The latest results from the Tevatron collider at Fermilab near Chicago suggest that the Higgs boson is on the light side – which means that it could be harder to detect than a heavier particle. Predicted by the Standard Model of particle physics, the Higgs, if discovered, would provide an explanation for how elementary particles acquire mass. The Standard Model does not, however, say anything about what the exact mass of the Higgs boson ought to be and its eventual detection would be a massive achievement in high-energy physics.

Also in the race to detect the Higgs is the Large Hadron Collider (LHC) at CERN in Switzerland, which yesterday announced its first 7 TeV proton–proton collisions of the year. Although the LHC is expected to run continually until the end of 2012 before a year-long upgrade, researchers at Fermilab are keen to steal a march on their European rivals. Time is running out, though, as the Tevatron is due to close for good in September.

This makes the Tevatron the frontrunner in the hunt for the Standard Model Higgs boson Rob Roser, Fermilab

The new analysis of data from Tevatron's CDF and D0 experiments – along with earlier results – adds spice to that race, ruling out a Higgs mass of 156–183 GeV/C2. Much of this region is excluded to 95% confidence, with some excluded to 90%. The new analysis extends Tevatron's previous Higgs exclusion zone of 158–175 GeV/c2 (95%), which was reported in July 2010. "This makes the Tevatron the frontrunner in the hunt for the Standard Model Higgs boson," claims Fermilab physicist Rob Roser, who works on the CDF experiment.

When combined with previous searches for the Higgs and constraints imposed by the Standard Model, the Higgs mass is most likely to be in either a small sliver at about 183–185 GeV/C2 or somewhere between 114 and 156 GeV/C2 (see figure).

Lurking in the background

The ease with which the LHC can look for the Higgs depends partly on the particle's mass. If the Higgs is heavier than about 140 GeV/c2, it is more likely to decay into pairs of Z or W bosons, which would cause a distinct signal in the LHC's detectors. A lighter Higgs, in contrast, would favour a decay to b-quarks, which would be harder to see against the background of other events. Indeed, this difficulty is the reason why the Tevatron has not yet been able to extend its exclusion zone to lower Higgs masses.

Although it's the toughest region, the [LHC] experiments have been designed to do the job Greg Heath, University of Bristol, UK

If the Higgs weighs in towards the bottom of the theoretical range it could prove very difficult for the LHC to find the particle. However, Greg Heath of the University of Bristol in the UK, who works on the LHC's CMS experiment, points out that the collider is equipped for the job.

"In the LHC experiments we have a range of strategies for looking in the low-mass region, not all of which are available at the Tevatron because the LHC detectors are more powerful," he says. "Although it's the toughest region, the experiments have been designed to do the job there and we have a good chance of seeing at least the first signs with this year's data sample."

As for a higher mass Higgs, Heath points out that the LHC will also be looking at masses above 180 GeV/c2.

Colliders race for the Higgs

Despite the collider's imminent closure, researchers at Fermilab will continue operating the Tevatron until September 2011 in what is shaping up to be a race for the Higgs. "In the coming months our collaborations will focus on both the high-mass and low-mass scenarios and optimize our analysis techniques for the entire Higgs mass range," says CDF physicist Giovanni Punzi, of the University of Pisa and the National Institute of Nuclear Physics in Italy.