To solve a longstanding mystery about how long a neutron can “live” outside the nucleus of an atom, physicists have come up with a wild but testable theory that posits a right-handed version of our left-handed universe. They designed a mind-bending experiment at the Department of Energy’s Oak Ridge National Laboratory to attempt to detect a particle that was predicted but not detected. If found, a “neutron mirror” – a dark matter twin to a neutron – could explain the discrepancy between answers from two types of neutron lifetime experiments and provide the first observation of dark matter.
ORNL’s Leah Broussard, who led the study published in physical review messages.
Neutrons and protons make up the nucleus of an atom. However, it 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 measurement ever About how long free neutrons live before they decay, or turn into protons, electrons, and antineutrinos. The answer — 877.8 seconds, give or take 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 the neutron. Flipping quarks causes neutrons to decay into protons.
“The age of the neutron is an important factor 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 neutron oscillation probabilities in the ORNL study. “If quarks don’t mix up the way we expect them to, that hints at 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. Someone traps neutrons in a magnetic bottle and calculates their disappearance. The other numbers of protons appear in a bundle with the decay of neutrons. It turns out that neutrons appear to live in a beam nine seconds longer than they do in a bottle.
Over the years, bewildered physicists have considered many reasons for the discrepancy. One theory says that the neutron switches from one state to another and back again. “Oscillation is a quantum mechanical phenomenon,” Broussard said. “If a neutron can exist as a normal neutron or an inverse neutron, then you can have that kind of oscillation, swinging back and forth between the two states, as long as that transition isn’t forbidden.”
The team led by ORNL conducted the first search for oscillating neutrons dark matter Reversing neutrons using a new technique of disappearance and regeneration. The neutrons were made at the Spallation Neutron Source, a DOE Office of Science user facility. A beam of neutrons was directed at the SNS magnetometer. Michael Fitzsimmons, a co-designated physicist at 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 is actually oscillating between the normal and the mirror state, then when the neutron state hits the wall, it will interact with the atomic nuclei and get sucked into the wall. However, if it were in its theoretical inverse neutron state, it would be dark matter that would not interact.
So only the reverse neutrons will pass through the wall to the other side. It would be as if neutrinos had passed through a “gateway” to a dark sector – a symbolic concept used in the physics community. However, press reports on related prior work have been fun taking the liberty with the concept, comparing the mirror-theoretic universe that Broussard’s team is exploring to the alternate reality “Upside Down” in the TV series “Stranger Things.” The team’s experiments weren’t exploring an actual portal to a parallel universe.
Co-author Yuri Kamyshkov, a physicist from the University of Texas who and his colleagues have long pursued the ideas of neutron oscillations and mirror neutrons. “If we see any regenerating neutrons, it could be an indication that we’ve seen something really strange. Discovering the particle nature of dark matter will have huge implications.”
ORNL’s Matthew Frost, who earned his Ph.D. from UT working with Kamyshkov, conducted the experiment with Broussard and helped with data mining, minimization, and analysis. Frost and Broussard conducted initial tests with the help of Lisa Deber-Schmidt, a neutron scattering scientist at ORNL.
Lawrence Heilbronn, a nuclear engineer at the University of Texas, distinguished the backgrounds, while Eric Iverson, a physicist at ORNL, distinguished the neutron signals. Through the Department of Energy’s Office of Science Office of Science internship program, Ohio State University’s Michael Klein figured out how to calculate oscillations using graphics processing units—accelerators for certain types of computations in application codes—and performed independent analyzes of neutron beam intensity and statistics. East Tennessee University in preparing the experiment and analyzing background data, becoming one of the finalists in a competition for this work. University of Texas graduate students Josh Barrow, James Turnolo and Sean Favra contributed with undergraduates Adam Johnston, Peter Louise, and Christopher Mattison at various stages of experiment preparation and analysis. University of Chicago graduate student Luis Variano and a former UT Torchbearer holder assisted with quantum mechanics conceptual calculations of mirror neutron regeneration.
Conclusion: No evidence of neutron regeneration has been seen. “One hundred percent of the neutrons stopped, zero percent passed through the wall,” Broussard said. Regardless, the result is still important for the advancement of knowledge in the field.
With one particular theory of mirror matter debunked, scientists are turning to others to try and solve the mystery of the neutron’s age. “We will continue to look for the cause of the discrepancy,” Broussard said. She and her colleagues will use the High Flow Isotope Reactor, a Department of Energy user facility in ORNL’s Office of Science, for this. The ongoing upgrades at the HFIR will make more sensitive searches possible because the reactor will produce a much higher flux of neutrons, and the shielded detector in the small-angle neutron scattering diffractometer has a low background.
Since the rigorous experiment found no evidence of mirror neutrons, physicists were able to rule out a far-fetched theory. This brings them closer to solving the puzzle.
If it seems sad that a file neutron The mystery of life remains unsolved, take solace from Broussard: “Physics is difficult because we have done a very good job at it. Only really difficult problems – and lucky discoveries – remain”.
LJ Broussard et al, Experimental research on neutron reversal of neutron oscillations as an explanation for neutron lifetime anomalies, physical review messages (2022). DOI: 10.1103/ PhysRevLett.128.212503
Oak Ridge National Laboratory
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