Particles and Forces

Abstract

Radiation consists, ultimately, of subatomic matter and non-matter particles (e.g. photons) and nuclei. The mechanisms by which radiation impacts the surrounding world is determined by the laws that govern subatomic particle interactions. This chapter presents the elementary and composite particles discovered so far and introduces the forces by which these particles interact. The emphasis is placed on an overview of the current knowledge in this field, rather than an historical or theoretical account of the topic.

Glossary

Electronvolt (eV) Kinetic energy gained by an electron after accelerating through a potential difference of 1 volt

$$\begin{aligned} 1\,\mathrm{eV}=1.602\,176\, 565(35)\times 10^{-19}\,\mathrm{J}. \end{aligned}$$

SI International System of Units

Planck constant Fundamental constant that relates the energy of a photon with its frequency

$$\begin{aligned} h=E/\nu = 6.626\,069\,57(29)\times 10^{-34}\mathrm{\,J\,s} = 4.135\,667\,516(91)\times 10^{-15}\mathrm{\,eV\,s} \end{aligned}$$

Ångström (Å) Distance corresponding to $10^{-10}$ m which is approximately the scale of an atom

Antimatter Collective name for the antiparticles

Baryons Composite particles which are bound states of three quarks or three antiquarks

Boson Particle with spin 1, such as the photon and the Higgs particle

Feynman diagram Pictorial mathematical representation of particle interactions

Fermi (f) Distance corresponding to $10^{-15}$ m which is approximately the scale of the atomic nucleus

Fundamental forces The four known fundamental forces of nature: electromagnetic, weak nuclear, strong nuclear, and gravitational

Hadrons Composite particles which are bound states of quarks

Higgs boson Particle manifestation of the Higgs field, whose existence is key to explaining how leptons, quarks and the electroweak bosons acquire their own mass

Lepton Elementary matter particle with spin 1/2, such as the electron, the muon and the neutrino

Mesons Composite particles which are bound states of one quark and one antiquark

Muon Elementary unstable particle belonging to the second family of the leptons. Its mass is 106 MeV and mean lifetime of $\mathrm{2.2\,\upmu s}$

Quark Elementary matter particle, present for instance in the protons and neutrons of an atomic nucleus

Standard Model Quantum-mechanical relativistic theory of the electroweak and strong interactions between elementary particles

Speed of light Fundamental constant which in vacuum corresponds to

$$\begin{aligned} c = 299\,792\,458\,\mathrm{m \,s^{-1}}\simeq 3\times 10^{8}\mathrm{\,m \,s^{-1}} \end{aligned}$$

Top quark The heaviest of the quarks, with a mass of 173 GeV

Ultraviolet (UV) Part of the electromagnetic spectrum corresponding to wavelengths between 100 nm and 400 nm

X-ray (soft) Electromagnetic radiation corresponding to wavelengths between 0.01 nm and 10 nm

X-ray (hard) Electromagnetic radiation corresponding to energies of the order 10–100 keV