Experiments at Fermilab have narrowed the range of possible masses for the Higgs Boson – the elusive particle that physicists hope to see for the first time at the Large Hadron Collider (LHC). The results rule out about one quarter of the previous mass range for the Higgs and boost the chances that it will be rather lightweight – and therefore more difficult to detect.

The Higgs boson is predicted by the Standard Model of particle physics, and its existence provides an explanation for how elementary particles acquire mass. The Standard Model does not, however, predict the exact mass of the Higgs boson and its measurement is expected to be a major achievement of the LHC at CERN in Switzerland.

The ease with which the LHC will find the Higgs is partially dependent on the particle’s mass. A Higgs heavier than about 140 GeV/c2 is more likely to decay into pairs of Z or W bosons, which would cause a distinct signal in the LHC’s detectors. By contrast, a lighter Higgs would favour a decay to b–quarks, which would be more difficult to see against the background of other events.

The latest from the Tevatron collider at Fermilab in Chicago eliminates masses in the 158–175 GeV/c2 range to a confidence of 95%. When combined with previous searches for the Higgs and constraints imposed by the Standard Model, the Higgs mass is most likely be in the 114–158 GeV/c2 or the 175–185 GeV/c2 range.

Waiting around

If the Higgs weighs in towards the bottom of the 114–158 GeV/c2 range it could be five or more years before the LHC finds the particle. On the other hand, if it is at the top of this range – or at 175–185 GeV/c2 – its existence could be established within a year or so.

The latest constraint was a joint effort of Tevatron’s two main experiments – CDF and DZero. Each experiment has recorded about 500,000 billion proton-antiproton collisions since 2001 and the two experiment teams worked independently to sift through the data in search of evidence for the Higgs. While neither caught sight of the particle, the results were then combined to create the mass-exclusion limits.

There is still a chance that the Higgs could be spotted in the vast amount of data collected at the Tevatron. Indeed, scientists working at the collider are currently trying to persuade US funding bodies to run the facility for a few more years to boost the chances of spotting the Higgs – rather than shut it down in 2011 as planned.

The results were presented at the International Conference on High Energy Physics, which is currently taking place in Paris. A preprint of the paper is available at arXiv:1007.4587.