Recent Trends in Physics

This chapter is a brief, largely conceptual survey of ideas at the frontiers of physics: the particles that make up matter, and the large-scale behaviour of the universe as a whole.

Elementary particles and antiparticles

An elementary particle is a particle with no known substructure, i.e. it is not made up of smaller constituent particles (as far as current experiments can tell). Examples include electrons, quarks and photons. Every elementary particle of matter has a corresponding antiparticle with the same mass but opposite electric charge (and opposite values of certain other quantum properties). The antiparticle of the electron is the positron ($e^+$), identical to the electron in mass but carrying a positive charge. When a particle meets its antiparticle they can annihilate each other, converting their mass entirely into energy (usually gamma-ray photons).

Classification of particles

Elementary and composite particles are grouped into broad families based on the forces they interact with and their internal structure:

• Leptons: fundamental particles that do not experience the strong nuclear force. The electron, muon, tau and their associated neutrinos ($\nu_e$, $\nu_\mu$, $\nu_\tau$) are leptons.
• Quarks: fundamental particles that experience the strong nuclear force and combine to form composite particles called hadrons. Quarks are never found alone in nature; they are always bound together.
• Hadrons: composite particles built from quarks, split into two sub-groups:
  • Baryons: made of three quarks (e.g. the proton and the neutron).
  • Mesons: made of one quark and one antiquark (e.g. the pion, kaon).