Science in Mind

Searching for the “unparticle” and other new physics at Amherst College

courtesy of Steve Peck, Amherst College
courtesy of Steve Peck, Amherst CollegeCredit: The Amherst College experiment is trying to detect a new force -- long-range interactions between electrons in the Earth’s mantle and neutrons in the laboratory.

Inside a cylinder mounted on a rotating table top, Amherst College physicists are trying to detect tiny perturbations within mercury atoms, caused by long-range interactions with particles in the Earth’s interior.

Although it’s something of a scientific longshot, if the experiment detects anything, it could reveal new physics—exotic particles and a force never before seen—that would shake up the physics world.

In a study published Thursday in the journal Science, the Amherst team and a collaborator at the University of Texas at Austin reported that they had not discovered new physics yet, but had put limits on the energy associated with the force that would be carried by hypothetical particles—which include the existentially evocative “unparticle.” The team is honing in on the sensitive measurements it might need to detect new physics; the new results demonstrate that the theoretical force, if it exists, is extraordinarily weak—less than a millionth of the gravitational attraction between an electron and a neutron.

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“We are all doing what physicists do, which is we’re trying to check around the edges and see if things hang together. Theorists have proposed these particles, as possible things that we don’t have any reason to believe they don’t exist,” said Larry Hunter, a physics professor at Amherst. (In full disclosure, Hunter was one of my professors.)

“The probability of finding something has got to be regarded as not high, but the payoff is tremendous if you turn up a signal,” he said.

The modest-sized experiment fits easily in a room, and is an attempt to verify or rule out physics that lie beyond the widely accepted understanding of the fundamental building blocks of the universe, called the Standard Model. It’s an opposite tack from the one taken by the best-known physics experiments, such as the Large Hadron Collider, which require multinational teams of thousands of scientists and billions of dollars. The Amherst experiment uses precision measurements of a relatively small-scale setup, and drew on the efforts of a small team of five scientists.

What the physicists are probing is a theoretical type of distant interaction between ordinary particles, called long-range spin-spin interactions. Short-range spin-spin interactions are well known to anyone who has ever played with magnets, Hunter points out. The spins of electrons in magnets get lined up, causing attraction. But the theorized interactions between the spins of ordinary particles, such as electrons and neutrons, would be weaker and active over much longer distances. If such a force exists, it would potentially be transmitted through particles that have been postulated but never detected.

Other experiments have hunted for such interactions by creating a source of particles in the laboratory; helium atoms’ electron spins are all lined up using polarized light. But Hunter realized that right beneath his feet was a much larger source: electrons in the Earth’s mantle, the portion of the interior between the crust and the core. He did some back-of-the-envelope calculations to estimate how Earth’s magnetic field would influence electrons in its interior, and began to get very excited. But Hunter is not a geophysicist, so he turned to to an expert, Jung-Fu Lin of the University of Texas at Austin, for help. Using modern knowledge of geophysics, the Earth’s magnetic field, and the spin of electrons in the planet’s interior, they realized they could increase the size of the source—the number of electrons that could weakly interact with the neutrons in their laboratory detector—by more than a quadrillion-fold.

There was one problem: to detect spin-spin interactions, scientists needed to be able to reverse the magnetic field of the source. That’s the crucial switch that allows them to probe whether the source was interacting through the weak force with the particles in the detector. Because they couldn’t flip the magnetic field of the Earth, Hunter instead designed an experiment in which the detector could rotate, which would allow him to look for long-range interactions between electrons in the mantle and the neutrons, subatomic particles found in mercury, in the detector.

Frank Wilczek, a Nobel laureate and physics professor at the Massachusetts Institute of Technology, said the experiment was fascinating.

“It’s very exploratory, kind of speculative work,” Wilczek said. He noted that what he considers some of the most plausible hypothetical particles, called pseudoscalar bosons, can’t be detected by this experiment because their range of interaction drops off rapidly.

“Some of the most interesting possibilities elude them,” Wilczek said. “But they are breaking new ground in other directions.”