A joint Fermilab/SLAC publication

More than one way to search for SUSY

07/26/11

Experiments at the Large Hadron Collider have yet to find signs of supersymmetric particles, scientists announced at the European Physical Society conference this week in Grenoble.

Image courtesy of timlewisnm on Flickr via a Creative Commons license

But physicists will significantly improve their knowledge of SUSY in the coming year through indirect methods, which could include the discovery of the Higgs boson.

“For supersymmetry, this is a decisive moment in time,” said theorist Lars Bergstrom of Stockholm University.

Supersymmetry is a model that solves some significant problems of the Standard Model of particle physics. SUSY doubles the zoo of elementary particles by adding a partner for each of the particles we already know. It handily relates different types of particles in the Standard Model and offers an appealing candidate for dark matter. So far, scientists have found no evidence of SUSY particles at the LHC.

It’s looking bad for the simplest models of supersymmetry. Scientists at the LHC have been chipping away at two basic SUSY theories since they began analyzing data in 2010. But dozens of other models are still in the running, and scientists have more than one way to test them.

Discovering the mass of the Higgs boson, which scientists could do by next year, will reveal something about the likelihood that supersymmetry exists. Members of the ATLAS and CMS collaborations both displayed at the EPS conference tiny hints of a Higgs boson with a mass within the interval of 115-150 GeV.

To mesh well with the simplest models of supersymmetry, the Higgs would need to have a mass lower than 130 GeV, Bergstrom said. If its mass were more than 140, even the more complicated models of supersymmetry would fall out of favor.

“If it were higher than 140 GeV, you would have to twist things in weird ways to make supersymmetry work,” Bergstrom said. He added, “But one should never underestimate theorists.”

Scientists will also know more about SUSY by next year through improved studies of rare decays, which supersymmetric particles could influence.

Scientists from the CDF collaboration at the Tevatron and the CMS and LHCb collaborations at the LHC announced results in a study of the decay of a bottom-strange meson into two muons. According to the Standard Model, this decay should happen extremely rarely. However, if heavier, unseen particles exist, they could spur this decay to happen more frequently.

“This decay is one of the most powerful indirect searches for new physics,” said Guido Tonelli, spokesperson of the CMS experiment.

The CDF experiment saw the mesons decay onto a pair of muons more often than expected but only by a small amount with limited statistical significance. CDF scientists plan to update their result using 20-30 percent more data by next year.

“By then the LHC experiments will probably write the book on this, though,” said Rob Roser, spokesperson for the CDF experiment. “And that’s good; that’s what they’re supposed to do.”

The CMS and LHCb experiments did not see the same effect, but so far they do not have enough data to give a final verdict. They hope to have triple the data by next year.

In these types of decays, the heavier the unseen particles are, the less influence they have. It could be that the experiments do not see much of an effect because the particles causing the decay are extremely massive.

This could spell trouble for supersymmetry, Tonelli said. “The heavier they become, the more difficult it is to explain supersymmetry,” he said. “Above a certain mass it makes no sense.”

It could also be that the particles affecting the decay are not supersymmetric but rather something different. And it could be that nothing is affecting the decay at all.

The LHCb experiment, which was made to do this kind of physics, is looking into about six of these types of decays. Other accelerator experiments are looking into some of these as well.

The many versions of SUSY make it almost impossible to completely rule out, but physicists are spreading as wide as they can in their search.

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