Elementary particles
An «elementary» particle is, by name, the simplest particle with no internal structure; historically the name was given to many particles, some of which later turned out to be composite. First came the electron (J. J. Thomson, 1897), then the photon, the proton (Rutherford, 1919), the neutron (Chadwick, 1932) and the positron (1932); from the 1950s hundreds of new particles were discovered. Every particle has an antiparticle of equal mass and opposite charge (electron — positron, proton — antiproton). When a particle meets its antiparticle they annihilate: e⁻ + e⁺ → 2γ; for a pair at rest each photon has 0.511 MeV and the two fly apart in opposite directions (momentum is conserved, so at least two photons are needed). The reverse process is pair production from a photon with E_γ ≥ 2m_e c² = 1.022 MeV near a nucleus. In medicine positron emission tomography (PET) is based on detecting the photon pairs from electron–positron annihilation. In the present picture there are four fundamental interactions: strong, electromagnetic, weak and gravitational. Particles that take part in the strong interaction are hadrons (proton, neutron, mesons); those that do not are leptons (electron, muon, neutrino and their antiparticles). Hadrons are built from quarks: proton — uud, neutron — udd (the u quark has charge +2e/3, the d quark −e/3), mesons — a quark and an antiquark. The modern Standard Model has 6 quarks, 6 leptons and the force-carrying particles; the Higgs boson was discovered at CERN in 2012. The Standard Model does not include gravity.
In your notebook give one example for each of the four fundamental interactions (for example strong — holds nucleons in a nucleus; weak — β-decay; electromagnetic — an electron in an atom; gravitational — planets). A paper exercise.