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Particle lifetimes – live forever or die quickly?


Why is everyday matter so stable? We take it for granted that the lifetime of (non-radioactive) atoms is billions of years. Same for the protons and neutrons in their nuclei (barring a high-energy interaction).

Unlike protons, lifetimes are not the same for bound and unbound “free” neutrons. Beta decay. Sticking around for billions of years depends on togetherness (and nowhere to run, so to speak).

(Wiki) While a free neutron has a half life of about 10.2 min, most neutrons within nuclei are stable. According to the nuclear shell model, the protons and neutrons of a nuclide are a quantum mechanical system organized into discrete energy levels with unique quantum numbers. For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state. In stable nuclei the possible lower energy states are all filled, meaning they are each occupied by two protons with spin up and spin down. The Pauli exclusion principle therefore disallows the decay of a neutron to a proton within stable nuclei.

The Ultra Cold Neutrons tau experiments described in this article (below) are complicated. Isolating free neutrons is a technological challenge, and being sure of what you’re counting over time is even trickier.

• Caltech Weekly > “How Long Does a Neutron Live?” (October 13, 2021) – Physicists use “bottle” method to make most precise measurement yet of a neutron’s lifetime.

(quote) Now, in a new study published in the journal Physical Review Letters, a team of scientists has made the most precise measurement yet of a neutron’s lifetime using the bottle technique. The experiment, known as UCNtau (for Ultra Cold Neutrons tau, where tau refers to the neutron lifetime), has revealed that the neutron lives 14.629 minutes with an uncertainty of 0.005 minutes. This is a factor of two more precise than previous measurements made using either of the methods. While the results do not solve the mystery of why the bottle and beam methods disagree, they bring scientists closer to an answer.

Ordinary matter consists of only 3 types of so-called elementary particles: up and down quarks, and electrons. How stable are those?

Well, that’s something for follow-up. Up and down quarks can evidently decay into each other. Wiki says the mean lifetime of an electron is essentially forever: > 6.6×10^28 years! I’m curious about visualizations using quantum field theory on this topic.


Wiki: Elementary particle

• DOE > “DOE Explains…the Standard Model of Particle Physics

(quote) All ordinary matter, including every atom on the periodic table of elements, consists of only three types of matter particles: up and down quarks, which make up the protons and neutrons in the nucleus, and electrons that surround the nucleus.

• Symmetry Magazine > “Do protons decay?” (9-22-2015)

(quote) The stuff of daily existence is made of atoms, and all those atoms are made of the same three things: electrons, protons and neutrons.

Protons and neutrons are very similar particles in most respects. They’re made of the same quarks, which are even smaller particles, and they have almost exactly the same mass.

Yet neutrons appear to be different from protons in an important way: They aren’t stable. A neutron outside of an atomic nucleus decays in a matter of minutes into other particles.

A free proton is a pretty common sight in the cosmos. Much of the ordinary matter (as opposed to dark matter) in galaxies and beyond comes in the form of hydrogen plasma, a hot gas made of unattached protons and electrons. If protons were as unstable as neutrons, that plasma would eventually vanish.