Physicists confront the neutron lifetime puzzle


From left, ORNL’s Matthew Frost and Leah Broussard utilized a neutron spreading instrument at the Spallation Neutron Source to look for a dark matter twin to the neutron. Credit: Genevieve Martin/ORNL, U.S. Dept. of Energy

To resolve an enduring puzzle about for how long a neutron can “live” outside an atomic nucleus, physicists amused a wild however testable theory presuming the presence of a right-handed variation of our left-handed universe. They developed a mind-bending experiment at the Department of Energy’s Oak Ridge National Laboratory to attempt to spot a particle that has actually been hypothesized however not identified. If discovered, the thought “mirror neutron”—a dark-matter twin to the neutron—might describe a disparity in between responses from 2 kinds of neutron lifetime experiments and offer the initially observation of dark matter.


“Dark matter remains one of the most important and puzzling questions in science—clear evidence we don’t understand all matter in nature,” stated ORNL’s Leah Broussard, who led the research study released in Physical Review Letters.

Neutrons and protons comprise an atom’s nucleus. However, they likewise can exist outdoors nuclei. Last year, utilizing the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, led the most exact measurement ever of for how long totally free neutrons live prior to they decay, or develop into protons, electrons and anti-neutrinos. The response—877.8 seconds, provide or take 0.3 seconds, or a little under 15 minutes—meant a fracture in the Standard Model of particle physics. That design explains the habits of subatomic particles, such as the 3 quarks that comprise a neutron. The turning of quarks starts neutron decay into protons.

“The neutron lifetime is an important parameter in the Standard Model because it is used as an input for calculating the quark mixing matrix, which describes quark decay rates,” stated Gonzalez, who determined likelihoods of neutrons oscillating for the ORNL research study. “If the quarks don’t mix as we expect them to, that hints at new physics beyond the Standard Model.”

To procedure the lifetime of a complimentary neutron, researchers take 2 methods that need to reach the very same response. One traps neutrons in a magnetic bottle and counts their disappearance. The other counts protons appearing in a beam as neutrons decay. It ends up neutrons appear to live 9 seconds longer in a beam than in a bottle.

Physicists confront the neutron lifetime puzzle
Oak Ridge National Laboratory’s Leah Broussard reveals a neutron-taking in “wall” that stops all neutrons however in theory would permit theoretical mirror neutrons to travel through. Credit: Genevieve Martin/ORNL, U.S. Dept. of Energy

Over the years, perplexed physicists have actually thought about numerous factors for the disparity. One theory is that the neutron changes from one state to another and back once again. “Oscillation is a quantum mechanical phenomenon,” Broussard stated. “If a neutron can exist as either a regular or a mirror neutron, then you can get this sort of oscillation, a rocking back and forth between the two states, as long as that transition isn’t forbidden.”

The ORNL-led group carried out the very first look for neutrons oscillating into dark-matter mirror neutrons utilizing an unique disappearance and regrowth strategy. The neutrons were made at the Spallation Neutron Source, a DOE Office of Science user center. A beam of neutrons was assisted to SNS’s magnetism reflectometer. Michael Fitzsimmons, a physicist with a joint consultation at ORNL and the University of Tennessee, Knoxville, utilized the instrument to use a strong electromagnetic field to improve oscillations in between neutron states. Then the beam struck a “wall” made from boron carbide, which is a strong neutron absorber.

If the neutron carries out in reality oscillate in between routine and mirror states, when the neutron state strikes the wall, it will engage with atomic nuclei and get soaked up into the wall. If it remains in its thought mirror neutron state, nevertheless, it is dark matter that will not engage.

So just mirror neutrons would make it through the wall to the opposite. It would be as if the neutrons had actually gone through a “portal” to some dark sector—a metaphorical principle utilized in the physics neighborhood. Yet, the press reporting on previous associated work had a good time taking liberties with the principle, comparing the thought mirror universe Broussard’s group is checking out to the “Upside Down” alternate reality in the television series “Stranger Things.” The group’s experiments were not checking out an actual website to a parallel universe.

“The dynamics are the same on the other side of the wall, where we try to induce what are presumably mirror neutrons—the dark-matter twin state—to turn back into regular neutrons,” stated co-author Yuri Kamyshkov, a UT physicist who with associates has actually long pursued the concepts of neutron oscillations and mirror neutrons. “If we see any regenerated neutrons, that could be a signal that we’ve seen something really exotic. The discovery of the particle nature of dark matter would have tremendous implications.”

Credit: ORNL

Matthew Frost of ORNL, who got his doctorate from UT dealing with Kamyshkov, carried out the explore Broussard and helped with information extraction, decrease and analysis. Frost and Broussard carried out initial tests with assistance from Lisa DeBeer-Schmitt, a neutron spreading researcher at ORNL.

Lawrence Heilbronn, a nuclear engineer at UT, defined backgrounds, whereas Erik Iverson, a physicist at ORNL, defined neutron signals. Through the DOE Office of Science Scientific Undergraduate Laboratory Internships Program, Michael Kline of The Ohio State University found out how to determine oscillations utilizing graphics processing systems—accelerators of particular kinds of estimations in application codes—and carried out independent analyses of neutron beam strength and data, and Taylor Dennis of East Tennessee State University assisted establish the experiment and examined background information, ending up being a finalist in a competitors for this work. UT college students Josh Barrow, James Ternullo and Shaun Vavra with undergrads Adam Johnston, Peter Lewiz and Christopher Matteson contributed at numerous phases of experiment preparation and analysis. University of Chicago college student Louis Varriano, a previous UT Torchbearer, assisted with conceptual quantum-mechanical estimations of mirror-neutron regrowth.

The conclusion: No proof of neutron regrowth was seen. “One hundred percent of the neutrons stopped; zero percent passed through the wall,” Broussard stated. Regardless, the result is still crucial to the improvement of understanding in this field.

With one specific mirror-matter theory unmasked, the researchers rely on others to attempt to resolve the neutron lifetime puzzle. “We’re going to keep looking for the reason for the discrepancy,” Broussard stated. She and associates will utilize the High Flux Isotope Reactor, a DOE Office of Science user center at ORNL, for that. Ongoing upgrades at HFIR will make more delicate searches possible since the reactor will produce a much greater flux of neutrons, and the protected detector at its small-angle neutron spreading diffractometer has a lower background.

Because the strenuous experiment did not discover proof of mirror neutrons, the physicists had the ability to dismiss an improbable theory. And that takes them closer to resolving the puzzle.

If it appears unfortunate that the neutron lifetime puzzle stays unsolved, take solace from Broussard: “Physics is hard because we’ve done too good a job at it. Only the really hard problems—and lucky discoveries—are left.”


Understanding the early universe depends upon estimating the life-span of neutrons


More info:
L. J. Broussard et al, Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.212503

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Oak Ridge National Laboratory

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Physicists confront the neutron lifetime puzzle (2022, June 28)
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