The humble neutron could be hiding a deep, dark secret. For 20 years, two experiments that measure the lifetime of a neutron have been at odds. Now it seems that disconnect may be the result of neutrons occasionally decaying into particles of dark matter, the stuff that is thought to make up most of the unseen mass of the universe.
“What we thought were our most likely ideas for what dark matter might be, none of those have worked,” says at Fermilab near Batavia, Illinois, who wasn’t involved in the research. “It seems well worthwhile to consider the seemingly unlikely ideas.”
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We know that a neutron can morph into a proton in a process called beta decay, which also spits out an electron and an antineutrino.
The “beam experiment” involves counting the number of protons produced by a beam of neutrons. This method assumes that a neutron only undergoes beta decay, and it gives the particle a lifetime of 888 seconds.
The neutron’s lifespan can also be calculated via the bottle experiment: a container of ultracold neutrons is monitored for varying lengths of time, and those left after each stage are used to estimate the length of the particle’s life. This method makes no assumptions about how a neutron decays, and it comes up with a lifetime of 879.6 seconds.
The discrepancy in the two values generated by the beam and bottle experiments is difficult for particle physicists to reconcile.
Decaying into the dark
So, and at the University of California, San Diego, wondered whether neutrons were decaying in other ways as well. This could explain the longer lifetime of the beam method, because it only accounts for beta decay. “The more ways the particle can decay, the shorter it lives,” says Fornal.
If this idea is correct, a new hypothetical process of neutron decay would have to produce some mystery particle. “It has to be something that doesn’t interact strongly with our matter,” says Fornal. Otherwise, we’d have seen it by now.
That’s also how physicists think of dark matter, the stuff that keeps galaxies from flying apart. Dark matter may not interact with normal matter, except gravitationally.
Fornal and Grinstein analysed various ways in which a neutron might decay into a “dark” particle, a process that would happen about once for every 100 neutron decays.
They showed that if such a particle were to be a candidate for dark matter, its mass would need to be just slightly less than that of the neutron. In some scenarios, the difference in mass would show up as a photon with an energy between 0.8 and 1.7 megaelectronvolts – something that could potentially be detected.
Weighing the options
Hooper says it’s unlikely that a dark matter particle would have the same mass as that of a neutron, yet he is intrigued.
“If I were convinced that there was really a neutron decay anomaly and it required some decay to an exotic state, the 64,000 dollar question that it makes you want to ask is: why is this state almost exactly the same mass as the neutron?” says Hooper.
The answer may lie in theories called asymmetric dark matter models, in which neutrons and dark matter particles have comparable mass because they were produced by similar processes in the early universe. Such models would get a boost if Fornal and Grinstein’s ideas can be experimentally verified.
Besides, knowing a neutron’s precise lifetime would influence our understanding of how elements such as helium came into being soon after the big bang.
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Read more: Ancient black holes ruled out as source of all dark matter
Article amended on 18 January 2018
We clarified how dark matter may interact with other matter
