Measuring the neutron’s lifetime from space could solve an enduring mystery

To really understand neutrons, physicists
may have to take to space.

When outside the confines of an atomic nucleus,
a neutron decays into other particles in about 15 minutes on average. But exactly
how long it lives has been tough to pin down, because two different methods of measuring disagree (SN: 2/1/17).

Now, scientists have made a third type of measurement, using data from NASA’s MESSENGER spacecraft.
Although the new measurement isn’t sensitive enough to resolve the discrepancy,
a future space mission could help overcome the impasse, physicists report June
11 in Physical Review Research. Determining
the neutron’s lifetime is essential to understanding cosmic questions like how certain elements formed after the Big Bang (SN: 5/9/18).

The MESSENGER spacecraft orbited Mercury
from 2011 to 2015 and flew by Venus on the way there, taking measurements of
neutrons in the vicinity of the two planets. Neutrons are produced in reactions
set off by high-energy particles from space slamming into the planets. By
measuring how the number of neutrons detected decreased as the spacecraft got
farther from each planet, the researchers estimated how quickly those particles
were disappearing.

That analysis suggests a shorter
lifetime of about 13 minutes, but the large experimental uncertainty on that
figure means that it is still consistent with both previous types of
measurements.

MESSENGER wasn’t intended to measure the
neutron’s lifetime, so a future space mission dedicated to the task could do
much better, Jack Wilson of the Johns Hopkins Applied Physics Laboratory in Laurel,
Md., and colleagues report.

“Having a third technique to break the
tie could be critically important,” Wilson says.

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