By Geoff Brumfiel of Nature magazine

The Higgs boson, the most sought-after particle in all of physics, is proving tougher to find than physicists had hoped.

Last month, a flurry of "excess events" hinted that the Higgs could be popping up inside the Large Hadron Collider (LHC), the world's most powerful particle accelerator located at CERN, Europe's high-energy physics lab near Geneva, Switzerland. But new data presented today at the Lepton Photon conference in Mumbai, India, show the signal fading. It means that "this excess is probably just a statistical fluctuation," says Adam Falkowski, a theorist at the University of Paris-South in Orsay, France.

For almost 50 years, scientists have been chasing after the Higgs. The boson is thought to be a key part of the mechanism that endows other particles with mass. Just as importantly for physicists, it allows for the unification of electricity and magnetism with the weak nuclear force into a single "electroweak" force. Most believe that the particle is the last key part of the standard model of particle physics.

Many physicists hoped that the LHC might be able to catch the particle early on in its run, and there was cause for optimism. Last month in Grenoble, France, scientists announced evidence for what seemed to be enticing hints of a Higgs boson inside their detectors. The faint signal, at around 144 gigaelectronvolts (GeV), was seen independently by both the ATLAS and the Compact Muon Solenoid (CMS) detectors, the two largest detectors at the collider.

Moreover, the events appeared as an excess of W bosons, particles that moderate the weak nuclear force. Higher-than-expected numbers of W bosons were predicted by many theorists to be an early indicator of a Higgs boson (see "Collider sees tantalizing hints of Higgs").

Higgs hopes fade

But the latest results, which use about twice the data, seem to show that signal weakening. Both detectors now report that the significance of the find has dropped from around 2.8 sigma to 2 sigma. That means the odds of it being the real Higgs have fallen, from more than 99 percent to 95 percent, the opposite of what researchers would hope with additional data.

The drop in confidence came because of that additional data, and because physicists have improved their understanding of the other processes that can make W bosons, according to Richard Hawkings, deputy physics coordinator at ATLAS. It is not entirely unexpected, adds Joe Incandela, deputy spokesperson for the CMS. "The veterans who have been in the trenches know that in the early goings there are often things that are unclear," he says.

What researchers can say for sure is where the Higgs isn't. The CMS experiment has ruled out the boson's existence for energies between 145 and 400 GeV; ATLAS has eliminated several large patches between 146 and 466 GeV.

"There's still certainly plenty of room for a Higgs to be hiding in there," says Hawkings. Many physicists believe that the Higgs, if it exists, is likely to be at the lower mass end of the energy spectrum, perhaps between about 120 and 140 GeV. Those lower energy ranges will require more data to find a signal. It may be possible to rule out the Higgs in these regions by the end of the year, but confirmation of a discovery, which requires more data, will probably have to wait until the end of 2012.

This article is reproduced with permission from the magazine Nature. The article was first published on August 22, 2011.