The neutron lifetime problem and its possible solution

Neutrons are one of the basic elements of matter. As long as they are part of a stable atomic nucleus, they can stay there for arbitrary periods of time. However, the situation is different for free neutrons: they decay after about 15 minutes on average.

Interestingly, however, different contradictory results have been obtained for this average lifetime of free neutrons, depending on whether the neutrons are measured in a neutron beam or in some sort of “bottle”.

A research team from TU Wien has proposed a possible explanation: there could be previously unknown excited states of the neutron. This would mean that some neutrons could be in a state in which they have a little more energy and a slightly different lifespan. This could explain the differences measured.

The proposal is published in the journal Physical examination D. And the team already has ideas on how to detect this neutron state.

Two measurement methods, two results

By pure chance, without any reason, neutrons can spontaneously decay according to the laws of quantum theory, transforming into protons, electrons and antineutrinos. This is particularly likely if it is a free neutron. If the neutron combines with other particles to form an atomic nucleus, it can be stable.

The average lifetime of free neutrons is surprisingly difficult to measure. “For almost 30 years, physicists have been intrigued by the contradictory results on this topic,” explains Benjamin Koch from the Institute for Theoretical Physics at TU Wien.

He analyzed this enigma with his colleague Felix Hummel. The two men also work closely with the neutron research team led by Hartmut Abele at the Atomic Institute of TU Wien.

“For such measurements, a nuclear reactor is often used as a neutron source,” explains Koch. “Free neutrons are produced during radioactive decay in the reactor. These free neutrons can then be channeled into a neutron beam where they can be precisely measured.”

We can measure how many neutrons are present at the start of the neutron beam and how many protons are produced by the decay process. If these values ​​are determined very precisely, the average lifetime of the neutrons in the beam can be calculated.

But it is also possible to take a different approach and try to store neutrons in a sort of “bottle”, for example using magnetic fields. “This shows that the neutrons emitted by the neutron beam live about eight seconds longer than the neutrons in a bottle,” says Koch.

“With an average lifespan of just under 900 seconds, this is a significant difference, far too large to be explained by simple measurement inaccuracy.”

A new unknown state?

According to Koch and Hummel, this discrepancy can be explained by assuming that neutrons can have excited states – previously unknown states with slightly higher energy. Such states are well known for atoms and constitute the basis of lasers, for example.

“With neutrons, it is much more difficult to calculate such states precisely,” says Koch. “However, we can estimate what properties they should have in order to explain the different results of neutron lifetime measurements.”

The researchers’ hypothesis is that when free neutrons emerge from radioactive decay, they are initially in a mixture of different states: some of them are ordinary neutrons in the so-called ground state, but d Others are in an excited state, with a little more energy. However, over time, these excited neutrons gradually transition to the ground state.

“You can think of it like a bubble bath,” says Hummel. “If I add energy and bubble it, a lot of foam is created. You could say I put the bubble bath into an excited state. But if I wait, the bubbles burst and the bath suddenly returns to its original state itself.

If the theory about excited neutron states is correct, this would mean that in a neutron beam, several different neutron states are present in significant numbers. The neutrons contained in the bottle, on the other hand, would be almost exclusively ground state neutrons. After all, it takes time for neutrons to cool and be captured in a bottle. At this point, the vast majority will have already returned to their ground state.

“According to our model, the probability of a neutron decaying strongly depends on its state,” explains Hummel. Logically, this also results in different average lifetimes for the neutrons in the neutron beam and for the neutrons in the neutron bottle.

Further experiments are needed

“Our calculation model shows the parameter range in which we need to search,” explains Koch. “The excited state lifetime must be less than 300 seconds, otherwise the difference cannot be explained. But it must also be greater than 5 milliseconds, otherwise the neutrons would already be back to the ground state before d ‘reach the beam experience.’

The hypothesis of previously unknown neutron states can be tested using data from previous experiments. However, these data need to be re-evaluated and additional experiments will be required to obtain convincing proof. Such experiments are now being planned.

To this end, the researchers work closely with teams at the Institute of Atomic and Subatomic Physics at TU Wien, whose PERC and PERKEO experiments are well suited to this task. Research groups from Switzerland and Los Alamos in the United States have also already expressed interest in using their measurement methods to test the new hypothesis.

Technically and conceptually, nothing stands in the way of the necessary measures. We can therefore hope to know soon whether the new thesis has actually solved a decades-old physics problem.