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Shrinking Higgs brings optimism to US lab

January 9, 2007 By Jenny Hogan This article courtesy of Nature News.

Tevatron gains renewed hope of bagging the particle that endows mass.

Physicists shooting to find the Higgs boson the particle thought to endow all others with mass have seen their target move, again. A new measurement of the mass of another subatomic particle, the W boson, has lowered the predicted mass of the Higgs.

This reassures scientists working at the Tevatron particle collider at Fermilab in Batavia, Illinois, that they still have a chance of finding the Higgs possibly even before the more powerful collider under construction at CERN (the European particle-physics laboratory near Geneva in Switzerland) can claim the discovery.

CERN's Large Hadron Collider (LHC) is expected to find the Higgs within a few years if the particle exists at all. But there is still a race between instruments. And the stakes for those involved are high.

"As they say, nobody remembers the person who came second," says Mark Lancaster, one of around 700 scientists working on the Collider Detector at Fermilab (CDF). CDF is one of the instruments collecting and analysing the debris from the proton-antiproton collisions in the Tevatron.

Weighty issue

Whether the Tevatron is likely to scoop the prize or not depends heavily on the mass of the elusive Higgs, for which scientists can make a good estimate based on the masses of other subatomic particles.

On 8 January, the estimate was tightened when the CDF announced it had pinned down the mass of the W boson, which mediates the weak nuclear force involved in processes such as radioactive decay. The new measurement is in agreement with previous estimates, but towards the upper end of the range. This, along with the added precision in the measurement, brings the upper limit for the Higgs' mass down to 153 giga electronvolts from 166 GeV. Previous experiments have shown that the Higgs must be heavier than 114 GeV.

A lighter Higgs suits the Tevatron, which is only capable of finding the particle if its mass is less than around 170 GeV. The closer a particle's mass is to this upper limit, the harder it would be to find.

Anything heavier than 170 GeV would certainly have to wait for the LHC, which will smash protons together harder to probe higher energies.

Ups and downs

The estimated mass of the Higgs has changed quite a bit over the past few years. In 2004, a revised measurement of the mass of the heaviest quark, known as the top quark, had bumped the predicted upper mass limit for the Higgs way up, to around 250 GeV. Since then a succession of measurements of top-quark and W mass has dragged the upper limit on the Higgs' mass down, with the latest measurement of the W boson bringing it to its lowest point yet.

In 2004, chances for the Tevatron looked slim. Not only was the Higgs possibly too heavy to be seen, but the collider was also struggling to reach its design specifications (see ' Below-par performance hampers Fermilab quest for Higgs' boson').

Now things are looking better for the Tevatron - on both fronts. The Higgs is thought to be lighter, and the instrument's performance has improved.

Race to a tie?

Lancaster is hopeful the Tevatron has a chance of finding the Higgs. Although, he admits: "I'm biased because I'm at the Tevatron". Still, he's hedging his bets about who will get there first. If the particle does turn up, Lancaster thinks the two projects may end up sharing the glory, with any hints of the Higgs from the Tevatron's late data being confirmed by early results from the LHC. "My gut reaction is that if it's around 120 GeV, the Tevatron and the LHC will probably find it around the same time."

They have some time to play with. Although the LHC is due to switch on later this year, it will take months or years for it to accumulate enough data to be sure of any detection. Meanwhile, the Tevatron will keep running until 2009 (though recent budget cuts mean that it may be forced to close temporarily during this time). "We need to increase our data size by a factor of five and improve our analysis techniques, but we've got three years to do that," says Lancaster.

But nature could, of course, still trump both teams. If there's physics beyond the Standard Model, it could wink the Higgs out of existence.

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