Precise measurement of neutron lifetime

The scientific purpose of the experiment, which IU has led for over a decade, is to measure how long, on average, a free neutron lives outside the confines of atomic nuclei.

The results from the team, which encompasses scientists from over 10 national labs and universities in the United States and abroad, represent a more than two-fold improvement over previous measurements -- with an uncertainty of less than one-tenth of a percent.

The work is reported in the Oct. 13 issue of the journal Physical Review Letters. It was also the subject of a live news briefing at the 2021 Fall Meeting of the American Physical Society Division of Nuclear Physics. A pre-print version of the paper is available.

"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 David Baxter, chair of the IU Bloomington College of Arts and Sciences' Department of Physics. "We're proud of IU's long-time role as a leading institution on this work."

IU-affiliated authors at the time of the study were graduate students Nathan Callahan, Maria Dawid and Francisco Gonzalez; engineer Walt Fox; Rudy Professor of Physics Chen-Yu Liu; research scientist Daniel Salvat; and mechanical technician John Vanderwerp. (Callahan and Gonzalez are currently affiliated with Argonne National Laboratory and Oak Ridge National Laboratory, respectively.) The research was conducted at Los Alamos National Laboratory.

"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," said Salvat, who led the experiments at Los Alamos. "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 neutrons used in the study are produced by the Los Alamos Neutron Science Center Ultracold Neutron source at Los Alamos National Lab. The UCNtau experiment captures these neutrons, whose temperatures are lowered to nearly absolute zero, inside a "bathtub" lined with about 4,000 magnets. After waiting 30 to 90 minutes, researchers count the surviving neutrons in the tub as they're levitated against gravity by the force of the magnets.

The unique design of the UCNtau 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 study's researchers counted approximately 40 million neutrons captured using this method. These efforts were the thesis work of Gonzalez, who collected the data at Los Alamos as an IU graduate student from 2017 to 2019, and led the analysis of the published result.

Salvat said the experiment's 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. 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," Salvat said. "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."