To answer the big questions, we sometimes have to look at the very small ones. Researchers at the ultracold neutron source at the Los Alamos Neutron Science Center within Los Alamos National Laboratory have been passing the baton for more than a decade, working in increasingly cold temperatures to study the behavior of neutrons. Now, an international collaboration of scientists has announced the most accurate measurement ever taken of the life of a free neutron, within an uncertainty of less than two-tenths percent.
Neutrons are the simplest particle that is radioactive, that is, they break down into other particles. Inside the nuclei, the neutrons are stable. Outside of a core, however, it is a completely different game. When it is outside the nucleus, the decay of neutrons does not take long: previous work estimated the half-life of a free neutron at about fifteen minutes, plus or minus a few tens of seconds between the high-end and low-end estimates. But that “give or take” is enough to make or break a theory.
“The process by which a neutron ‘decays’ into a proton, with the emission of an electron of light and an almost massless neutrino, is one of the most fascinating processes known to physicists,” said Daniel Salvat, who led the experiments in Los Alamos. . “The effort to measure this value very precisely is significant because understanding the precise lifespan of the neutron can shed light on how the universe developed, as well as allow physicists to discover flaws in our model of the subatomic universe that we know exist but that no one has. I have still been able to find “.
One way that scientists can study free neutrons is by using a particle beam. First, they measure the number of neutrons in a specific volume of the beam. They then direct the beam into a “particle trap” made up of an EM field. Like a mousetrap, they set it and come back later. The number of protons left from neutron decay is evidence of how many neutrons decayed in that time.
Another important way to study free neutrons is by using a “bottle.” Ultracold neutrons move very slowly, a few meters per second, compared to neutrons in fission reactions, which move at speeds on the order of millions of kilometers per second. Scientists measure how many ultracold neutrons are in a container at the beginning of the experiment and then again at the end. This is a measure of “live” neutrons, whereas beam experiments measure “dead” ones.
If the “beam” and “bottle” experiments agreed, it would be it: the lifespan of a measured neutron, game over. But the readings just didn’t match, so the scientists went to work to eliminate the discrepancies. One physicist, Chen-Yu Liu, paid special attention to the interactions between ultracold neutrons and their bottle. In earlier work at Los Alamos, Liu and his colleagues completely dispensed with the physical container for their neutrons, instead moving into an electromagnetic field. “I was in the field of, if we do that, we could get one neutron to live longer and agree with the life of the beam,” said Liu, an Indiana University physics professor who led that experiment. “That was my personal bias.” But the difference remained. “That was a huge shock to me,” he said. said of the 2018 work. Continuing to search for sources of error, Liu also participated in this current ultra-cold experiment.
In this experiment, the UCNtau researchers trap neutrons from the ultracold neutron source in an antigravity “magnetic bathtub” lined with about 4,000 magnets. After the neutrons are counted, they are soaked in the bathtub for 30 to 90 minutes and then counted again to see how many neutrons survived. Over two years, the authors of this study counted about 40 million free neutrons. The study reports that the half-life of a free neutron is 877.75 +/- 0.28 seconds, with an uncertainty of 0.34 seconds. However, to remove the uncertainty, the UCNtau trap can allow the neutrons to get all the pruning in the bath – it can keep the neutrons near absolute zero for eleven days or more. This means that the experiment can account for even long-lived outliers, allowing for the most accurate measurements yet.
The half-life problem is not yet solved, but experiments like UCNtau are quickly closing the gap. Meanwhile, complementary efforts are being made using space-based measurement techniques, in hopes of confirming or correcting even this very accurate ground-based measurement. In 2020, the results were released from a collaboration between NASA and another international group of researchers, which used the MESSENGER spacecraft to measure neutron leakage from Mercury and Venus. Its reported neutron half-life was shorter than that reported in the UCNtau experiments, but MESSENGER was not designed to be a neutron collector.
Ultimately, these measurements can help us answer fundamental questions, such as the relative abundance of elements in the early universe. Salvat explained that the results of this experiment can confirm or challenge the “Cabibbo-Kobayashi-Maskawa matrix”, which refers to the nature of quarks and plays a key role in the “standard model” of particle physics. “The underlying model that explains neutron decay implies that quarks change their identities, but recently improved calculations suggest that this process may not occur as previously predicted,” Salvat said. “Our new measurement of the lifespan of neutrons will provide an independent assessment to solve this problem, or it will provide much-sought evidence for the discovery of new physics.”