The neutron darkness in a bottle (and a beam)

All matter around us is constituted of particles that are stable. This is a very good thing. It means that the constituents of our world are stable and will not decay. We indeed do not want to see our favorite cup of coffee decaying into something else before having the time to drink it…

For once, I will not discuss elementary particles, but restrict myself to a larger scale. I hence start my discussion at the level of atoms.

To summarize what can be observed in the picture on the left, atoms are made of an atomic nucleus and of electrons that orbit around the nucleus.

We however know for more than a century that the atomic nucleus is a composite object whose building blocks are protons and neutrons.

Here are thus our stable particles: protons, neutrons and electrons. To this list, we can add neutrinos (that appear in decay processes of an atomic species into another) and photons (that are connected to the electromagnetic interactions of our atoms).

In this post, I will focus on the neutron. Although I have stated that the neutron is stable, it actually decays after 15 minutes (wow, a first contradiction?!). Moreover, the measurements of its lifetime exhibit a puzzle that lasts for more than 20 years (a second contradiction!).

This puzzle attracted a lot of interest recently, as it could potentially be solved by dark matter, although neutron stars made this challenging.

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WHY HAVE I SAID THAT THE NEUTRON IS STABLE?

I started this post with a pair of contradictions. Let us focus on the first of these.

Within a couple of sentences, I have mentioned that neutrons can be classified as stable particles, and also that they decay in about 15 minutes.

The second part of the above statement may seem frightening: neutrons decay in 15 minutes. Knowing that neutrons are everywhere in normal matter, this may indeed be worrisome.

But there is actually no reason to panic…

It is true that neutrons are unstable. One can indeed take a neutron, wait for about 15 minutes, and one will get a proton, an electron and a neutrino.

No more neutron! The beta decay mechanism did the rest (as sketched on the picture).

What saves matter is that within an atomic core, neutrons are not alone. They are surrounded by other protons and neutrons, which collectively makes the atomic nucleus a stable system.

I hope now that this contradiction about the stability of the neutron is settled. Let us thus go back to the topic: neutrons are unstable.

Consequently, experiments have been conducted for decades in order to try to measure the neutron lifetime. This is after all one of the crucial properties of the neutron and is thus need to be measured.

We have two independent classes of experiments here, using respectively the bottle and beam methods.

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NEUTRONS IN A BOTTLE

One of the two classes of experiments aiming to measure the lifetime of the neutrons is very simple. It is called the bottle method, and for a very good reason.

First, we collect a bunch of neutrons and store them in a bottle. In fact, we actually trap the neutrons, by means of a combination of magnetic fields or material walls for instance, so that we can count them.

We have thus a kind of bottle with neutrons in it, and we know exactly how many neutrons we have at the beginning of the experiment.

Then, we wait… wait… and wait a bit more.

After some time, we check how many neutrons remain in the trap. From that number and the initial number of neutrons, we can evaluate the decay rate of the neutrons, and thus the neutron lifetime.

This is what has been done in particular at the Laue-Langevin Institute in Grenoble. Ultra-cold neutrons have been trapped, as detailed in this open-access publication. Thanks to a bunch of neutron counters, it was possible to follow the evolution of the number of neutrons with time.

The measured neutron lifetime has been found to be 879.6 ± 0.6 seconds. Roughly 14 minutes and a half. And it can be seen that the measure is damned precise.

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NEUTRON BEAMS

The second class of experiments targeting the measurement of the neutron lifetime relies on neutron beams.

For a very inventive reason, the method is known as the neutron beam method. Yeah, physicists are good with names ;)

Here, we prepare a beam of neutrons (in case you did not guess from the name of the method) and we measure the amount of neutrons decaying over time in a given volume of the beam.

This number is derived by counting the rate of protons that are emitted by the decaying neutrons. This in fact measures the beta-decay-rate of the neutron, as each neutron is assumed to decay into a single proton, an invisible neutrino and an electron.

The most precise measurement of the neutron lifetime within this method can be found in this publication, and it is found to be 888.0 ± 2.0 seconds.

This number is very close to the previous one, but different enough!

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DISCUSSION: A DISCREPANCY SOLVED BY DARK MATTER?

As I have detailed in this post, one can use two independent methods to extract the neutron lifetime (the so-called bottle and beam methods).

Until now, both methods have been found to disagree with a difference of about 8.5 seconds. Even after accounting for the size of the error bars, the two measurements still disagree. We are talking of a disagreement of more than 4 standard deviations (i.e. a disagreement at the 99.993666% level).

One possible catch is that with the neutron beam method, one only focuses on neutron decays into protons. In contrast, with the neutron bottle method we focus on all neutron decays.

‘All’ is the important word. This means, also non-standard decays that no one is expecting and that could not be observed.

Physicists have tried to investigate this path, and have proposed models where neutrons could decay into a very light dark matter particle, as shown for instance in this work.

The interesting fact is that even if this scenario is challenged experimentally, it is still viable. In particular, neutron stars can be problematic, as I have written in this old post.

Therefore, there is only one wait to know more about this puzzle: wait for more data.

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