Particle Physics


Could this extreme magnetic shield reveal new physics?

Next to a nuclear reactor in Germany is a four ton, ten cubic meter metal box surrounded by lasers. The box is the most magnetically-shielded room ever built: the magnetic field inside is just a tenth of a billionth of a tesla, weaker than anywhere else in the solar system. This sanctuary could help physicists probe infinitesimal effects that may have profoundly influenced the newborn universe, and help solve one of the greatest secrets of the cosmos: why the universe has more matter than antimatter.

This image shows the inner three layers of the magnetic shield (also known as the insert), and the cylindrical layer with 780 copper rods. The cylindrical layer is used to generate a very uniform magnetic field, which is necessary for EDM experiments. Also shown are two measurement devices: A mercury magnetometer is placed between the wooden support structure, and a high precision pendulum device, which is used for the absolute alignment of conventional magnetic field probes, is on top of the support table. Credit: Technische Universität Müchen

The answer to this mystery could come from a decades-old quest exploring the structure of the neutron. Although neutrons are electrically neutral, they are made of particles known as quarks that are positively or negatively charged. Overall, these electric charges cancel out in neutrons, but there is the possibility they are not equally distributed—for instance, perhaps the north poles of neutrons are slightly positive, while the south poles are negative, explains Tim Chupp, a physicist at the University of Michigan who helped test the new shield and design experiments for it. The distribution of charge within the neutron is called its electric dipole moment.

If there is a tiny imbalance of electric charge within the neutron, this could be evidence of a special kind of asymmetry, called a charge parity (or “CP”) violation, that explains why there is more matter than antimatter in the universe. “This asymmetry would have had a strong effect in the early universe, shaping how the universe evolved a microsecond to a nanosecond after the Big Bang,” says physicist Peter Fierlinger of the Technical University of Munich, who led the overall effort that developed the new shield. But to detect it, physicists must work with neutrons that are shielded from the natural magnetic field of the Earth and the artificial magnetic fields of motors, electronics, and all the ubiquitous devices that produce their own magnetic fields.

Experiments conducted since the 1950s have not found a neutron electric dipole moment, but the new shield could improve the sensitivity of such measurements a hundredfold, down to the theoretically predicted scale of this phenomenon.

These exquisitely precise measurements are made possible by a magnetic shield that is ten times better than previous state-of-the-art shields. Previous research typically used a trial-and-error approach to find designs that magnetically shielded their interiors. Instead, Fierlinger and his colleagues used computer models of how magnetic fields interact with matter to optimize elements of their room’s design, including the spacing and thickness of the layers making up the walls.

The result is shield constructed like a Russian nesting doll, made of four one-millimeter-thick sheets of a soft nickel-iron alloy that are extraordinarily easy to magnetize and demagnetize, and act like a sponge to absorb outside magnetic fields. Within a one-cubic-meter space inside this haven, items would experience as little as a tenth of a billionth of a tesla—about 500,000 times less than the magnetic field typically felt at the surface of the Earth, and some ten times less than the magnetic field that suffuses “empty” space in the solar system. The scientists detailed their research May 12 in the Journal of Applied Physics.

The neutron is slightly magnetic, and so experiences a torque in the presence of an external magnetic field. If the neutron has an electric dipole moment, the amount of torque it experiences from an external magnetic field will change if an electric field is also applied on it. The lasers surrounding the shield are part of instruments known as optical atomic magnetometers that help measure and correct for the effects of extraneous magnetic fields.

“This magnetic shield will help us perform tests of the Standard Model with unprecedented precision,” says physicist Wolfgang Korsch at the University of Kentucky, who did not take part in this research. “Any deviation from the predictions of the Standard Model will clearly hint at new physics.”

“These low-energy precision measurements are very complementary with the experiments conducted at high-energy atom smashers such as the LHC,” says theoretical physicist Michael Ramsey-Musolf at the University of Massachusetts at Amherst, who did not participate in this study. “They’re both ways to solve the mystery of physics beyond the Standard Model.”

There are many other discoveries this magnetic shield could lead to, the researchers add. For example, it could help detect magnetic monopoles, long-theorized, hitherto-unseen particles that each only possess one magnetic pole, either north or south, unlike every known particle, which each have two. Magnetic sensors inside the shield could detect the faint, unique magnetic signals from any monopoles zipping through the room.

“The symmetries in nature may make beautiful patterns, but what are really interesting are the imperfections or breaking of symmetries—that’s where interesting physics lies,” says Chupp.

The shield may also open up unexpected avenues of research. “It could help detect the magnetic fields generated by electric currents in the brain,” Korsch says. “If they could actually put a person in there to do those measurements, I’d find that quite exciting.”

Go Deeper
Editor’s picks for further reading

PhysicsWorld: Extraordinary magnetic shield could reveal neutron’s electric dipole moment
A profile of the new magnetic shield and its scientific potential.

Symmetry: Explain it in 60 seconds: CP violation
A concise explanation of CP violation from Yosef Nir of the Weizmann Institute of Science.

Wikipedia: Neutron Electric Dipole Moment
An overview of the theoretical implications of the neutron electric dipole moment and the experimental searches for its value.

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Charles Choi

    Charles Q. Choi has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.