Physicists confront the puzzle of neutron lifespan

To solve a long-standing puzzle about how long a neutron can “live” outside an atomic nucleus, physicists have developed a wild but testable theory that posits the existence of a right-handed version of our left-handed universe. They designed a mind-blowing experiment at the Department of Energy’s Oak Ridge National Laboratory to try to detect a particle that has been speculated but undetected. If found, the theorized “mirror neutron” — a dark matter twin of the neutron — could explain a discrepancy between the responses of two types of neutron lifetime experiments and provide the first dark matter observation.

“Dark matter remains one of the most important and puzzling 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 Assessment Letters†

Neutrons and protons make up the nucleus of an atom. However, they can also exist outside of nuclei. Last year, co-author Frank Gonzalez, now at ORNL, with the help of the Los Alamos Neutron Science Center led the most accurate measurement ever how long free neutrons live before they decay, or turn into protons, electrons, and anti-neutrinos. The answer — 877.8 seconds, give or take 0.3 seconds, or just under 15 minutes — pointed to a crack in the Standard Model of particle physics. That model describes the behavior of subatomic particles, such as the three quarks that make up a neutron. The flipping of quarks initiates the decay of neutrons into protons.

“The lifetime of the neutrons is an important parameter in the Standard Model because it is used as an input to calculate the quark mixing matrix, which describes the decay rates of quarks,” said Gonzalez, who calculated the probabilities of neutrons oscillating for the ORNL- study. “If the quarks don’t mix as we expect, that indicates new physics beyond the Standard Model.”

To measure the lifetime of a free neutron, scientists take two approaches that should arrive at the same answer. Neutrons are captured in a magnetic bottle and their disappearance counts. The other counts protons appearing in a beam as neutrons decay. It turns out that neutrons seem to 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 is that the neutron transforms from one state to another and back again. “Oscillation is a quantum mechanical phenomenon,” Broussard said. “If a neutron can exist as a regular or a mirror neutron, then you can get this kind of oscillation, a rocking 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 that oscillate in dark matter mirror neutrons using a new vanishing and regeneration technique. The neutrons were created in the Spallation Neutron Source, a user facility of the DOE Office of Science. A beam of neutrons was directed to the magnetism reflectometer of SNS. Michael Fitzsimmons, a physicist jointly appointed with ORNL and the University of Tennessee, Knoxville, used the instrument to apply a strong magnetic field to enhance oscillations between neutron states. Then the beam hit a “wall” made of boron carbide, which is a strong neutron absorber.

If the neutron does indeed oscillate between normal and mirror states, when the neutron state hits the wall, it will interact with atomic nuclei and be absorbed into the wall. However, if it is in its theorized mirror neutron state, it is dark matter that will not interact.

So only mirror neutrons would come through the wall to the other side. It would be as if the neutrons had passed through a “portal” into a dark sector – a figurative concept used in the physics community. But the press reporting on related past work had fun taking liberties with the concept, comparing the theorized mirror universe Broussard’s team explores to the alternate reality “Upside Down” in the TV series “Stranger Things.” . The team’s experiments weren’t exploring a literal portal to a parallel universe.

“The dynamics are the same on the other side of the wall, where we’re trying to convert what are believed to be mirror neutrons — the twin state of dark matter — into regular neutrons,” said co-author Yuri Kamyshkov, a UT physicist who works with colleagues long pursued the ideas of neutron oscillations and mirror neutrons. “If we see regenerated neutrons, that could be a signal that we’ve seen something very exotic. The discovery of the particle nature of dark matter would have huge implications.”

Matthew Frost of ORNL, who obtained his PhD in collaboration with Kamyshkov at the UT, conducted the experiment with Broussard and assisted with data extraction, reduction and analysis. Frost and Broussard conducted preliminary tests with help from Lisa DeBeer-Schmitt, a neutron scattering scientist at ORNL.

Lawrence Heilbronn, a nuclear engineer at the 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 discovered how to calculate oscillations using graphics processing units — accelerators of specific types of computation in application codes — and performed independent analyzes of neutron beam intensity and statistics, and Taylor Dennis of East Tennessee State University helped set up the experiment and analyze background data, making him a finalist in a competition for this work. UT graduates Josh Barrow, James Ternullo and Shaun Vavra with undergraduates Adam Johnston, Peter Lewiz and Christopher Matteson contributed at various stages of experiment preparation and analysis. Louis Varriano, a graduate student at the University of Chicago, a former UT Torchbearer, helped with conceptual quantum mechanical calculations of mirror neutron regeneration.

The conclusion: No evidence of neutron regeneration was seen. “One hundred percent of the neutrons stopped, zero percent went through the wall,” Broussard said. Anyway, the result is still important for knowledge development in this area.

Now that one theory about mirror matter has been debunked, the scientists are turning to others to solve the puzzle about the lifespan of neutrons. “We continue to look for the reason for the discrepancy,” Broussard said. She and colleagues will use the High Flux Isotope Reactor for this, a DOE Office of Science user facility at ORNL. Ongoing upgrades at HFIR will allow for more sensitive searches because the reactor will produce a much higher neutron current, and the shielded detector on its small-angle neutron scattering diffractometer has a lower background.

Since 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 puzzle.

If it seems sad that the neutron lifelong puzzle remains unsolved, Broussard comforts: “Physics is hard because we’ve done it too well. Only the really difficult problems – and happy discoveries – remain.”