Oak Ridge National Laboratory

Physicists confront neutron lifetime puzzle

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 oscillating in dark matter mirror neutrons using a novel disappearance 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 SNSs magnetism reflectometer† 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, 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. Still, the press reporting on related past work had fun taking liberties with the concept, comparing the theoretical mirror universe Broussard’s team explores to the “Upside Down” alternate reality 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 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-Schmitta 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, Michael Kline of Ohio State University figured out how to calculate oscillations using graphics processing units — accelerators of specific types of calculations in application codes — and performed independent analyzes of the neutron beam’s intensity and statistics, and Taylor Dennis from East Tennessee State University helped set up the experiment and analyzed background data, making it 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. University of Chicago graduate student Louis Varriano, a former UT Torchbearerassisted 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.

With a certain mirror matter theory debunked, the scientists turn to others to try to solve the neutron lifespan puzzle. “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 as the reactor will produce a much higher neutron current and the shielded detector will run out 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 puzzle of neutron lifespans remains unsolved, take solace in Broussard: “Physics is difficult because we did too good a job at it. Only the really difficult problems – and happy discoveries – remain.”

The title of the article is “Experimental Search for Neutrons to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly.”

DOE’s Office of Science and ORNL’s Laboratory Directed Research and Development Program supported the work. The study used resources from the Spallation Neutron Source, a DOE Office of Science user facility at ORNL.

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