Wednesday, November 13, 2013

Still No Electric Dipole Moment For Electron

I would prefer to comment on something like this once it has been properly published, but this has been in the news media for the past couple of days. And since I've already highlighted an earlier result on this, I might as well mention it here.

A while back, a study to find the electric dipole moment of the electron didn't find any. This result is consistent with the Standard Model picture of the electron being a symmetric, point particle with no internal structure. Now along comes a study that tries to measure this property again with even greater accuracy, and they still found nothing.

The Standard Model of particle physics, which describes all the known particles in the universe, predicts a practically zero electric dipole moment for the electron. Yet theories that include additional, yet-to-be-detected particles predict a much larger dipole moment. Physicists have been searching for this dipole moment for 50 years. Now a group called the ACME collaboration, led by David DeMille of Yale University and John Doyle and Gerald Gabrielse of Harvard University, has performed a test 10 times more sensitive than previous experiments, and still found no signs of an electric dipole moment in the electron. The electron appears to be spherical to within 0.00000000000000000000000000001 centimeter, according to ACME’s results, which were posted on the preprint site arXiv. “It’s a surprise,” says Ed Hinds, also of Imperial College London, who worked with Hudson on the previous best limit, set in 2011. “Why on Earth is it still zero?”
Of course, this new result is another blow to some classes of Supersymmetry, as if that theory isn't in trouble already from what we have (or didn't have) out of the LHC.

The new result deals a significant blow to many new physics theories, most notably supersymmetry, a favored idea that suggests each known particle in the universe has a supersymmetric twin particle that has yet to be discovered. “Supersymmetry is so elegant and somehow feels so natural that many people were starting to believe it was right,” Hinds says. But if they exist, all these twin particles should arise as virtual phantoms in the cloud around electrons, giving it a measurable electric dipole moment. The lack of one so far backs supersymmetry into a pretty tight corner. “It’s getting close to the point where it’s make or break for supersymmetry,” Hudson says. Although some basic models of the theory have been ruled out by the latest measurement, more complex models predict a small electric dipole moment that could be hiding in the range physicists have yet to search. “You can endlessly make models of supersymmetry,” says Eugene Commins, an emeritus professor of physics at the University of California, Berkeley, who led the last search for the dipole moment in atoms. “A good theorist can invent a model in half an hour, and it takes an experimentalist 20 years to kill it.”
I'll try to update this post with the proper citation when this gets published.


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