Fundamental Particles

Two families of fundamental particles — quarks and leptons. Fundamental means having no internal structure: they are not made of anything smaller.

Quarks — the ones needed at A-level are up, down and strange, with charges +2/3, −1/3 and −1/3 of the elementary charge respectively.

Leptons — the electron, the muon, and their neutrinos. Leptons are not affected by the strong nuclear force.

Hadrons — particles made of quarks. They come in two kinds:

Baryons, made of three quarks. The proton is uud (charge +2/3 +2/3 −1/3 = +1) and the neutron is udd (+2/3 −1/3 −1/3 = 0).

Mesons, made of a quark and an antiquark.

The proton and neutron are not fundamental — this is the central point of the topic. They are composite, which is exactly why beta decay can turn one into the other.

Antiparticles — every particle has an antiparticle with the same mass but opposite charge: the positron for the electron, the antiproton for the proton. When a particle meets its antiparticle they annihilate, their entire mass converting to energy.

Beta-minus decay — a neutron becomes a proton:

n → p + e⁻ + ν̄_e

At the quark level, a down quark changes into an up quark, which is what udd → uud means.

Beta-plus decay — a proton becomes a neutron:

p → n + e⁺ + ν_e

with an up quark changing to a down quark.

Example: the neutrino was proposed before it was detected, purely to save conservation of energy. Beta particles emerged with a range of energies rather than the single value the decay should give, so energy appeared to be going missing. Rather than abandon conservation, Pauli proposed an unseen particle carrying the remainder. It took twenty-five years to detect — an instance of a conservation law being trusted over the evidence, and being right.

Conservation rules — charge, baryon number and lepton number are all conserved in every interaction, and checking them is how decay equations are verified.