How long do neutrons live? Physicists are approaching a puzzle that is decades old

Denver, Colorado

Physicists are approaching more and more of a mystery of the universe: the lifespan of a neutron.

Neutrons are electrically neutral particles that usually combine with protons to form atomic nuclei. Some neutrons are not linked to atoms; these free neutrons disintegrate radioactively in other particles in a few minutes.

But physicists can not agree on how long it takes a neutron to die. Using a laboratory approach, they measure the average lifetime of neutrons at 14 minutes 39 seconds. Using a different approach, they have 8 seconds more. The divergence has been harming researchers for almost 15 years.

"We do not know why they are different," said Shannon Hoogerheide, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. "We really need to understand and eliminate this discrepancy." She and other scientists have been discussing new ways to solve the problem on April 13 and 14 at a meeting of the US. American Physical Society in Denver, Colorado.

Determining the lifetime of a neutron is important to understand the amount of hydrogen, helium and other light elements formed during the first few minutes after the birth of the neutron. 39, Universe at the Big Bang, 13.8 billion years ago. Scientists also believe that they can search for new types of physics if they can better determine the lifetime of neutrons, as this would help to constrain measurements of other subatomic particles.

Subatomic clock

James Chadwick discovered the neutron in 1932, but it was only in 1951 that the researchers first reported measuring the lifetime of the particle, using nuclear reactors that made free neutrons and followed them. 'evolution. Physicists continued to work closer to the answer – until 2005, when their measurements became precise enough to reveal the confusing difference of eight seconds. Then the scientists worried.

One way to measure the neutron lifetime is to put some of the particles in a bottle and count the number of particles that remain after a certain period. This "vial" method has been tried in several laboratories, including the Los Alamos National Laboratory in New Mexico.¹ and the Institut Laue – Langevin (ILL) in Grenoble, France^2,3. On average, they have a neutron lifetime of 14 minutes 39 seconds.

The other method is to introduce neutrons into a detector that counts the protons created during the decay of neutrons. This "beam" method was used at NIST and the Japanese Proton Accelerator research complex in Tokai. Japanese work has just begun, but the collaboration with NIST revealed in 2013 that their neutrons lived eight seconds longer, on average, than what we see in the bottle method.⁴.

This is a big problem because beam and bottle measurements are so accurate that they do not overlap, even when their margins of error are taken into account. Physicists have therefore looked for ways to explain why neutrons could disappear from cylinders faster than beams.

Quantum Strangeness

One possibility is that one of the two methods does something wrong. In this case, researchers may want to combine the beam and the bottle in a single device. At the meeting, physicist Zhaowen Tang of the Los Alamos Laboratory explained how researchers could place a particle detector in a bottled neutron trap and count neutrons using both methods. His team got funding to start building the device.

Another possibility is that the beam and bottle approaches correctly measure the lifetime of the neutrons, but that invisible factors explain the difference between the two. The main idea is that neutrons can sometimes be broken down into non-protons but also into black matter, the mysterious invisible material that makes up most of the matter of the Universe.

"It would be amazing if the good old neutron turns out to be the particle that opens the doors of the dark sector for us," said Bartosz Fornal, theoretical physicist at the University of California at San Diego, who helped to propose the Idea last year⁵. But so far, the experimenters have not been able to confirm whether this would happen, several teams announced at the Denver meeting.

Meanwhile, the NIST beam experiment has been collecting new data since last year, using sensitive sensors and other components that will make it more accurate than previous tests – measuring the duration neutron life to one second instead of three to four seconds so far. "Everyone is waiting for the results," says Nadia Fomin, a physicist at the University of Tennessee in Knoxville. The team is already designing its next-generation experiment, which aims to define the lifetime of neutrons in less than 0.3 seconds.

"We are about to solve this problem," said Peter Geltenbort, physicist at ILL.