Rare particle's decay confounds hunt for new physics

Physicists have for the first time pinned down one of the rarest particle transformations glimpsed ? the decay of a B_s meson into a muon and its antiparticle. The sighting might not spark celebration, though, as it skewers some leading candidates for a much-needed new theory of the fundamental particles and forces.

The current leading theory ? the standard model of particle physics ? predicts that this decay is extremely rare, as turns out to be the case. But the search for an extension to the standard model is one of the top priorities in particle physics. While the theory explains most of the known particles and forces, it must be incomplete, as it says nothing about dark matter or gravity.

One such extension is supersymmetry, or SUSY, a collection of theories that encompasses dark matter and predicts that all elementary particles have a heavy, "super" partner. As many SUSY models predict the B_s (pronounced B-sub-s) meson decay to a pair of muons will be more common than the rate given by the standard model, searching for these transformations is one way to test SUSY.

Previous results from both the Large Hadron Collider at CERN near Geneva in Switzerland, and the now defunct Tevatron accelerator at Fermilab in Batavia, Illinois, had constrained the rate of decay to within a range close to that predicted by the standard model. But the constraints were not tight enough to rule out other theories such as the SUSY variants or other exotic theories that predicted a rate even lower.

Hot under the collar

That changed earlier today, when Johannes Albrecht of the LHCb experiment told the Hadron Collider Physics symposium in Kyoto, Japan, that his experiment has finally amassed enough data to rule out the rates predicted by those theories. LHCb sets the decay rate at one in every 300 million B_s mesons ? in close agreement with the standard model.

Maddeningly, that leaves SUSY neither confirmed nor disproven, as there are versions of the theory that allow this rate too. "This measurement certainly further shrinks the allowed parameter space for SUSY, but unfortunately, one cannot fully rule out SUSY," says Albrecht.

The result certainly reduces the SUSY options, though. "It is actually ruling out a lot of places for new physics," says Val Gibson, another LHCb researcher at the University of Cambridge, UK. "It is going to make theorists very hot under the collar."

Albert De Roeck, who works on another LHC experiment ? CMS ? looks on the bright side: "The SUSY space is vast, so having it reduced is of course helpful."

No smoking gun

He admits, though, that the lack of evidence for SUSY is disappointing. "There is no obvious smoking gun that we might have hoped for." He adds: "Most people were probably expecting this, but also hoping it would not be the case."

The CMS team is currently working on its own measurement of the B_s meson decay and expects to be able to confirm the LHCb rate in a couple of months.

Later this week at the same conference, results are expected about the true nature of the Higgs boson. Though the particle was discovered in July, the extent to which it fits the predictions of the standard model is still under question. Many theorists are hoping it will have some surprise qualities that may lead to a new theory, though the space for new physics around the Higgs is closing fast too.

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