Physicists Measure Neutron Lifetime with Unprecedented Precision


Physicists have performed an improved measurement of the free neutron lifetime using the UCNτ apparatus at the Los Alamos Neutron Science Center. Their results, published in the journal Physical Review Letters, represent a more than two-fold improvement over previous measurements.

Gonzalez et al. report an improved measurement of the free neutron lifetime. Image credit: Tomislav Jakupec.

Gonzalez et al. report an improved measurement of the free neutron lifetime. Image credit: Tomislav Jakupec.

“This work sets a new gold-standard for a measurement that has fundamental importance to such questions as the relative abundances of the elements created in the early Universe,” said Dr. David Baxter, chair of the Department of Physics at Indiana University Bloomington.

“The process by which a neutron decays into a proton — with an emission of a light electron and an almost massless neutrino — is one of the most fascinating processes known to physicists,” added Dr. Daniel Salvat, also from the Department of Physics at Indiana University Bloomington.

“The effort to measure this value very precisely is significant because understanding the precise lifetime of the neutron can shed light on how the Universe developed — as well as allow physicists to discover flaws in our model of the subatomic Universe that we know exist but nobody has yet been able to find.”

The UCNτ experiment captures neutrons, whose temperatures are lowered to nearly absolute zero, inside a ‘bathtub’ lined with about 4,000 magnets.

After waiting 30 to 90 min, the physicists count the surviving neutrons in the tub as they’re levitated against gravity by the force of the magnets.

The unique design of the UCNτ trap allows neutrons to remain stored for more than 11 days, a significantly longer time than earlier designs, minimizing the need for systematic corrections that could skew the results of the lifetime measurements.

Over two years, the authors counted 38 million neutrons captured using this method.

Their experiment gave a value for the free neutron lifetime of 877.75 seconds.

“The results will help physicists confirm or deny the validity of the Cabibbo-Kobayashi-Maskawa matrix, which concerns subatomic particles called quarks and plays an important role in the widely accepted Standard Model of particle physics,” Dr. Salvat said.

“It will also help physicists understand the potential role that new ideas in physics, such as neutrons decaying into dark matter, may play in evolving theories about the Universe, as well as possibly help explain how the first atomic nuclei were formed.”

“The underlying model explaining neutron decay involves the quarks changing their identities, but recently improved calculations suggest this process may not occur as previously predicted,” he added.

“Our new measurement of the neutron lifetime will provide an independent assessment to settle this issue, or provide much-searched-for evidence for the discovery of new physics.”

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F.M. Gonzalez et al. (UCNτ Collaboration). 2021. Improved Neutron Lifetime Measurement with UCNτ. Phys. Rev. Lett 127 (16): 162501; doi: 10.1103/PhysRevLett.127.162501

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