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Home Tech How long do neutrons live? Physicists join a decades-old puzzle

# How long do neutrons live? Physicists join a decades-old puzzle

Physicists move closer to answering a long-standing mystery of the universe: how long a neutron lives.

Neutrons are electrically neutral particles that normally form nuclei with protons. Some neutrons are not bound in atoms; These free-floating neutrons radioactively decay into other particles within a few minutes.

However, physicists can not agree on how long it takes for a neutron to die. Using a laboratory approach, they measure the average neutron life with 14 minutes 39 seconds. With another approach, they will be 8 seconds longer. The discrepancy has annoyed the researchers for almost 15 years.

"We do not know why they are different," says 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 discussed new ways to solve the problem at a meeting of the American Physical Society in Denver, Colorado on April 13 and 14.

Determining the lifetime of a neutron is important in understanding how much hydrogen, helium and other light elements formed in the Big Bang in the first few minutes after the birth of the universe 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 the neutron, as this would limit the measurement of other subatomic particles.

## Subatomic clock

James Chadwick discovered the neutron in 1932, but it was not until 1951 that researchers first reported measuring the lifetime of the particle using nuclear reactors that produced free neutrons and watched them decay. The physicists worked ever closer to the answer – until 2005, when their measurements were accurate enough to reveal the enigmatic difference of eight seconds. Then the scientists worried.

One way to determine the lifetime of the neutron is to put some of the particles in a bottle and count how many are left after a certain amount of time. This "bottle" method has already been tried in several laboratories, including the Los Alamos National Laboratory in New Mexico1 and the Institut Laue-Langevin (ILL) in Grenoble, France2,3, On average, they have a neutron life of 14 minutes 39 seconds.

The other possibility is to feed neutrons into a detector that counts the protons that arise when the neutrons decay. This beam method was used at NIST and the Japan Proton Accelerator Research Complex in Tokai. The Japanese work has just begun, but the NIST collaboration reported in 2013 that their neutrons live on average eight seconds longer than the bottle method4,

This is a big problem because the jet and bottle dimensions are each so accurate that they do not overlap, even if their error limits are considered. That's why physicists have been looking for ways to explain why neutrons can disappear from bottles faster than rays.

## Quantum weirdness

One possibility is that one of the two methods does something wrong. In this case, researchers may want to combine the jet and bottle in a single device. At the meeting, physicist Zhaowen Tang from the laboratory in Los Alamos described how researchers can place a particle detector in a bottle neutron trap and count neutrons with both methods. His team has received funding to begin building the device.

Another possibility is that the jet and bottle approaches have correctly measured the neutron lifetime, but that an unseen factor is responsible for the discrepancy between the two. A guiding idea is that neutrons occasionally disintegrate not only into protons, but also into dark matter, the mysterious, invisible material that makes up much of the matter in the universe.

"It would be amazing if the good old neutron turns out to be the particle that opens the gates of the dark sector," says Bartosz Fornal, a theoretical physicist at the University of California, San Diego, who helped suggest the idea last year5, So far, however, the experimenters have not yet determined whether this could be possible, several teams reported at the meeting in Denver.

In the meantime, since last year, the NIST beam experiment has been collecting new data using sensitive detectors and other components that make it more accurate than previous runs. The neutron lifetime was measured within one second and not three to four seconds as usual. "Everyone is waiting for the results," says Nadia Fomin, a physicist at the University of Tennessee in Knoxville. The team is already designing a next-generation experiment to reach neutron life within 0.3 seconds.

"We are on the way to pinpoint this," says Peter Geltenbort, a physicist at the ILL.

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