To answer the big questions, sometimes we must look to the very small. Researchers at the Los Alamos Neutron Science Center’s Ultracold Neutron Source, within Los Alamos National Lab, have been passing the cryo-baton for more than a decade, working at ever colder temperatures in order to study the behavior of neutrons. Now, an international collaboration of scientists has announced the most precise measurement ever taken of a free neutron’s lifetime, within an uncertainty of less than two tenths of a percent.
Neutrons are the simplest particle that is radioactive, which is to say that they break down into other particles. Within nuclei, neutrons are stable. Outside of a nucleus, however, it’s a different game entirely. When outside the nucleus, neutron decay doesn’t take long: prior work estimated a free neutron’s half-life at about fifteen minutes, give or take 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.
Caption: “A schematic of the nucleus of an atom indicating β− radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge. : The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.” Image and caption by InductiveLoad, Public Domain.
“The process by which a neutron ‘decays’ into a proton — with an emission of a light electron and an almost massless neutrino — is one of the most fascinating processes known to physicists,” said Daniel Salvat, who led the experiments at Los Alamos. “The effort to measure this value very precisely is significant because understanding the precise lifetime 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 nobody has yet been able to find.”
One way scientists can study free neutrons is 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 set it and come back later. The number of protons remaining from the neutrons’ decay is evidence of how many neutrons decayed in that time.
Another major way to study free neutrons is by using a “bottle.” Ultra-cold neutrons move very slowly — a few meters per second, compared to neutrons in fission reactions, which move at velocities on the order of millions of kilometers per second. Scientists take a measurement of how many ultra-cold neutrons are in a container at the beginning of the experiment, and then again at the end. This is a measurement of “living” neutrons, while the beam experiments measure the “dead.”
If the “beam” and “bottle” experiments agreed, that would be it: lifetime of a neutron measured, game over. But the readings just wouldn’t match, so scientists set to work to eliminate the discrepancies. One physicist, Chen-Yu Liu, paid specific attention to interactions between the ultracold neutrons and their bottle. In previous work at Los Alamos, Liu and colleagues dispensed altogether with the physical container for their neutrons, moving instead to an electromagnetic field. “I was in the camp of, if we do that, we might get a neutron to live longer and agree with the beam lifetime,” said Liu, an Indiana University physics professor who led that experiment. “That was my personal bias.” But the difference remained. “That was a big shock to me,” she said of the 2018 work. Continuing to hunt down sources of error, Liu also participated in this current ultracold experiment.
In this experiment, the UCNtau researchers trap neutrons from the Ultracold Neutron source in an antigravity “magnetic bathtub” lined with some 4,000 magnets. After the neutrons are counted, they’re left to soak in their 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 forty million free neutrons. The study reports the half-life of a free neutron to be 877.75 +/- 0.28 seconds, with an uncertainty of 0.34 seconds. To eliminate uncertainty, though, the UCNtau trap can actually allow neutrons to get all prune-y in the bath: it can hold neutrons close to absolute zero for eleven days or more. This means that the experiment can account for even super-long-lived outliers, allowing for the most precise measurements yet.
The half-life issue still isn’t settled, but experiments like UCNtau are rapidly closing the gap. Meanwhile, complementary efforts are underway using space-based measurement techniques, hoping to confirm or correct even this very precise terrestrial measurement. In 2020, 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. Their reported neutron half-life was shorter than what was reported in the UCNtau experiments, but MESSENGER wasn’t 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 this experiment’s results stand to 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 the quarks changing their identities, but recently improved calculations suggest this process may not occur as previously predicted,” said Salvat. “Our new measurement of the neutron lifetime will provide an independent assessment to settle this issue, or provide much-searched-for evidence for the discovery of new physics.”
The research is reported in the Oct. 13 issue of Physical Review Letters. A pre-print version of the work is also available on arXiv.
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