Neutron

The neutron $n$ is a baryonic hadron with no electric charge. It is one of the two components of the nucleus and therefore a fundamental part of matter. It is a baryon composed of one up quark and two down quarks. Its antiparticle is the antineutron $\bar{p}$.

Discoveries#

Neutron decay#

The process of neutron free particle decay eluded physicists for decades. While the beta minus decay was well known, the innerworkings of how it worked internally would not come to light until the quark model was introduced. Fermi originally thought beta decay was a four-particle contact interaction and that's not completely wrong. Using quark notation:

What's happening is that the down quark is being converted into an up quark, releasing the electron and neutrino with it. This occurs through the weak force, specifically through the emission of a negative W boson and its subsequent decay into the electron/neutrino. However, the mean lifetime of the $W^{-}$ is so small that it is imperceptible at any reasonable energy scale, meaning it can be fully considered a contact interaction.

The antineutron#

The antineutron followed the discovery of the antiproton by about a year, also being discovered at the Bevatron, and actually in a rather similar fashion. In a photograph taken of a bubble chamber (a liquid-based, improved cloud chamber), scientists saw the presence of a weird track. An track simply vanished, but if you followed the track were it would've gone, it led to a starburst decay into four particles. This piqued interest, as the track that vanished couldn't have done it due to being stopped. This is because due to how stopping power works, particles lose more and more energy as the slow down, meaning that the track would've gotten thicker and thicker. Also, it would've curled really heavily due to being in a magnetic field. Neither were observed: it just vanished. The assumption was that an antiproton struck an ordinary proton in some nucleus and annihilated into

which is a valid decay path (plus possibly other particles). The neutrons are undetectable by the bubble chamber, so the normal one simply got stuck in the material while the antineutron traveled for a bit, then struck an ordinary neutron in some nucleus. This triggered annihilation into a four-particle starburst pattern (probably all pions), which is a reasonable path. This was proof that the antineutron was likely to exist. An experiment was devised by Cork, Lambertson and Piccioni at the Bevatron to intentionally produce a beam of antiprotons with

and channel them into a series of detectors to artificially reproduce this event, which proved successful.