New test of electron’s roundness could help explain universe’s matter/antimatter imbalance

When it comes to measuring how round the electron is, physicists hate uncertainty. Much depends on the most precise measurement possible, including a potential answer to a major scientific puzzle: why the universe contains any matter at all.

In a series of ever-more-sensitive experiments over the past 30 years, researchers have established that if the shape of the electron has any distortion at all, the bulge must be smaller than 1 thousand trillion trillionths of a millimeter (10-27 mm). Now, a group at the JILA research institute in Boulder, Colorado, has demonstrated what it describes as a "radically different" approach that probes electrons inside larger charged particles. Ed Hinds of Imperial College London calls the approach "brilliant" for the field, because it promises to help reduce the uncertainty still further—and perhaps reveal an actual distortion.

The electron's egg shape, if real, would be quantified by what is known as the electric dipole moment (EDM). Whereas scientists usually think of the electron as an exceedingly, if not infinitely, small and uniform sphere of negative charge, a nonzero EDM would mean that charge is distributed unevenly—forming one region fractionally more negative than the particle's average charge and one slightly less negative.

This tiny spatial asymmetry would have far-reaching implications, because it would contradict the idea that all physical processes look the same whether time runs forward or backward. Whereas time reversal would flip the direction of another property of the electron, its magnetic spin, it would leave any EDM unaffected, changing the relationship between the two. This breakdown in time-reversal symmetry would, in turn, "blow a hole" in particle physicists' simplest model of particles and forces, Hinds says. Instead, he adds, it would require a model in which nature contains many more fundamental particles than have been seen to date. It would also imply a fundamental asymmetry between matter and antimatter that would go some way toward explaining why the universe today contains far more matter than antimatter, even though equal amounts of each should have been made in the big bang.

According to David Weiss, an atomic physicist at Pennsylvania State University (Penn State) in State College, the cosmic matter excess implies that the electron's EDM is "very likely" to exist. And although the size of the EDM remains unknown, the most popular theories predict it is big enough to detect.

Because an EDM would cause an electron—or, more precisely, its spin axis—to rotate when placed in an electric field, simply sticking an electron between positive and negative electrodes should reveal it, in principle. But the resulting rotational force would be extremely weak—so weak that the electron would barely begin to turn before it crashed into the positive electrode. Scientists usually get around this problem by studying electrons within certain neutral atoms and molecules, in which internal fields far stronger than any external field can be induced. Researchers probe beams of these atoms or molecules for signs that certain electrons wobble, or precess—evidence of an EDM. But the motion of a beam limits the measurement time.

In the latest work, reported in Physical Review Letters this month, Eric Cornell and colleagues at JILA opt for an audacious alternative. Instead of probing a beam of neutral particles, they confine molecular ions of hafnium fluoride in a rotating electric field, which causes the ions to trace out little circles rather than flying away. After overcoming a few technical hurdles related to this circular motion, they tracked electrons' spin precession over the course of 0.7 seconds—about 1000 times longer than was previously possible with beams, which should open the way to greater sensitivity.

Cornell's group hasn't yet improved on the best existing measurement of the electron's sphericity, because the grouped ions disturb each other's spin and limit the number the trap can contain. The team's upper limit of 1.3 × 10-28 centimeters is some 1.5 times higher than the current best limit set with molecular beams, from the ACME collaboration at Harvard and Yale universities in 2014. (Harvard's Gerald Gabrielse says that by next year, the ACME team could reduce the uncertainty by a further factor of 20.)

Last month, however, the JILA group started up a new version of its experiment with higher electric fields in order to trap more ions simultaneously. Combined with other "nickel and dime" improvements, Cornell says this could boost sensitivity by about a factor of 10 over the next couple years. Eventually, he adds, the group plans to start using thorium fluoride, which is harder to measure than hafnium fluoride, but whose greater stability offers even longer precession times.

Other groups are deploying new measurement strategies that could also crank up the sensitivity. Weiss and colleagues at Penn State aim for a 30-fold improvement over the ACME result by creating a trap with lasers rather than electric fields. They hope to confine and measure cold neutral cesium atoms for several seconds. Physicists at Imperial College, meanwhile, hope to study a fountain of laser-cooled ytterbium fluoride molecules, which could yield a 1000-fold gain in the next 5 years. If no asymmetry appears at that sensitivity level, says group leader Hinds, the team should be able to "rule out a whole range of theories" predicting an electron EDM. But that, he adds, "won't stop theorists from coming up with new ideas."