Sizing up the Electron

Long thought to be a simple speck of negative charge, the humble electron may be hiding one more surprise in its depths.

The electron was the first fundamental particle discovered. It was the first to have its charge measured, and it inspired the mathematical equation that first hinted at the existence of antimatter, the exotic, oppositely charged counterpart to ordinary matter.

Now the electron is poised to go one step further, by helping scientists understand why matter triumphed over antimatter in the early universe. In theory, the Big Bang should have created matter and antimatter in equal amounts, but if so they would have annihilated each other and left nothing behind.

Though the standard model of particle physics, the mathematical framework for explaining how stuff is held together, can’t quite account for how matter beat out antimatter, some theories that go beyond the standard model do. By carefully measuring the shape of the electron, through a particular property known as the electric dipole moment, scientists think they can narrow those theories down to get at the one that best reflects reality.

“The electron EDM is one of the places where there should be a good chance of seeing some new phenomena that can’t be explained in the standard model, and could in turn help to explain this matter-antimatter imbalance in the universe,” says physicist David DeMille of Yale University.

Spotting the dipole moment would mean that the electron has some kind of internal structure, a bizarre concept for a particle that is supposed to buzz around the nuclear hearts of atoms and molecules with its mass concentrated into an essentially sizeless point. Although no one has yet measured the electron’s electric dipole moment, researchers think it should exist and could be within reach of today’s modern laboratory setups.

“There are good theoretical reasons to think that it isn’t too far away,” says physicist Larry Hunter of Amherst College in Massachusetts, who has been hunting the electron’s electric dipole moment since the 1980s. “What has made us all dedicate our lives to it is the real good chance that something might emerge soon.”

Researchers are now betting on several ways they might succeed, from studying ultracold atoms to lopsided molecules to magnetized ceramics. Within the next few months, scientists at Imperial College London are expected to report the latest limit on the size of the electron electric dipole moment, the first such improvement in a decade.

An actual measurement of that dipole moment “would be a big, big discovery,” says Eugene Commins, a physicist now retired from the University of California, Berkeley. “That would be a Nobel Prize.”

What lies within

Physicists suspect that electric dipole moments exist because they allow particles to violate what’s known as time-reversal symmetry. Although symmetry sounds like a good thing, scientists know that processes involving other particles (such as B mesons) behave differently whether running forward or backward, a violation of time-reversal symmetry. In order for this to happen, the electron (and other fundamental particles) must have an internal structure, something an electric dipole moment can reveal.

To envision an electron electric dipole moment, imagine that the electron has “a cloud of stuff following it, like the Pig-Pen character in the old Charlie Brown cartoons,” says DeMille. Blow that electron cloud up to the size of Earth, and extra positive charge would appear as a tiny dent on the north pole while extra negative charge would be a tiny bulge on the south pole. Given current limits, the size of that dent or bulge would correspond to adding or subtracting no more than about one-thousandth the width of a human hair from either end of the planet.

The standard model predicts that the electron’s electric dipole moment is less than 10^–38 in units of electron charge times centimeters. That’s equivalent to separating an electron and a similar charged particle by a distance of 10^–38 centimeters, or a hundred trillionth of a trillionth of a trillionth of a centimeter. But extensions of the standard model predict the electric dipole moment to be bigger, between 10^–25 and 10^–30. In 2002, Commins’ team published the most stringent limit yet: 1.6 – 10^–27. That means researchers are well within the hunting grounds where they might find the beast.

Each time experimentalists fail to detect the dipole moment despite increasing the sensitivity of their tests, they tighten the limit on how big it could be, like lowering the bar in a game of limbo. When the bar drops, theorists have to rule out more of their ideas on how the universe works. “A good theorist can make a model in an hour, but it takes us 20 years to destroy it,” Commins says.

The current limit has already ruled out the simplest version of a popular idea known as supersymmetry, which tries to explain the cosmic matter/antimatter imbalance by suggesting that every particle has an as-yet-unseen “superpartner.” If researchers can push the limit to 10^–29, that would rule out another extension to the standard model that tries to solve the matter problem by postulating multiple kinds of the particle known as the Higgs boson, which Europe’s Large Hadron Collider was designed to detect.

To measure the electron electric dipole moment, physicists need to watch an electron closely as they flip on an electric field. They then scrutinize whether a property known as the particle’s spin responds differently when the field is switched on in different orientations, which would mean the electron possesses an electric dipole moment. Seeing that difference is the hard part. In particular, because of the deep link between electricity and magnetism (moving electrons produce a magnetic field), it’s easy to accidentally change the magnetic field when the external electric field is applied. If this happens, the electron’s spin changes in unwanted ways that mimic how it should respond if it had an electric dipole moment.

Scientists have thus developed a bag of tricks to maximize their chances of detecting an electric dipole moment — by watching the electron for as long as possible, by enhancing its reaction and by removing as many sources of outside error as possible. The work is finicky and frustrating. At Amherst, Hunter spent years fine-tuning an experiment with cesium atoms and published a limit in 1989, only to be overtaken the next year by Commins. That work, at Berkeley, looked for the electric dipole moment in thallium atoms in the wee hours of the morning, when nearby trains that could disturb the measurement weren’t running.

Material of choice

Since those days, though, breakthroughs in trapping and cooling atoms using lasers have made atomic studies much more sensitive. One promising atom-based search today is in the lab of David Weiss at Pennsylvania State University in University Park. There he has developed a way to trap cesium atoms in two regions, each with an oppositely oriented electric field. Applying an external electric field to the entire thing should cause electrons in both regions to react equally. And because the fields had opposite orientations to start, any problems that might arise as an artifact of the test should be obvious.

Weiss’ team is building equipment now and hopes to start putting atoms into it soon. From there, though, analysis could take years. “The real question is how well you can ultimately control for systematic errors,” Weiss says. “You have to be sure that it’s right.”

Some scientists are taking a different tack by looking for the electron electric dipole moment in molecules. Polar molecules, which have one end with a slightly positive charge and the other with a slightly negative charge, look particularly promising. In polar molecules with one heavy atom and one light atom, electrons zoom around the heavy end quickly, like comets zipping into the solar system and past the sun. This gets the electrons going at nearly the speed of light, which naturally enhances the way the electron responds to an applied electric field, ramping up any dipole moment signal.

Las Vegas gamblers would do well to put their chips on ytterbium fluoride as the molecule most likely to yield a new limit on the electron electric dipole moment. A team led by physicist Edward Hinds, now at Imperial College London, has been using YbF in the hunt since 1993, and has submitted a paper describing its latest limit for publication.

Unlike atoms that can be trapped in one spot for a while, heavy molecules can be studied only in flight: A research team makes a beam of them and looks for the electric dipole moment signal as they fly by. Hinds’ group can currently detect only about one in every 100 YbF molecules that zip past, but is working on a new source that sends 10 times more molecules past and sends them at one-third the speed. Because the experiment’s sensitivity is proportional to how long researchers can study the molecule, the next generation should be 10 times better at spotting the electric dipole moment, Hinds says.

The group plans to have the new source up and running soon and, within the next few years, to lower the limit to 10^–29, where the electron electric dipole moment might be detected at last. “It’s conceivable that it’s just not there,” Hinds says. “But there should be a dipole moment unless there is some extraordinary accident.”

Hot on Hinds’ heels is another molecular experiment. A team led by DeMille along with Gerald Gabrielse and John Doyle of Harvard chose the thorium monoxide molecule because it would naturally enhance the electric dipole moment signal by quite a lot. The team first vaporizes some thorium dioxide with a pulse of laser light, then lines up the resulting thorium monoxide molecules in a beam line so they are all spinning in the same direction. Then, applying an electric field, the researchers try to figure out if the electron spin shifted within the molecule as it would if it had an electric dipole moment.

“These are incredibly tiny signals that we’re looking for,” says DeMille. “It’s not hard to imagine that effects can mimic the tiny thing you’re looking for that have nothing to do with the electric dipole moment.… If you accidentally apply a small magnetic field that changes along with the electric field, it can really be a dangerous type of error.”

The way forward

Yet another group is taking a fresh approach to molecules: stripping off one electron so that the molecule has a positive charge. Because they are electrically charged, such molecular ions can be easily confined and studied.

Eric Cornell, a Nobel-winning physicist at JILA in Boulder, Colo., started the molecular ion trend seven years ago, when he wondered how he could design an experiment from scratch to detect the electron electric dipole moment. He walked down the hall and took the elevators to the office of JILA theorist John Bohn. Handing Bohn a thick stack of manila folders, one per candidate molecule, Cornell asked him to calculate which molecular ion had the best chance of being studied for an electron electric dipole moment.

Several detailed papers later, Bohn had a list of candidates. All of them had the peculiar property of having electrons in what’s called a “triplet delta” state. That property makes the dipole moment easier to measure because when scientists apply the electric field, they can also simultaneously measure any magnetic field that might result — the very sorts of fields that can trip up the measurements. “There’s sort of a built-in way in the molecule to monitor what the magnetic field is doing,” says Aaron Leanhardt, a former postdoc in Cornell’s lab who now works at the University of Michigan in Ann Arbor.

Cornell is now starting to build an experiment to measure one of the ions, known as hafnium fluoride plus, in this triplet delta state. But so little is known about these molecular ions that his team must first do basic studies on the ions’ physical properties. “We’re mapping out this terra incognita,” Cornell says. Once he gets to measuring the dipole moment, he says, “I think I can do it better.”

Some scientists can’t be bothered fussing with individual atoms and molecules, and instead are trying big chunks of solids. Such materials contain untold numbers of electrons to measure; the idea is to apply an electric field that would line up a fraction of the electron spins in the same direction as the electric field. The researchers try to detect the resulting magnetization — which in this case is not a problem, but the actual signal they are trying to measure.

At Yale, physicists Steve Lamoreaux and Alex Sushkov think they can succeed with ceramic materials whose electron spins naturally align, a property that enhances the effect of an applied electric field. The researchers apply high voltage to a sample of material, about the size of a quarter, sitting in liquid helium. Using a supersensitive magnetometer, they detect magnetization in the ceramic as the electric field reverses. “We study all the physical effects going on in the sample to make sure that what we detect is from the EDM rather than something else,” Sushkov says.

Whether any of these approaches to hunting the electric dipole moment will succeed, and when, remains to be seen. After the Imperial team reports its results, the race will be on to see who can lower the bar next — or even detect the electric dipole moment altogether. Many are betting on DeMille’s collaboration using thorium monoxide, though that team, too, has run into unexpected challenges lately.

Even if one team does manage to detect the electric dipole moment, the work could be far from over. At least one other group, preferably working in a totally different system, would need to confirm the result to ensure the dipole moment really had been found. “It’s easy to screw up the experiments in subtle ways,” Cornell says. “You would really want two very different groups to see it to have any credibility.”

Hunter recently shut down another experiment using ceramics because he didn’t think it could be competitive. He says he can’t wait to see which team manages to cross the finish line first.

“It’s a high-risk field,” he notes. “You could easily spend your entire life working at it, like I have, and come up fairly empty-handed in the end. On the other hand, if you are the person who manages to improve the sensitivity to the point that you can unambiguously see the electron EDM, that’s just really exciting. It kind of shakes the foundation of physics, and that’s what we all dream of doing.”

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Electric past

The electron was at the heart of many scientific discoveries at the turn of the 20th century. Detecting its electric dipole moment could once again put the particle in the spotlight.

1897 J.J. Thomson discovers electrons, calling them “corpuscles,” revealing that atoms are divisible.

1900 Henri Becquerel, who discovered radioactivity, finds that beta particles are in fact electrons.

1913 Robert Millikan publishes results of his famous oil-drop experiments, which determine the charge of the electron.

1925 Samuel Goudsmit and George Uhlenbeck propose that an electron has an intrinsic angular momentum, called spin.

1927 Lester Germer and Clinton Davisson (left to right, above) find that electrons scatter from the surface of a crystal the same way X-rays do, proving particles can act like waves.

1928 Paul Dirac formulates his electron equation, which implies the existence of antielectrons — particles with the same mass as electrons but opposite charge.

1932 Carl Anderson discovers the antielectron, or positron, confirming the existence of antimatter (electron-positron pair formation shown above).

Image credit, from top: Goronwy Tudor Jones, Univ. of Birmingham/Photo Researchers; Bell Laboratories, courtesy AIP Emilio Segrè Visual Archives

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Beyond the standard

Though the electron’s electric dipole moment, or EDM, hasn’t been detected, experiments keep lowering the bar on how big it could be. Reducing that limit can rule out theories that go beyond physics’ standard model. In further experiments, scientists hope to reject some leading ideas.

1. The multi-Higgs model calls for multiple types of the Higgs particle, the as-yet-undiscovered particle thought to imbue others with mass.

2. In left-right symmetric models, particles behave the same way even if their direction of spin (or other qualities that are left-right dependent) is reversed.

3. The minimal supersymmetric standard model, or MSSM, is a standard model extension that holds that every elementary particle has a “superpartner.” One of the simplest versions has been ruled out by the current limit.

4. Another version of MSSM that sets a parameter dubbed phi to a different value is still a possibility, but it too may be ruled out when researchers lower the bar further.

Source: Imperial College London
Credit (graph): Centre for Cold Matter/Imperial College London, www3.imperial.ac.uk/ccm, adapted by T. Dubé