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SLAC theorist shares Sakurai Prize

SLAC National Accelerator Laboratory particle theorist Lance Dixon has been selected to receive the 2014 J.J. Sakurai Prize for Theoretical Particle Physics, one of the most prestigious awards in the field. The American Physical Society prize recognizes and encourages outstanding achievements in particle theory. Dixon shares the award with Zvi Bern of the University of Southern California, US, and David Kosower of the French research institute CEA-Saclay.

Lance Dixon, theoretical physicist at SLAC. Image courtesy SLAC/Brad Plummer.

Dixon will join his colleagues and collaborators, Bern and Kosower, to accept the $10,000 award in Savannah, Georgia, US, next April. The prize recognizes their work developing the Unitarity Method, a way of calculating probable outcomes for complicated particle interactions that are almost impossible to solve using standard methods.

"We essentially tried to reformulate how people do problems they used to attack with Feynman diagrams," Dixon says. Introduced by Nobel Prize-winning physicist Richard Feynman more than 60 years ago, Feynman diagrams are a way to visualize the math behind the behavior of subatomic particles. Diagram by diagram, they can produce as final states, particles that are physically impossible. You have to add all the possible diagrams together to demonstrate the probability for those physically impossible states is actually zero.

By the early 1990s, when particle colliders around the world were on the hunt for missing fundamental particles and experimentalists needed help interpreting the data, Feynman diagrams were reaching the limits of their usefulness. ‘All possible diagrams’ could number in the hundreds, even for simple collisions where only two particles interact to produce only a few more.

Each of these hundreds of diagrams produces many terms in the mathematical formula representing how particles interact. This makes it infeasible to do the calculations by hand. These problems can now only be handled using powerful distributed computers and supercomputers. For more complex collisions, the Unitarity Method can help.

"We looked at the problem and realized there were more patterns – some terms we expected to see didn't show up and some coefficients canceled," Dixon said. With help assistance from British theorist David Dunbar, they turned the patterns into generalizations based on the principle of unitarity, which ensures that the probabilities for all possible outcomes of an interaction add up to one. This helped solve more diagrams, and the new approach was simpler and more powerful than one based on string theory.

"It's become especially important now that the Large Hadron Collider hasn't yet found any new particles, aside from the Higgs," Dixon says. "We'll probably need to understand the Standard Model to an unprecedented level to recognize new physics when it appears in the LHC data."

Unitarity can also yield insights into some versions of supersymmetry – including one that includes gravity, the last fundamental force to join the quantum landscape. It may even be able to describe the graviton, the quantum mechanical expression of gravity.

Dixon follows SLAC Professor Emeritus Helen Quinn, who last year shared the award with her collaborator Roberto Peccei for their development of the theory of axions.


- Amber Harmon

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