Higgs boson is too saintly and supersymmetry too shy

HOPES of using the Higgs boson and the elegant theory of supersymmetry as shortcuts to discovering the mysteries of the universe are evaporating fast. That's the verdict of a major update from the Large Hadron Collider in CERN, near Geneva, Switzerland - the first since a boson resembling the Higgs was spotted there earlier this year.

"If our understanding of nature is correct, then the details of what happens next are more complicated than we had hoped," says Matthew Walker of CMS, one of the major LHC detectors.

In July, when CMS and its sister detector, ATLAS, announced the discovery of the boson, anomalies in the data hinted at physics beyond the standard model, the well-established description of the universe's particles and forces.

Such new physics is urgently needed because the standard model contains no mention of dark matter, makes incorrect predictions about the universe's antimatter and requires awkward "fine-tuning" to incorporate the Higgs mass reported in July.

The Higgs isn't searched for directly, but spotted via a slew of particles that the standard model predicts it decays to. One anomaly in July's particle debris was insufficient tau leptons, which could have implied the existence of non-standard particles (see "diagram").

But on 14 November, armed with twice as much data, CMS and ATLAS researchers told the Hadron Collider Physics Symposium in Kyoto, Japan, that the number of taus has crept up, removing the hint of deviant physics. CMS also reported a signal suggesting that the boson behaves the same when viewed in a mirror, giving it the property of positive parity, which the standard model also predicts.

There's still one anomaly left. In July, the newly discovered boson seemed to decay twice as often as predicted into pairs of photons, which could be the signature of an extra, non-standard particle, or of a non-standard Higgs. If that anomaly disappears too, the probable Higgs boson will look very standard indeed, which is strange because of all the known possible extensions to the standard model, none predicts a completely standard Higgs.

One explanation could lie in a theory called the Neutrino Minimal Standard Model (nuMSM), in which dark matter is actually a neutrino and the Higgs behaves so similarly to the standard model that the differences would be unobservable. Instead nuMSM might be discovered via space-based detectors that look for its proposed dark-matter particles, but it's a long shot. Just because it seems to fit right now doesn't mean nuMSM is the most plausible scenario, says Raymond Volkas of the University of Melbourne, Australia.

As if the boson's good behaviour wasn't frustrating enough, the LHC's searches for particles predicted by supersymmetry (SUSY) have turned up nothing. As SUSY - which proposes a heavier superpartner for each known particle - extends the standard model to include dark matter and other omissions, this failure deals a further blow to possible sources of new physics at the LHC. It is also stoking exchanges between SUSY supporters and sceptics (see "SUSY no-show fuels debate").

SUSY particles could show up at the higher LHC energies scheduled for 2014, after its year-long planned rest next year. But that is cold comfort to those hoping to have gleaned clues already. "I would, as a hunter of new physics, have liked to see it different to what we have now," says Albert De Roeck of CMS. "But the data is the data."

SUSY no-show fuels debate

Does a lack of evidence for supersymmetry at the LHC (main story) count against this elegant extension to the standard model?

"SUSY's plausibility is reduced," says Nobel laureate Steven Weinberg, "but not to zero." Others say the theory is flexible; that the latest results merely help to whittle down a list of possible incarnations.

Still the results have stoked a debate about the attention SUSY gets. "The theory, specifically as something we would observe at the LHC, was wildly over-promoted," wrote Matthew Strassler from Rutgers University in New Jersey on his blog.

That might have led other promising theories to suffer, says Raymond Volkas of the University of Melbourne, Australia, as popular theories can reduce interest in others. "Many people feel they have to work on the bandwagon ideas."

Michael Peskin of Stanford University in California disagrees. "I think that the serious effort given to SUSY is appropriate," he says, adding that SUSY is attractive because its predictions can be tested. Perhaps only time will tell, but if SUSY eludes the next round of searches in 2014, it will become much harder to test.

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