From the left, ORNL’s Matthew Frost and Leah Broussard used a neutron scattering instrument at the Spallation neutron source to look for dark matter bent at the neutron. Credit: Genevieve Martin / ORNL, U.S. Department of Energy
To solve a long-standing puzzle about how long a neutron can “live” outside an atomic nucleus, physicists entertained a wild but verifiable theory that postulated the existence of a right-wing version of our left-wing universe. They designed a mind-blowing experiment at the Oak Ridge National Laboratory in the Department of Energy to try to detect a particle that has been speculated but not detected. If found, the theorized “mirror neutron” —a twin of dark matter to the neutron — could explain a discrepancy between the responses of two types of neutron life 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 do not understand all of nature’s matter,” said Leah Broussard of ORNL, who led the study published in Physical review letters.
Neutrons and protons form the nucleus of an atom. However, they can also exist outside the nuclei. Last year, using the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, led the most accurate measure of how long free neutrons live before they disintegrate or become protons, electrons, and antineutrinos. The answer: 877.8 seconds, 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 form a neutron. The reversal of quarks initiates the decay of neutrons into protons.
“The service life of 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 González, who calculated the probabilities that neutrons oscillate for the ORNL study. “If the quarks do not mix as expected, this indicates a new physics beyond the standard model.”
To measure the lifespan of a free neutron, scientists adopt two approaches that should arrive at the same response. One traps neutrons in a magnetic bottle and counts their disappearance. The other counts the protons that appear in a beam as the neutrons disintegrate. It turns out that neutrons appear to live nine seconds longer in a beam than in a bottle.
Leah Broussard of Oak Ridge National Laboratory shows a neutron-absorbing “wall” that stops all neutrons, but in theory would allow hypothetical mirror neutrons to pass through. Credit: Genevieve Martin / ORNL, U.S. Department of Energy
Over the years, perplexed physicists have considered many reasons for the discrepancy. One theory is that the neutron transforms from one state to another and returns again. “Oscillation is a phenomenon of quantum mechanics,” Broussard said. “If a neutron can exist as a regular neutron or mirror, then you can get that kind of oscillation, a swing between the two states, as long as that transition isn’t forbidden.”
The ORNL-led team conducted the first search for neutrons that oscillate into dark matter mirror neutrons using a new disappearance and regeneration technique. The neutrons were made at the Spallation neutron source, a user facility of the DOE Science Office. A neutron beam was guided to the SNS magnetism reflectometer. Michael Fitzsimmons, a physicist with a joint appointment at the ORNL and the University of Tennessee, Knoxville, used the instrument to apply a strong magnetic field to improve oscillations between neutron states. The beam then impacted against a “wall” made of boron carbide, which is a strong neutron absorber.
If the neutron actually oscillates between the regular and mirror states, when the neutron state touches the wall, it will interact with the atomic nuclei and be absorbed into the wall. If it is in its state of theorized mirror neutrons, however, it is dark matter that will not interact.
So only mirror neutrons could cross the wall to the other side. It would be as if neutrons had passed through a “portal” into some dark sector, a figurative concept used in the physical community. However, the press reporting on previous related work had fun taking liberties with the concept, comparing the theorized mirror universe that Broussard’s team is exploring with the alternative reality “Upside Down” from the TV series “Stranger Things “. The team’s experiments were not exploring a literal portal 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 twin state of dark matter, to become regular neutrons again,” said co-author Yuri Kamyshkov, UT physicist. who with his colleagues has long pursued the ideas of neutron oscillations and mirror neutrons. “If we see regenerated neutrons, that could be a sign that we’ve seen something really exotic. The discovery of the nature of dark matter particles would have huge implications.”
Credit: ORNL
Matthew Frost of the ORNL, who received his doctorate from UT working with Kamyshkov, conducted the experiment with Broussard and helped with data extraction, reduction, and analysis. Frost and Broussard performed preliminary tests with the help of Lisa DeBeer-Schmitt, an ORNL neutron scattering scientist.
Lawrence Heilbronn, a UT nuclear engineer, characterized the background, while Erik Iverson, an ORNL physicist, characterized the neutron signals. Using the DOE Office’s scientific-grade lab internship program, Michael Kline of Ohio State University discovered how to calculate oscillations using graphics processing units (accelerators of specific types of calculations in codes application) 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 the background data, becoming a finalist in a competition for this work. UT graduate students Josh Barrow, James Ternullo and Shaun Vavra along with undergraduate students Adam Johnston, Peter Lewiz and Christopher Matteson contributed to various stages of preparation and analysis of experiments. Louis Varriano, a former torchbearer at the University of Chicago, helped with conceptual calculations of quantum mechanics of mirror neutron regeneration.
The conclusion: no evidence of neutron regeneration was observed. “One hundred percent of the neutrons stopped; zero percent passed through the wall,” Broussard said. Regardless, the outcome remains important for the advancement of knowledge in this field.
With a particular theory of mirror matter disproved, scientists turn to others to try to solve the neutron life puzzle. “We will continue to look for the reason for the discrepancy,” Broussard said. She and her colleagues will use the high-flow isotope reactor, a user facility of the DOE Science Office at ORNL. Ongoing upgrades to HFIR will make possible more sensitive searches because the reactor will produce a much higher neutron flux, and the armored detector of its small-angle neutron scattering diffractometer has a lower background.
Because the rigorous experiment found no evidence of mirror neutrons, 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 lifespan puzzle remains unresolved, I console myself with Broussard: “Physics is difficult because we have done too good a job at it. Only the really difficult problems and the lucky discoveries remain.”
Understanding the early universe depends on estimating the life of neutrons. More information: LJ 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 Provided by Oak Ridge National Laboratory
Quote: Physicists confront the neutron lifetime puzzle (2022, June 28) retrieved June 29, 2022 from …