Particle Physics

Abstract

Just as nuclear physics became a distinct branch of atomic physics, so a new field called particle physics has evolved from nuclear physics. It is not concerned with the structure of nuclei as such, but with the relations between nucleons and other “elementary” particles. This field is one of the major frontiers of physics today, and not surprisingly it is in a far from tidy state. The quotation marks about the word elementary are an admission that it is not known just what are the basic building-blocks of the universe. More than thirty kinds of subatomic particles are known. Most of them are unstable and decay very quickly, sometimes directly into electrons, sometimes first into other unstable particles, and then into electrons or protons. It sounds easy. The final stable particles are the elementary ones. But consider the neutron, which is the constituent making up more than half the mass of most atoms. A free neutron, not bound in a nucleus, is unstable, and disintegrates according to the equation

$$_0{n^1}{ \to _1}{p^1}{ + _{ - 1}}{e^0} + v$$

where p is used for a proton rather than H1, with a half-life of about 13 min. A free proton is stable, but a bound proton in an excited nucleus can sometimes decay into a neutron and a positron. So, is the proton more elementary than the neutron?