Physicists seek neutron lifetime’s secret

New experiments aim to pin down subatomic particle’s decay rate

NIST Center for Neutron Research

TIME’S UP  New experiments aim to resolve a puzzle about how long the neutron takes to decay. At the National Institute of Standards and Technology’s Center for Neutron Research (shown), physicists will monitor a beam of neutrons to determine the neutron’s lifetime.

Ronald Cappelletti/NIST

WASHINGTON — Lone neutrons quickly decay, but scientists don’t agree on how long the particles stick around before their demise. New experiments could resolve the dispute — or deepen the mystery.

Outside of a nucleus, neutrons survive only about 15 minutes on average. They quickly decay into a proton, an electron and an antineutrino. Two methods used for measuring the neutron lifetime disagree, leaving scientists uncertain about the subatomic particle’s true longevity (SN: 5/19/12, p. 20).

One technique involves containing chilled neutrons in a trap, or “bottle,” waiting awhile, and counting the remaining neutrons to determine how many decayed. Other experiments monitor beams of neutrons and count the number of decays by detecting the protons emitted. Bottle measurements come up with lifetimes about 9 seconds shorter than beam measurements.

“This is actually fairly important for a number of things,” physicist Robert Pattie said January 29 at a meeting of the American Physical Society. In particular, pinning down the neutron’s lifetime is necessary for understanding how atomic nuclei began forming after the Big Bang. Scientists’ befuddlement makes it harder to calculate the properties of the early universe.

One drawback of typical bottle experiments is that neutrons can be absorbed or otherwise lost when they hit the wall of the bottle. So Pattie, of the Los Alamos National Laboratory in New Mexico, and colleagues are working on an updated bottle-style measurement using a magnetic field to keep neutrons from hitting the bottom of the trap, while gravity keeps them from flying out of the top.

Physicist Craig Huffer of North Carolina State University in Raleigh and colleagues are working on a bottle experiment that uses a magnetic field to trap neutrons. Rather than counting neutrons at the end, the researchers detect flashes of light produced as neutrons inside the bottle decay away.

In beam experiments, accuracy depends on making a careful count of the neutrons beamed in and the protons produced in the decays, physicist Shannon Fogwell Hoogerheide of the National Institute of Standards and Technology in Gaithersburg, Md., explained at the meeting on January 30. She and colleagues are refining their beam measurement to better enumerate protons and neutrons.

Some scientists have suggested the discrepancy could have deeper meaning. The short lifetime in bottle measurements could indicate that neutrons are somehow disappearing unexpectedly, making the lifetime appear shorter than it really is. “Kind of an out-there mechanism is that they’ve gone into some kind of alternative reality, which we call the mirror world,” says physicist Ben Rybolt of the University of Tennessee. In such a world, all the particles we know of would be duplicated — mirror protons, neutrons and electrons could exist, which would interact only very slightly with the particles we know.

Jumping to such an explanation for the neutron lifetime discrepancy is “a little bit of a leap,” Rybolt acknowledges, but such mirror particles could also explain the conundrum of dark matter, an unseen substance indicated by the motions of stars inside galaxies. To test the idea, Rybolt and colleagues are proposing to shoot beams of neutrons at a barrier and check if any make it through, which could indicate the particles had briefly become mirror neutrons.

The problem of measuring the neutron’s lifetime is complex enough that a variety of new techniques are under preparation to unravel the issue. “I don’t think one additional experiment can resolve the discrepancy,” says Huffer. Instead, multiple new measurements with different techniques should eventually converge on the correct value.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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