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MEG experiment may give boost to supersymmetry

The MEG experiment saw the possible decay of muons into electrons and gamma rays in candidate events such as this one. <em>Image courtesy of University of Tokyo.</em>

The MEG experiment saw the possible decay of muons into electrons and gamma rays in candidate events such as this one. Image courtesy of University of Tokyo.

Scientists at the Large Hadron Collider came up empty-handed in their first searches for evidence of the theory of supersymmetry. But preliminary results from MEG, an experiment initiated by the University of Tokyo, have given SUSY fans a glimmer of hope.

The MEG experiment, located at the Paul Scherrer Institute in Switzerland, does not search for supersymmetric particles directly. Instead, it searches for a process never observed before in nature and perhaps best explained by the theory of supersymmetry.

In supersymmetry, every fundamental particle in the Standard Model of physics has a heavier superpartner. Up until now, no man-made particle accelerator has succeeded in creating one of these theoretical particles, as the task would require huge amounts of energy. However, scientists at the Large Hadron Collider hope that, if supersymmetry exists, they will be able to produce hefty superparticles with their powerful machine.

Scientists at the MEG experiment want to catch nature in the act of doing something the Standard Model forbids. They are looking for muons decaying into their much lighter cousins, electrons, with excess energy flowing out as gamma rays. Theorists think that, if superparticles exist, they may make muon-to-electron conversion possible.

In their first run, MEG scientists found some candidate events but not enough to be sure of what they were seeing. This summer, the MEG experiment will announce new results using twice as much data.

“If there’s a signal, it should be evident,” said Toshinori Mori, spokesperson for the MEG experiment.

Muons and electrons are grouped together in the Standard Model, along with heavier particles called taus. Scientists expect particles in this group, known as charged leptons, to be able to transform into one another because they see something similar happening in a much lighter clan of particles -- neutrinos.

Neutrinos come in three types, each associated with one of the charged leptons: the electron neutrino, the muon neutrino and the tau neutrino. Scientists have already observed neutrinos changing from one type to another, a process the Standard Model cannot explain. If charged leptons transform, too, all particles might communicate with one another in a way that scientists have not seen thus far. Theorists say this would be possible in the framework of supersymmetry.

“Of course we can cook up lots of crazy models to explain this process,” said theorist Takeo Moroi of the University of Tokyo. “But from a theorist’s point of view, this is very interesting because it’s predicted by an existing model with its own motivations.”

The results of the MEG experiment will interest not only SUSY-watchers but also scientists on Fermilab’s planned Mu2e experiment, which could start construction in 2013.

If muons change into electrons, the process could occur in many different ways. MEG is looking for one of them. Mu2e would look for all of them. If MEG observes muon-to-electron conversion, Mu2e is sure to see it as well. But Mu2e scientists need not despair if the MEG experiment’s signal evaporates in its new data; it will still be possible for them to find muon-to-electron conversion occurring in another way.