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NATURE|Vol 445|18 January 2007 NEWS

Behind the hunt for the Higgs

Tension is building on both sides of the Atlan- bound of the boson’s hovered around tic in the race to find the , the 250 gigaelectron volts (GeV), which was beyond elusive predicted by physicists’ under- the ’s upper limit of about 170 GeV. standing of mass. But estimates of the Higgs’ mass are based Scientists working at the Tevatron particle on the mass of other , such as the top collider at in Batavia, Illinois, wel- and W boson, and updates in these meas- comed the new year by measuring the mass of urements have since dragged the upper limit on another subatomic particle, the W boson. This the Higgs down. Most recently, on 8 January, the finding lowers the expected mass of the Higgs, CDF measured the mass of the W boson more bringing it further into the range in which the precisely than ever before, at 80.413 GeV. Tevatron might be able to find it. That figure is consistent with previous Meanwhile, the Tevatron’s more power- results, but towards the upper end of the range, ful competitor, the Large Collider and brings the upper limit for the Higgs down (LHC), is entering its final stages of construc- to 153 GeV from 166 GeV. The Large Elec- tion at CERN, the European particle- tron– (LEP) collider, which CERN lab near Geneva in hosted until it was Switzerland. If the shut down in 2000, WHERE IS LEP saw a hint of the Higgs exists, the LHC Higgs at 115 GeV, before ruled out of up should find it, but the THE HIGGS? it shut down in 2000 to 114.4 GeV — leav- spark of a chance that ing just a small win- the Tevatron might get dow still to explore there first is energiz- (see Graphic). ing both teams. “We At the same , are pulling out all the the Tevatron has over- stops,” says Mark Lan- come some of the per- caster, one of around formance shortfalls 700 scientists working that had dampened on the Collider Detec- hopes of finding it at tor at Fermilab (CDF) ExcludedMasses any mass. So research- — an instrument that by LEPstill to ers there, despite the Probability of Higgs at this mass this at Higgs of Probability collects and analyses collidersearch threat of budget cuts debris from – 30 114.4 153 forcing a temporary collisions Higgs mass (GeV) closure, are allowing in the Tevatron. themselves some opti- Jim Virdee at CERN is the spokesman for mism. “We have got about three years,” says one of the LHC’s experiments, the Compact Lancaster. The Tevatron will run until 2009 . “We are aware of this and, although the LHC is due to switch on for competition,” he says. “It makes us make sure a warm-up run in November, it will not start to the experiments and the accelerator are ready collect data seriously until spring 2008. on time.” The enormous parts of the spectrom- Ironically, the Tevatron’s next Higgs eter are currently being lowered delicately into announcement might be about not finding place in the LHC’s tunnels. it. Just before LEP closed to make way for The Higgs is thought to be a missing piece the LHC, it saw hints of a Higgs at around of the of , 115 GeV. The thought that the Higgs had nar- which lumps together what is known about rowly escaped their grasp has haunted CERN’s the fundamental and particles. Theo- physicists ever since, but the Tevatron is on the ries that extend the standard model without verge of having enough data to rule out a par- the Higgs tend to be more complex than those ticle of that mass. Unless something emerges that include it — leaving physicists hopeful that in the next few months, the LEP will be it will turn up. laid to rest. “I would say that we would be able Now that estimates of the Higgs’ mass have to rule out this hint from LEP before the LHC shrunk, the odds have improved that the Teva- gets going in earnest,” says Lancaster. ■ tron will be able to detect it. In 2004, the upper Jenny Hogan

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