A leaked internal memo from physicists working at the Large Hadron Collider near Geneva reports a whiff of the Higgs boson, the long-sought theoretical particle that could make or break the standard model of particle physics.

The preliminary note, which is still under review, was posted April 21 in an anonymous comment on physicist Peter Woit's blog, "Not Even Wrong." Four physicists claim that ATLAS, one of the LHC's all-purpose particle hunting experiments, caught a Higgs particle decaying into two high-energy photons – but at a much higher rate than the standard model predicts.

"The present result is the first definitive observation of physics beyond the standard model," the note says. "Exciting new physics, including new particles, may be expected to be found in the very near future."

The word from CERN, which operates the LHC, is that the leaked note is not an official result, and hasn't been backed up by the cast of thousands that makes up the rest of the ATLAS collaboration.

"It's way, way too early to say if there's anything in it or not," said CERN spokesman James Gillies. "The vast majority of these notes get knocked down before they ever see the light of day."

A member of the ATLAS collaboration who wished to remain anonymous noted that unexpected signals show up in the data pretty frequently, and turn out to be due to errors or biases that went uncorrected. The signal is much more likely to be a fluke than anything else.

The mood in the physics blogosphere is mixed between cautious excitement and outright denial.

"It may well turn out to be a false alarm ... or it could be the discovery of the century ... stay tuned," wrote a blogger called Jester at Résonaances, a blog that covers particle theory from Paris.

But graduate student Sarah Kavassalis at The Language of Bad Physics counters, "Until there is an official statement from the collaboration, or even one of the co-authors, this is just gossip. Don’t get excited. Seriously."

This isn't the first time a Higgs rumor has swept the physics community, either. A possible detection came from the CDF experiment at the Tevatron, a particle accelerator at Fermilab in Illinois, in July 2010. Blogger and physicist Tommaso Dorigo notes that CDF ought to have seen this new signal if it's really there.

Whether the Higgs is there or not, the paper is real. Physicists with access to the paper say it begins, "It is the purpose of this Note to report the first experimental observation at the Large Hadron Collider (LHC) of the Higgs particle."

"It's exciting stuff if it's true," Gillies said.

The standard model of particle physics is widely regarded as a theory of almost everything, explaining most of what we know about matter with 17 subatomic particles. But only 16 of those particles have been observed. The holdout is the Higgs boson, which was introduced in the 1960s to explain why matter has mass.

Finding the Higgs is one of the main goals of the LHC, a 17-mile-long underground tunnel near the border of France and Switzerland. Protons traveling around this tunnel at near-light speeds smash into each other and create new particles that only exist at very high energies.

These particles quickly decay into a flurry of other, more mundane particles like photons. Detectors like ATLAS and its twin, called CMS, can track the masses and paths that those ordinary particles take, and use their paths to reconstruct what happened in the collision.

The authors of the note, led by physicist Sau Lan Wu of the University of Wisconsin-Madison, say that ATLAS saw two photons whose energies add up to 115 gigaelectronvolts (GeV). That's the sort of thing you might expect if the Higgs boson had a mass of 115 GeV divided by the speed of light squared. (Because energy and mass are related by Einstein's famous E=mc2 equation, particle physicists often speak of mass and energy interchangeably. For comparison, a proton has a mass of about 0.9 GeV/c2.)

That mass is suggestive. The LHC's predecessor, the Large Electron-Positron Collider, or LEP, also may have seen a hint of a Higgs with the same mass in 2000, just weeks before LEP was shut down to make way for the LHC. Wu was also involved in that possible detection.

But if ATLAS really saw something, it's something decidedly unusual, the researchers report. If an experiment produces the standard model's version of the Higgs boson, only one in 100,000 of them will decay into two photons. The signal at ATLAS is 30 times bigger than the standard model predicts, meaning either they produced 30 times more Higgs bosons than expected, or 30 in 100,000 of them turned into two photons.

The purported signal could be a signature of supersymmetry, an extension of the standard model in which every particle has a "superpartner" that differs only in a quantum mechanical property called spin. Or it could be a particle that goes beyond the standard model altogether. One candidate is a hypothetical particle called the radion, which is associated with extra dimensions.

"Everybody wants to see something that takes us beyond the standard model," Gillies said. "Finding a standard-model Higgs at the LHC ... from a physics perspective it would be quite a boring thing. There's a lot of hope and expectation that we can find something beyond the standard model."

In the meantime, the LHC is charging forward into new territory. Around midnight Swiss time April 22, the LHC set a new record for beam intensity. The collider is scheduled to run at its current energy level, which is only half of what it's capable of, until the end of 2012.

That should be plenty of time to tell if the standard model Higgs exists or not, said ATLAS collaboration member Gustaaf Brooijmans of Columbia University in a press briefing in February.

"With the 2012 run added, in principle we believe we can exclude the existence of the Higgs boson over the full range, of course if it doesn’t exist," he said. "If it exists we would see a small signal. Then it depends on what the mass is, what kind of signal we will see."

Image: The ATLAS experiment at CERN.

See Also: