Physicists Capture Most Accurate Measure of Neutron Lifespan Yet

We now know, to the nearest tenth of a percent, how long a neutron can survive outside the atomic nucleus before decaying into a proton.

This is the most accurate measurement to date of the lifespan of these fundamental particles, representing an improvement of more than twice over previous measurements. This has implications for our understanding of how the first matter in the Universe was created from a soup of protons and neutrons within minutes of being released. big Bang.

“The process by which a neutron” decays “into a proton – with an emission of a light electron and almost no mass. neutrinos – is one of the most fascinating processes known to physicists “, said nuclear physicist Daniel Salvat from Indiana University Bloomington.

“The effort to measure this value very precisely is important because understanding the precise lifespan 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 but that no one has yet been able to find. “

The research was conducted at the Los Alamos National Science Center, where a special experiment is being set up just to try to measure the lifetimes of neutrons. This is the UCNtau project, and it involves ultra-cold neutrons (UCN) stored in a magneto-gravitational trap.

The neutrons are cooled to almost absolute zero and placed in the trap, a bowl-shaped chamber lined with thousands of permanent magnets, which levitate the neutrons, inside a vacuum jacket.

The magnetic field prevents neutrons from depolarizing and, combined with gravity, prevents neutrons from escaping. This design allows neutrons to be stored for up to 11 days.

The researchers stored their neutrons in the UCNtau trap for 30 to 90 minutes, then counted the remaining particles after the allotted time. In repeated experiments, carried out between 2017 and 2019, they counted more than 40 million neutrons, obtaining enough statistical data to determine the lifespan of particles with the greatest precision to date.

This lifespan is approximately 877.75 ± 0.28 seconds (14 minutes and 38 seconds), according to the researchers’ analysis. The fine-grained measurement can help impose significant physical constraints on the Universe, including the formation of matter and black matter.

After the Big Bang, things happened quite quickly. In the very first moments, the hot, ultra-dense matter that filled the Universe cooled into quarks and electrons; a few millionths of a second later, the quarks have merged into protons and neutrons.

Knowing the lifespan of the neutron can help physicists understand what role, if any, decaying neutrons play in forming the mysterious mass in the Universe known as dark matter. This information can also help test the validity of something called the Cabibbo-Kobayashi-Maskawa matrix, which makes it possible to explain the behavior of quarks under Standard model of physics, the researchers said.

“The underlying model for neutron decay involves quarks changing identity, but recently improved calculations suggest this process may not occur as previously expected. Salvat said.

“Our new measure of neutron lifetimes will provide an independent assessment to address this problem, or provide much-needed evidence for the discovery of new physics.”

The research was accepted in Physical examination letters, and is available on arXiv.

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