s a healthy competition,” she says.

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The Tevatron did not perform well in the first few years after a major upgrade in 2001. Structural instabilities, along with magnet and beam problems, meant that collision rates lagged. Publications were similarly slow to appear. But tweaks to the machine and improvements in data analysis have given the two main experiments the statistics they had initially set out for. Roser says that it is vindicating to finally be in a position to start directly excluding the Higgs. “Some people left the Tevatron because they didn’t think we’d be at this point now.”

Each experiment has now logged and analysed about 200 trillion events. Theory suggests that they might expect a few thousand Higgs events in the wreckage of those collisions. The Higgs events, if they exist, have probably been logged — they just need to be sifted from background noise. “It’s a piece of hay in a haystack,” says Wood. “It’s even harder than finding a needle.” 

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