The Field Guide to Particle Physics : Season 3
https://pasayten.org/the-field-guide-to-particle-physics
©2022 The Pasayten Institute cc by-sa-4.0
The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov.

The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.

The Antineutron

Like the antiproton, the antineutron is a composite particle made up of antiquarks. It looks a lot like the neutron, and that’s pretty interesting because both of those particles have no electric charge!

The antineutron is made from two antidown quarks and an antiup quark. The antineutron’s mass is a bit over 939 MeV, and the mass difference ratio between the neutron and the antineutron is essentially consistent with zero.

Because it’s electrically neutral, it is really hard to measure properties of the antineutron. You can’t really use electric or magnetic fields to confine, shape or cool collections of antineutrons in any meaningful way.

We don’t have a working measurement of the antineutron’s magnetic dipole moment. We haven’t really studied their decay. 

Left to its own devices, the neutron decays in about 15 minutes to a proton, and electron and a neutrino. We’d expect the antineutron to decay similarly, but with a positron. But again. It’s a serious experimental challenge.

 We barely have a handle on the antineutron’s mass! But there have been experimental antineutron beams and there is still plenty of interesting physics that can be done with them.

Antineutron beams

Antiproton and antineutron technologies are linked. The antiproton was discovered in 1955 , and the antineutron was found in 1956. In the 1980s, The Low Energy Antiproton Ring at CERN fired a slow beam of antiprotons at liquid hydrogen to create a secondary beam of anti neutrons.

Low energy proton-antiproton collisions proceed by the exchange of a single pion. Because the hydrogen was kept super cold, and the antiprotons had such low energy, the two particles exchanged a single, virtual neutral pion, which afforded a conversion of the proton antiproton pair to a neutron antineutron pair.

This secondary beam of neutron/antineutron pairs was aimed at an iron slab for a target. The neutron and antineutron interact with iron differently, but expecting to find both particles simultaneously made the measurement pretty tractable.

Again. Antineutrons are hard to work with, so any trick you can find to help is welcome!

Antinuclei

Of course, there’s more.

Antineutrons have been created in atomic nuclei. Or antinuclei, if you like. Deuterium - a hydrogen atom with a bonus neutron in the nucleus has a theoretical antimatter cousin, antideuterium. The nucleus of anti deuterium was created in experiments way back in the 60s, although cooling those nuclei down enough to accept an orbiting positron has not yet occurred. But hey, ,antihydrogen was only really successfully studied in 2016!

The relativistic heavy ion collider has observed the anti helium-4 nucleus. In other words, there’s also an anti alpha particle!

All these discoveries point to to the fact that there is very little difference between matter and antimatter, which makes the overall dearth of antimatter in our observed universe even more confusing.