How long can a neutron survive outside an atom?

Fundamental physics, as we’ve seen, finds itself in a difficult situation. Nothing unexpected has turned up at the Large Hadron Collider. We have phenomena like dark matter and dark energy that are defying explanation. And some of the most exciting ideas that theoreticians are coming up with have steadfastly refused to submit to any form of experimental testing.

On possible route out of this mess is to focus on some of the oddities in the data that we already have. For example, there are a few measurements that seem to show particle behavior that’s inconsistent with physics’ Standard Model. And there are other cases where two different routes to the same measurement give different results, a possible sign that some new physics is influencing one experimental approach but not another. But before we pursue these oddities, the first step is to confirm that something unexpected is really happening.

This is exactly the situation we have with the decay of neutrons. We have two different ways of measuring the neutron’s half-life, and the values they produce disagree by an appreciable amount. To find out whether this disagreement is real, however, we have to up the precision of the measurements. And that’s precisely what a large US-Russian collaboration has done.


Neutrons are probably best known for being a chargeless component of the nucleus of all atoms other than hydrogen. In that context, they can be extremely stable—you probably noted the fact that your body wasn’t decaying around you. But pull neutrons out of that context, and they become very unhappy. They’ll decay into a proton, an electron, and a neutrino. That decay has a half life—the time it would take half the neutrons in a large sample to decay—of a bit under 15 minutes. But just how much less isn’t clear.

That’s not for lack of measurements. We have plenty of them, many with error bars of less than three seconds. The problem is that these measurements systematically disagree.

Neutrinos have no charge, so they’re a bit difficult to control And they tend to undergo reactions with atoms that they happen to bump into. This makes them a challenge to track, but scientists have settled on two methods. One is to produce a beam of neutrons and watch the beam for signs of the decay products. This produces a value for the half life of 887.7 ± 2.2 seconds. A second is to try to store the neutrinos for a length of time and see how many of them decay. Annoyingly, this produces a value of 878.5 ± 0.8. Those are over nine seconds apart, a difference of about four standard deviations.

Getting that to the point where the difference is five standard deviations—a value that physics accepts as indicating an effect is real—requires cutting down on those errors. And that’s exactly what the new US-Russian work aims to do.

Neutrons in a bottle

This experiment relies on storing a bunch of neutrons in a container. By knowing how many you put in and measuring them some time later, you can figure out how many of them decayed in that time. Get enough of these measurements and you can figure out what the half life is.

This method may be conceptually simple, but implementing it is another matter entirely. With no charge, neutrons are difficult to control, and they can easily bump into the walls of the container or pick up enough energy to go flying out of it entirely. Plus you have to know how many are there.

To manage this, the researchers start with a beam of slow-moving neutrons. Part of that beam is diverted to a detector, which registers how many neutrons hit it; this gives us a measure of how many neutrons are left in the beam. The remaining neutrons are then dumped into a container, at which point the challenge of keeping them in the container starts.

First, there’s keeping them from hitting the walls. Neutrons may not have charge, but they do have spin, allowing them to be influenced by a magnetic field. The container uses an external magnetic field to align the spin of all the neutrons, and magnets lining the walls of the container gently repel the particles, keeping them from hitting the walls. The top of the container is kept open, but the neutrons will typically end up circulating near the bottom of the container due to gravity.

A few of the particles might have high-enough energies to escape the container against the pull of gravity, but the researchers use what they call a “cleaner” to handle them. The cleaner is simply a piece of plastic that gets inserted near the top of the container. Any neutrons with enough energy to escape quickly run into it and are removed from the experiment. The container is also asymmetric, which causes other high-energy neutrons to bump up against the magnets and bounce up to the cleaner.

When the researchers were ready to perform a measurement, they simply lowered a detector into the middle of the trap, at which point the neutrons promptly ran into it.

Still disagreeable

The key thing about this setup is that it makes measuring all sources of neutron loss during the experiment relatively easy. By counting the losses, the researchers were able to better estimate the population of neutrons that were available to decay, and therefore they got a better measurement of how many did. Six hundred and sixty-four measurements later, they calculated the half life.

In an ideal world, the slightly different measurement technique and higher precision of the measurement would get rid of the discrepancy. In case you hadn’t noticed, however, our world is far from ideal. The new value, 877 ± 0.7 seconds, actually makes the disagreement a tiny bit worse. Those willing to scroll up to the top of the article would also notice that the ± 0.7 seconds isn’t a whole lot better than the previous precision record, ± 0.8 seconds.

But the authors say that this uncertainty is dominated by statistics, not noise, which means it will go down with further measurements. They expect it should simply be a matter of doing more measurements to reduce the uncertainty below ± 0.5 seconds.

, 2018. DOI: 10.1126/science.aan8895  (About DOIs).

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