Physicists confront the mystery of the neutron lifetime

To solve a long-standing mystery of how long a neutron can “live” outside of an atomic nucleus, physicists have come up with a wild but testable theory that postulates the existence of a right-handed version of our left-handed Universe. They designed an intriguing experiment at the Department of Energy’s Oak Ridge National Laboratory to try to detect a particle that had been speculated but not discovered. If found, the theorized ‘mirror neutron’ – a dark matter twin of the neutron – could explain a discrepancy between answers from two types of neutron lifetime experiments and provide the first observation of dark matter.

“Dark matter remains one of the most important and enigmatic questions in science – clear evidence that we don’t understand all matter in nature,” said Leah Broussard of ORNL, who led the study published in Physical Verification Letters.

Neutrons and protons form the nucleus of an atom. However, they can also exist outside of cores. Last year, with the help of the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, made the most precise measurement yet of how long free neutrons live before they decay or turn into protons, electrons, and antineutrinos. The answer – 877.8 seconds plus or minus 0.3 seconds, or a little less than 15 minutes – indicated a crack in the Standard Model of particle physics. This model describes the behavior of subatomic particles, such as the three quarks that make up a neutron. Quark flipping initiates the decay of neutrons into protons.

“Neutron lifetimes are an important parameter in the Standard Model because they are used as input to calculate the quark mixing matrix, which describes quark decay rates,” said Gonzalez, who calculated the probabilities that neutrons will oscillate for the ORNL study. “If the quarks don’t mix in the way we expect, it points to new physics beyond the Standard Model.”

To measure the lifetime of a free neutron, scientists are taking two approaches that should lead to the same answer. You trap neutrons in a magnetic bottle and count their disappearance. The other counts protons that appear in a beam when neutrons decay. It turns out that neutrons live nine seconds longer in a beam than in a bottle.

Over the years, baffled physicists have considered many reasons for the discrepancy. One theory says that the neutron goes from one state to another and back again. “Oscillation is a quantum mechanical phenomenon,” Broussard said. “If a neutron can exist as either a normal or a mirror neutron, then you can get this kind of oscillation, a swing back and forth between the two states, as long as that transition isn’t forbidden.”

The ORNL-led team conducted the first search for neutrons oscillating in dark matter mirror neutrons using a novel disappearance and regeneration technique. The neutrons were produced at the Spallation Neutron Source, a user facility of the DOE Office of Science. A beam of neutrons was directed to the SNS magnetism reflectometer. Michael Fitzsimmons, a physicist with joint vocations at ORNL and the University of Tennessee, Knoxville, used the instrument to apply a powerful magnetic field to amplify the oscillations between neutron states. Then the beam hit a “wall” of boron carbide, which is a strong neutron absorber.

If the neutron is indeed oscillating between regular and mirror states, when the neutron state hits the wall, it will interact with atomic nuclei and be absorbed by the wall. However, when it’s in its theorized mirror neutron state, it’s dark matter, which won’t interact.

So only mirror neutrons would get through the wall to the other side. It would be as if the neutrons went through a “portal” to a dark sector – a figurative concept used in the physics community. But the press covering past work enjoyed taking liberties with the concept, comparing the theorized mirror universe Broussard’s team is exploring to the alternate reality “Upside Down” in the TV series Stranger Things . The team’s experiments did not explore a literal portal to a parallel universe.

“The dynamics are the same on the other side of the wall, where we’re trying to get what are believed to be mirror neutrons — the twin state of dark matter — to transform back into regular neutrons,” said co-author Yuri Kamyshkov, a UT -Physicist who has been pursuing the ideas of neutron oscillations and mirror neutrons with colleagues for a long time. “If we see regenerated neutrons, it could be a signal that we’ve seen something really exotic. Discovering the particle nature of dark matter would have enormous implications.”

ORNL’s Matthew Frost, who did his PhD at UT in collaboration with Kamyshkov, performed the experiment with Broussard and helped with data extraction, reduction and analysis. Frost and Broussard conducted preliminary experiments with the help of Lisa DeBeer-Schmitt, a neutron scattering scientist at ORNL.

Lawrence Heilbronn, a nuclear engineer at UT, characterized backgrounds, while Erik Iverson, a physicist at ORNL, characterized neutron signals. Through the DOE Office of Science Scientific Undergraduate Laboratory Internships Program, Ohio State University’s Michael Kline figured out how to calculate oscillations using graphics processing units – accelerators of certain types of calculations in application code – and performed independent analyzes of neutron beam intensity and statistics, and Taylor Dennis of East Tennessee State University helped set up the experiment and analyzed background data and became a finalist in a competition for this work. UT students Josh Barrow, James Ternullo, and Shaun Vavra with students Adam Johnston, Peter Lewiz, and Christopher Matteson contributed at various stages of experiment preparation and analysis. University of Chicago graduate student Louis Varriano, a former UT torchbearer, helped conceptual quantum mechanical calculations of mirror neutron regeneration.

The conclusion: no signs of neutron regeneration were seen. “One hundred percent of the neutrons were stopped, zero percent went through the wall,” Broussard said. Regardless, the result is still important for advancing knowledge in the field.

After one particular mirror matter theory is debunked, scientists turn to others to try and solve the mystery of neutron lifetimes. “We will continue to investigate the reason for the discrepancy,” Broussard said. She and her colleagues will use the High Flux Isotope Reactor, a user facility of the DOE Office of Science at ORNL. Ongoing upgrades to the HFIR will allow for more sensitive searches as the reactor will produce a much higher neutron flux and the shielded detector on its small-angle neutron scattering diffractometer will have a lower background.

Because the rigorous experiment found no evidence of mirror neutrons, the physicists were able to rule out a far-fetched theory. And that brings them closer to solving the mystery.

If it seems sad that the mystery of neutron lifetimes remains unsolved, you can take comfort from Broussard: “Physics is difficult because we’ve done too good a job at it. Only the really difficult problems—and happy discoveries—remain.”