Neutron lifetime mystery could now find a solution in ‘excited state’

When neutrons are inside an atomic nucleus, they are stable and can continue to exist for an indefinite period. However, this isn’t the case with free neutrons. Although they are known to decay in nearly 15 minutes, their lifetime has been a mystery for scientists.

This is because when their average lifetime is measured via two different methods, the results are also different. This is puzzling because a particle should have a single, consistent lifetime.

“The average lifetime of free neutrons is surprisingly difficult to measure. For almost thirty years, physicists have been puzzled by contradictory results on this topic,” Benjamin Koch, a researcher at Vienna University of Technology (TU Wien), said.

In their new study, Koch and his colleague Felix Hummel present a hypothesis that can clear all the confusion regarding the contradictory average lifetimes of neutrons .

The beam vs bottle lifetime problem

When neutrons are free, within minutes, they undergo decay and become an electron, proton, or antineutrino. The first method that scientists generally use to measure the lifetime of free neutrons before the decay involves the use of a neutron beam.

“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,” Koch explained.

Scientists count the number of neutrons before the experiment and then compare that with the number of protons formed as a result of the decay. Using these values, they calculate the average time for which neutrons last in the neutron beam.

In the second method, free neutrons are trapped in a specially designed container (or bottle) from which they can’t escape. Scientists monitor how many neutrons remain in the bottle over time and measure the duration until a certain number decays — allowing them to calculate the average lifetime of the neutrons while they are contained.

Koch and his colleagues measured the average lifetime of neutrons using both the beam and bottle methods. Their findings revealed that “neutrons from the neutron beam live around eight seconds longer than neutrons in a bottle.”

“With an average lifespan of just under 900 seconds (15 mins), this is a significant difference – far too big to be explained by mere measurement inaccuracy,” Koch added.

The “different states” hypothesis

According to the study authors, free neutrons have a secret that causes them to have different lifetimes. These particles are formed as a result of radioactive decay, and it is possible that they don’t all start in the same condition.

Some neutrons initially possess high energy, placing them in an excited state. Conversely, other neutrons begin in the ground state. However, eventually, all neutrons lose energy and enter the ground state.

“You can think of it like a bubble bath. If I add energy and bubble it up, a lot of foam is created – you could say I’ve put the bubble bath into an excited state. But if I wait, the bubbles burst and the bath returns to its original state all by itself,” Felix Hummel, study co-author and a scientist at TU Wien, explained.

During the neutron beam process, possibly many neutrons are in the excited state, whereas in bottle measurement, since neutrons are trapped after they’re cooled down, they are probably in their ground state. The difference in their states is what causes them to have different average lifetimes.

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

Their model also predicts that in the excited state, neutrons must have a lifetime ranging between five milliseconds to 300 seconds. However, these predictions are based on a hypothesis that is yet to be experimentally proved.

The study is published in the journal Physical Review D .