Neutrons shouldn’t be all that mysterious. Found inside every atomic nucleus, they may seem downright mundane—but they have long confounded physicists who try to measure how long these particles can live outside of atoms. For more than 10 years researchers have tried two types of experiments that have yielded conflicting results. Scientists have struggled to explain the discrepancy, but a new proposal suggests the culprit may be one of the biggest mysteries of all: dark matter.
Scientists are pretty sure the universe contains more matter than the stuff we can see, and their best guess is that it takes the form of invisible particles. What if neutrons are decaying into these invisible particles? This idea, put forward by University of California, San Diego, physicists Bartosz Fornal and Benjamin Grinstein in a paper posted this month to the physics preprint site arXiv.org, would explain why one type of neutron experiment consistently measures a different value than the other. If true, it could also provide the first way to access the dark matter particles physicists have long been seeking to no avail.
The idea has already gripped many researchers making neutron lifetime measurements, and some have quickly scrambled to look for evidence of it in their experiments. If neutrons are turning into dark matter, the process could also produce gamma-ray photons, according to Fornal and Grinstein’s calculations. “We have some germanium gamma-ray detectors lying around,” says Christopher Morris, who runs neutron experiments at Los Alamos National Laboratory. By serendipity, he and his team just recently installed a large tank to collect neutrons on their way from the start of the experiment to the point where physicists try to measure their lifetimes. This tank provided a large holding cell where many neutrons might decay into dark particles, if the process in fact occurs, and produce gamma-rays as a by-product. “When we heard about this paper, we took our detector and set it up next to our big tank and started looking for gamma rays.” He and his team are still analyzing the results of this trial, but hope to have a paper out in a few weeks reporting on what they found.
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Only one of the two types of neutron decay experiments would be sensitive to neutrons decaying into dark matter. This type, called “bottle experiments,” essentially puts a given number of neutrons into a “bottle” with magnetic walls that holds them inside, then counts how many are left after a certain amount of time. Through many measurements the researchers can calculate how long an average neutron lives.
The other type of experiment looks for the main product of neutron decays. Through a well-known process called beta decay, a neutron outside of an atomic nucleus will break down into a proton, an electron and an antimatter neutrino. So-called “beam” experiments shoot a beam of neutrons into a magnetic trap that catches positively charged protons. Researchers count how many neutrons go in and how many protons come out after a given time, then infer the average time it takes a neutron to decay.
Both classes of experiments find neutrons can last for only about 15 minutes outside of atoms. But bottle experiments measure an average of 879.6 seconds plus or minus 0.6 second, according to the Particle Data Group, an international statistics-keeping collaboration. Beam experiments get a value of 888.0 seconds plus or minus 2.0 seconds. The 8.4-second difference may not seem like much, but it is larger than either of the calculations’ margins of error—which are based on the experimenters’ understanding of all the sources of uncertainty in their measurements. The difference leaves the two figures with a statistically significant “4-sigma” deviation. Experimenters behind both methods have scoured their setups for overlooked problems and sources of uncertainty, with no luck so far.
But if neutrons can transform in more ways than just beta decay, it would explain why bottle and beam experiments do not find the same answers. Fornal and Grinstein suggest that occasionally neutrons turn into some type of dark particle, undetectable by traditional means. The bottle experiments would then measure a slightly shorter lifetime for the neutron than beam experiments, because the former would be counting the dark matter decays in addition to the beta decays—and thus detecting a larger number of total decays in any given time period. The beam setup, however, only measures how long it takes neutrons to turn into protons, so their tally would not include dark matter decays and would therefore suggest neutrons can stick around slightly longer. And that is indeed what the two methods show.
“It would be nice to have an explanation,” says Peter Geltenbort, who runs bottle experiments at the Institut Laue–Langevin in France. If the dark particle idea is correct, “it means that we experimentalists are giving the right error for our measurements. People have written that maybe we are just too optimistic estimating our systematic [uncertainties], but it would confirm that we did a good job.” Geltenbort is also collaborating with Morris on the Los Alamos bottle experiments.
Perhaps the larger implication—if neutron experiments show any evidence for the dark particle hypothesis—is that physicists might then have a link to dark matter. The dark particle that Fornal and Grinstein propose could be the same particle that makes up the cosmos’ missing mass. It could also be a different invisible particle, perhaps part of some larger sector of numerous dark particles. “They [Fornal and Grinstein] are building a very specific set of models to explain the neutron lifetime discrepancy,” says dark matter theorist Peter Graham of Stanford University. “It’s not obvious that their models really fit into any other dark matter models that people have built for other reasons.” For the neutron to decay into a dark particle, for instance, that particle must be lighter than the neutron’s mass of around 940 MeV/c2 (mega–electron volts divided by the speed of light squared). On the other hand, one of the most popular classes of theorized dark matter particles, so-called weakly interacting massive particles (WIMPs), would weigh somewhere around 100 GeV/c2 (giga–electron volts divided by the speed of light squared)—roughly 100 times more than a neutron.
Fornal first started thinking about the neutron enigma about a year ago. “I ran into an article by Peter Geltenbort about this mysterious discrepancy between the neutron lifetime measurements,” and thought, “wow, that’s a really big thing to explain,” he says. The article was an adaptation of an April 2016 Scientific American story Geltenbort had authored with University of Tennessee Knoxville physicist Geoffrey Greene that was published in the Institut Laue–Langevin’s annual report. Fornal says he was reminded of the topic a few months ago, when he and Grinstein came across a reference to it. “We didn’t find any theoretical model explaining this, and thought it might be an interesting thing to do,” he says. The researchers worked on the hypothesis over the holidays and posted their paper online just after the new year. They are surprised—but thrilled—that they might know soon whether or not neutron decay experiments see evidence for their proposal. “[neutron researchers] started looking for this so quickly,” Fornal says. “It’s nice to hear that this theory is not disconnected from experiments.”