Particle physics

In particle physics, the 'zoo' of particles discovered in the 20th century is organised into two main families: leptons and hadrons. This classification is based on the fundamental forces they interact with. Leptons, like electrons, are considered truly fundamental particles, meaning they cannot be broken down further. Hadrons, such as protons and neutrons, are composite particles, meaning they are made up of smaller constituents called quarks. The crucial difference is that hadrons experience the strong nuclear force, which binds quarks together and holds the nucleus together, while leptons do not. Both groups are affected by gravity, the weak nuclear force, and, if charged, the electromagnetic force.

Leptons are a group of six fundamental particles and their antiparticles. For your exams, the most important ones to know are the electron and the neutrino. The electron (e⁻) is a stable, negatively charged particle. Its antiparticle is the positron (e⁺), which has the exact same mass but an opposite, positive charge. The neutrino (ν) is a very unusual particle. It has almost no mass, no electric charge, and interacts very weakly with other matter. It was first hypothesised to explain beta decay. When a nucleus undergoes beta decay, the emitted beta particle (electron) has a continuous range of kinetic energies. To conserve energy and momentum in each decay, another particle must be emitted – the neutrino (or, in beta-minus decay, an antineutrino, ν̅).

Hadrons are composite particles made of quarks, held together by the strong nuclear force. They are divided into two subgroups based on their quark composition: baryons and mesons. Baryons are particles made of three quarks (or three antiquarks for antibaryons). The most common examples are the protons and neutrons that make up atomic nuclei. Because they are made of three quarks, baryons have half-integer spin (a quantum property you don't need to worry about, but it's their defining feature for physicists). Mesons are the other type of hadron. They are unstable and are made of one quark and one antiquark. Pions (π) are a common example of mesons. Because they contain an antiparticle, mesons are their own antiparticles in some cases, or have distinct antiparticles where the quark and antiquark are swapped.

The properties of hadrons, like their charge, are determined by the quarks they contain. There are six 'flavours' of quark, but for your course, you mainly need to know about the up(u) and down(d) quarks. Quarks have fractional electric charges, which is a very strange property! The up quark has a charge of +2/3e, and the down quark has a charge of -1/3e, where 'e' is the elementary charge (the magnitude of an electron's charge). Antiquarks have the opposite charge; the anti-up (u̅) has charge -2/3e and the anti-down (d̅) has charge +1/3e. By combining these, we can construct familiar particles. A proton is a baryon made of two up quarks and one down quark (uud). A neutron is a baryon made of one up quark and two down quarks (udd).

We can now understand radioactive beta decay in terms of quarks and fundamental particles. It's not the nucleus as a whole that decays, but a single nucleon within it. Beta-minus (β⁻) decay occurs when a neutron turns into a proton. At the fundamental level, a down quark within the neutron changes into an up quark. This changes the nucleon from udd (neutron) to uud (proton). This transformation is governed by the weak nuclear force. To conserve charge, a particle with charge -1e must be emitted - this is the electron (the β⁻ particle). To conserve lepton number, an electron antineutrino (ν̅e) is also created and emitted. Beta-plus (β⁺) decay is the opposite process, occurring in proton-rich nuclei. A proton turns into a neutron. Here, an up quark changes into a down quark (u → d). A positron (e⁺) and an electron neutrino (νe) are emitted.