Physicists entertained a wild but testable theory to solve a long-standing puzzle about how long a neutron can live outside an atomic nucleus. An experiment was designed at the Department of Energy's Oak Ridge National Laboratory to try to detect a particle that has been speculated but not seen. The first observation of dark matter could be provided if the mirror neutron is found.
"Dark matter is one of the most important and puzzling questions in science, with clear evidence we don't understand all matter in nature," said ORNL's Broussard.
The nucleus of an atom is made up of two particles. They can also be found outside the nucleus. Frank Gonzalez, now at ORNL, led the most precise measurement of how long free neutrons live before they decay, or turn into protons, electrons and anti-neutrinos. The Standard Model of particle physics may have been affected by the answer. The quarks that make up a neutron are described in that model. The flip of quarks causes decay into protons.
The Standard Model uses the neutron lifetime as an input to calculate the quark mixing matrix, which describes quark decay rates. The Standard Model hints at new physics if the quarks don't mix as expected.
Scientists use two different approaches to measure the lifetime of a free neutron. A person counts the disappearance of neutrons by trapping them in a bottle. There are counts of protons in a beam. Nuclear particles appear to live nine seconds longer in a beam than in a bottle.
Physicists have considered a lot of reasons for the discrepancy. There is a theory that the neutron can change from one state to the next. "Oscillation is a quantum mechanical phenomenon." It's possible to get a rocking back and forth between the two states of a regular or mirror neutron if that transition is allowed.
The first search for dark-matter mirror neutrons was performed by the ORNL-led team. The Spallation Neutron Source is part of the Office of Science. The magnetism reflectometer was used to guide the beam of neutrons. Michael Fitzsimmons is a physicist with an appointment at ORNL and the University of Tennessee. The beam hit a wall made of boron carbide, which is a strong absorber of neutrons.
When the neutron state hits the wall, it will interact with atomic nucleus and get absorbed into the wall. It is dark matter that won't interact if it is in a mirror neutron state.
There was only one way to get through the wall to the other side. It would be like the neutrons went through a portal to a dark sector. The press took liberties with the concept and compared it to the alternate reality in the TV series "Stranger Things." The team didn't explore a portal to a parallel universe.
The dynamics on the other side of the wall are the same as they are on the other side. It could be a sign that we've seen something really exotic. The discovery of dark matter's particle nature would have huge ramifications.
Matthew Frost, who received his doctorate from UT, was involved in the experiment with Broussard. Lisa DeBeer-Schmitt is a scientist at ORNL.
Lawrence Heilbronn, a nuclear engineer at UT, characterized the background of his work. Through the DOE Office of Science Scientific undergraduate laboratory internship program, Michael Kline of The Ohio State University figured out how to calculate oscillations using graphics processing units, while Taylor Dennis of East did independent analyses of the intensity of the neutron beam. A group of UT graduate students and undergrads contributed to various stages of experiment preparation. Louis Varriano is a graduate student at the University of Chicago.
There was no evidence of the regeneration of the neutrons. Broussard said that one hundred percent of the particles stopped. The result is crucial to the advancement of knowledge in this field.
The scientists try to solve the neutron lifetime puzzle after one of the mirror-matter theories was found to be false. Broussard said that they would keep looking for the reason for the discrepancy. She and her colleagues will use a DOE facility at ORNL. The shielded detector at the small-angle neutron scattering diffractometer has a lower background due to the ongoing upgrades at HFIR.
Physicists were able to rule out a far-fetched theory because the experiment did not find evidence of mirror neutrons. That makes it easier for them to solve the puzzle.
Broussard said that physics is hard because we have done a good job. There are only the hard problems left.
More information: 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 Journal information: Physical Review Letters
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