Physicists tackle neutron lifetime puzzle

Physicists tackle neutron lifetime puzzle

Physicists tackle neutron lifetime puzzle

Left to right, Matthew Frost and Leah Broussard of ORNL used a neutron scattering instrument at the spallation neutron source to search for a neutron dark matter twin. Credit: Geneviève Martin/ORNL, US Department of Energy

To solve a long-standing puzzle about how long a neutron lives outside an atomic nucleus, physicists have come up with a wacky but testable theory postulating the existence of a right-handed version of our left-handed universe. They’ve 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 on but not spotted. If found, the theorized ‘mirror neutron’ – a dark matter twin to the neutron – could explain a discrepancy between responses from two types of neutron lifetime experiments and provide the first observation of dark matter .

“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 ORNL’s Leah Broussard, who led the study. study published in Physical examination letters.

Neutrons and protons make up the nucleus of an atom. However, they can also exist outside of nuclei. Last year, using the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, conducted the most precise measurement ever of the lifetime of free neutrons before they decay or die. transform into protons, electrons and antineutrinos. The answer – 877.8 seconds, plus or minus 0.3 seconds, or just under 15 minutes – hinted at 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. The tilting of quarks initiates the decay of neutrons into protons.

“The neutron lifetime is an important parameter in the Standard Model because it is used as input for the calculation of the quark mixing matrix, which describes the decay rates of quarks,” said Gonzalez, who calculated the neutron oscillation probabilities for ORNL study. “If the quarks don’t mix as expected, it hints at new physics beyond the Standard Model.”

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

Physicists tackle neutron lifetime puzzle

Leah Broussard of the Oak Ridge National Laboratory shows a neutron-absorbing “wall” that stops all neutrons but which, in theory, would allow the passage of hypothetical mirror neutrons. Credit: Geneviève Martin/ORNL, US Department of Energy

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

The ORNL-led team performed the first search for oscillating neutrons in dark matter mirror neutrons using a new technique of disappearance and regeneration. The neutrons were fabricated at the Spallation Neutron Source, a DOE Office of Science user facility. A neutron beam was guided to the SNS magnetism reflectometer. Michael Fitzsimmons, a physicist with a joint appointment at ORNL and the University of Tennessee at 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 powerful neutron absorber.

If the neutron actually oscillates between regular and mirror states, when the neutron state hits the wall, it will interact with the atomic nuclei and be absorbed by the wall. If it is in its theorized mirror neutron state, however, it is dark matter that will not interact.

Thus, only the mirror neutrons would pass through the wall to the other side. It would be as if the neutrons had passed through a “portal” to a dark sector – a figurative concept used in the physics community. Still, press reporting on related earlier work have had fun 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 induce what are presumably mirror neutrons – the twin state of dark matter – to become regular neutrons again,” said the co-author Yuri Kamyshkov, a physicist from UT. who, with colleagues, has long pursued the ideas of neutron oscillations and mirror neutrons. “If we see regenerated neutrons, it could be a signal that we’ve seen something really exotic. Discovering the particulate nature of dark matter would have huge implications.”

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ORNL’s Matthew Frost, who earned his Ph.D. from UT working with Kamyshkov, performed the experiment with Broussard and helped with data extraction, reduction, and analysis. Frost and Broussard conducted preliminary tests with the help of ORNL neutron scattering scientist Lisa DeBeer-Schmitt.

Lawrence Heilbronn, a nuclear engineer at UT, characterized background noise, while Erik Iverson, a physicist at ORNL, characterized neutron signals. Through the DOE Office of Science’s Undergraduate Science Lab Internship Program, Ohio State University’s Michael Kline figured out how to compute oscillations using graphics processing units – accelerators of specific types of computations. 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 the baseline data, becoming a finalist in a competition for the work. UT graduate students Josh Barrow, James Ternullo, and Shaun Vavra along with undergraduate 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, assisted with conceptual quantum mechanical calculations of mirror neutron regeneration.

Conclusion: No evidence of neutron regeneration was observed. “One hundred percent of the neutrons stopped; zero percent went through the wall,” Broussard said. Either way, the result is still important for the advancement of knowledge in this field.

With one particular theory of mirror matter debunked, scientists are turning to others to try to solve the neutron lifetime puzzle. “We will continue to investigate the reason for the discrepancy,” Broussard said. She and her colleagues will use the High Flux Isotope Reactor, a DOE Office of Science user facility at ORNL, for this. Ongoing upgrades at HFIR will make more sensitive searches possible because the reactor will produce a much higher neutron flux and the shielded detector of its small-angle neutron scattering diffractometer has a lower background noise.

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

If it seems sad that the neutron lifetime riddle remains unsolved, take reassurance from Broussard: “Physics is hard because we’ve done too good a job at it. Only the really difficult problems remain – and the happy discoveries.

Understanding the early universe depends on estimating neutron lifetimes

More information:
LJ Broussard et al, Experimental investigation of neutron-mirror oscillations as an explanation for the neutron lifetime anomaly, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.212503

Provided by Oak Ridge National Laboratory

Quote: Physicists confront the neutron life puzzle (June 28, 2022) retrieved June 28, 2022 from

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