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The Mystery of the Neutron Lifetime

September 2, 2020

ORNL staff Matthew Frost and Leah Broussard work at the Magnetism Reflectometer at the Spallation Neutron Source, used for a search for mirror neutrons. Image Credit: Genevieve Martin/ORNL

Nine seconds. An eternity in some scientific experiments; an unimaginably small amount in the grand scheme of the universe. And just long enough to confound nuclear physicists studying the lifetime of the neutron.

The neutron is one of the building blocks of matter, the neutral counterpart to the positive proton. Like many other subatomic particles, the neutron doesn’t last long outside of the nucleus. Over the course of about 15 minutes, it breaks apart into a proton, an electron, and a tiny particle called an anti-neutrino.

But how long the neutron takes to fall apart presents a bit of a mystery. One method measures it as 887.7 seconds, plus or minus 2.2 seconds. Another method measures it as 878.5 seconds, plus or minus 0.8 second. At first, this difference seemed to be a matter of measurement sensitivity. It may be just that. But as scientists continue to perform a series of ever-more-precise experiments to evaluate possible issues, the discrepancy remains.

This persistence leads to the possibility that the difference is pointing to some type of unknown physics. It could be revealing an unknown process in neutron decay. Or it could be pointing to science beyond the Standard Model scientists currently use to explain all of particle physics. There are a number of phenomena that the Standard Model doesn’t fully explain and this difference could point the way towards answering those questions.

To unravel this strange disparity, the Department of Energy’s (DOE) Office of Science is working with other federal agencies, national laboratories, and universities to nail down the duration of the neutron lifetime.


Related Publication: Broussard. L.J., (2019). New search for mirror neutron regeneration. EPJ Web of Conferences, 219.