Physicists have for the first time pinned down one of the rarest particle transformations glimpsed – the decay of a Bs 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 Bs (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.
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