Why the neutron lifetime is still unknown and how to change it

The mystery of the neutron lifetime has not been solved for decades. This is despite the fact that the neutron is a fundamental part of the universe. We will tell you why this is happening and how superfluid helium-4 will help correct the situation.

How long does a neutron live?

The neutron lifetime is so fundamental and important for understanding the Universe that it can be logical to assume that it has been known for a long time. However, it is not. This is not to say that scientists did not try to find out. Decades and hundreds of high-precision measurements have not given any specifics. Two fundamentally different types of experiment showed two results – 879.4 +/- 0.6 seconds of the bottle method for measuring the lifetime versus 888 +/- 2.0 seconds of the beam method.

The difference of eight to nine seconds is four times the measurement error of two seconds. The chance that they agree with each other is about 60 per million, which is almost impossible. These seconds constitute the enigma of the neutron lifetime.

Two methods, two results

So, scientists used two methods to determine the neutron life. How do they work?

Bottle method

In the bottle method, neutrons can be sealed in a vacuum bottle made of a neutron-safe material, or held back by magnetic fields and gravity. They have extremely low kinetic energy and move at a speed of several meters per second. They are called ultracold neutrons (UCNs). Physicists separate neutrons from atomic nuclei, place them in a bottle, and then count how many of them remain there after a while. As a result, scientists conclude that neutrons decay radioactively in an average of 14 minutes and 39 seconds.

Ray method

Beam experiments use machines that create fluxes of neutrons. Scientists measure the number of neutrons in a specific volume of the beam. They then direct the flow through a magnetic field into a particle trap created by the electric and magnetic fields. The neutrons decay in a trap where physicists measure the amount of protons that are left behind. In such experiments, they determine the average neutron lifetime at the level of 14 minutes 48 seconds.

Results

At the moment, there are seven high-precision bottle measurements with different settings and only two – beam measurements. In both beam measurements, the same method was used — the Penning trap. The decay product, protons, is captured by it and counted by a well-calibrated detector.

The Penning trap itself is a device that uses a uniform static magnetic field and a spatially inhomogeneous electric field to store charged particles. This type of trap is often used to accurately measure the properties of ions and stable subatomic particles that have an electrical charge.

There is no doubt that more experiments are required for comparison and verification, not only with the beam, but in general.

Are there other ways?

In the ray method, physicists determine how many neutrons have undergone beta decay. Let us recall that beta decay of a neutron is a spontaneous transformation of a free neutron into a proton with the emission of a beta particle (electron) and an electron antineutrino.

Precision measurements of the neutron beta decay parameters (lifetime, angular correlations between particle momenta and neutron spin) are important for determining the properties of the weak interaction. This is a fundamental interaction responsible, in particular, for the processes of beta decay of atomic nuclei and weak decays of elementary particles, as well as violations of the laws of conservation of spatial and combined parity in them. This interaction is called weak, since the other two interactions that are significant for nuclear and high-energy physics (strong and electromagnetic) are characterized by significantly higher intensity. However, it is much stronger than the fourth of the fundamental interactions, gravitational.

Antineutrinos are difficult to detect. The world’s leading detectors are often gigantic and target an intense source of flux such as the Sun or a nuclear power plant. However, only a few events happen in a year. So antineutrino won’t help here.

What about the proton? Until now, all results with the best accuracy in the ray method have been obtained by registering protons. Now active work is underway to improve the method. For example, a modernized BL3 experiment is under preparation at NIST, USA. Researchers at J-PARC recently announced their preliminary neutron lifetime result by detecting beta decay electrons using a time projection chamber (TPC). Such chambers are a combination of drift and proportional chambers. They are the most versatile instrument in high energy physics since they allow one to obtain a three-dimensional electronic image of a track with a comparable spatial resolution in all three coordinates. The work of Japanese scientists is a revival of an experiment first proposed by Kossakowski et al. In 1989. They are now working to improve its accuracy.

After decades of effort, it can be assumed that all possible pathways of the ray method should be carefully investigated.

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Alexandr Ivanov earned his Licentiate Engineer in Systems and Computer Engineering from the Free International University of Moldova. Since 2013, Alexandr has been working as a freelance web programmer.
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Alexandr Ivanov

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