To answer the big questions, you sometimes have to turn to the very small ones. Researchers at the ultra-cold neutron source at the Los Alamos Neutron Science Center, within the 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. Today, an international collaboration of scientists announced the most accurate measurement ever taken of the lifetime of a free neutron, with an uncertainty of less than two-tenths of a percent.
Neutrons are the simplest particle that is radioactive, that is, they break down into other particles. In nuclei, neutrons are stable. Outside of a core, however, it’s an entirely different game. Outside the nucleus, the decay of neutrons is rapid: previous work has estimated the half-life of a free neutron at around fifteen minutes, within a few tens of seconds between the high and low estimates. But this “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 a light electron and a nearly 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 important 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 but that no one has yet been able to find.
Scientists can study free neutrons in a particle beam. First, they measure the number of neutrons in a specific volume of the beam. Then, they direct the beam into a “particle trap” formed by an EM field. Like a mousetrap, they put it down and come back later. The number of protons remaining from the neutron decay is evidence of the number of neutrons that decayed during this time.
Another important way to study free neutrons is to use a “bottle”. Ultra-cold neutrons move very slowly – a few meters per second, compared to neutrons in fission reactions, which move at speeds of the order of millions of kilometers per second. Scientists measure the number of ultra-cold neutrons in a container at the start of the experiment, and then again at the end. This is a measurement of “living” neutrons, while beam experiments measure “dead”.
If the “beam” and “bottle” experiments were okay, that would be all: lifetime of a measured neutron, game over. But the readings just didn’t match, so scientists got to work to close the discrepancies. A physicist, Chen-Yu Liu, paid special attention to the interactions between ultra-cold neutrons and their bottle. In previous work at Los Alamos, Liu and his colleagues completely abstained from the physical container of their neutrons, instead moving towards an electromagnetic field. I was in the camp of, if we do this we could make a neutron live longer and be okay with the beam lifetime, said Liu, professor of physics at the University of Indiana who led this experiment. It was my personal bias. But the difference remained. “It was a big shock to me,” she said of the 2018 job. Continuing to track down sources of error, Liu also participated in this current ultra-cold experiment.
In this experiment, UCNtau researchers trap neutrons from the ultra-cold neutron source in an antigravity “magnetic tub” lined with some 4,000 magnets. After the neutrons are counted, they are left to soak in their tub for 30 to 90 minutes, then counted again to see how many neutrons have survived. Over two years, the authors of this study counted around 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. To eliminate the uncertainty, however, the UCNtau trap can actually allow neutrons to prune in the bath: it can hold neutrons near absolute zero for eleven days or more. This means that the experiment can take into account even very long-lived outliers, allowing the most accurate measurements to date.
The half-life issue is still not resolved, but experiments like UCNtau are quickly closing the gap. Meanwhile, further efforts are underway using spatial measurement techniques, in the hope of confirming or correcting even this very precise terrestrial measurement. In 2020, the results of a collaboration between NASA and another international group of researchers, who used the MESSENGER spacecraft to measure neutron leaks from Mercury and Venus were published. Their 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 should confirm or challenge the “Cabibbo-Kobayashi-Maskawa matrix”, which concerns the nature of quarks and plays a key role in the “standard model” of particle physics. “The underlying model explaining neutron decay involves quarks changing their identity, but recently improved calculations suggest that this process may not occur as expected,” Salvat said. “Our new measure of neutron lifetimes will provide an independent assessment to address this problem, or provide much-needed evidence for the discovery of new physics.”
The research is reported in the October 13 issue of Physical Review Letters. A pre-printed version of the book is also available on arXiv.
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