Historical Notes and Fundamental Concepts

Abstract

Elementary particle physics deals with the search and study of the ultimate constituents of matter as well as of their interactions. Currently, the term elementary particle denotes really elementary particles, as the electron (e ⁻), the muon (μ ⁻), the tau (τ ⁻) and the corresponding neutrinos (ν _e ,ν _μ ,ν _τ ), which are globally called leptons, and the quarks. Free quarks were never observed as they are confined inside hadrons. The term includes a stable object (the proton), several particles with relatively long lifetimes (as the neutron and the Λ ⁰ hyperon) and many resonances which have very short lifetimes. They are all composed by quarks and/or antiquarks. This first chapter deals with orders of magnitude, specific numbers and relevant measurement units used in particles physics. Most long-lived particles were discovered in the Cosmic Rays, before the advent of particle accelerators. Supplement 1.1 presents the connection between cosmic accelerators and the particles reaching the Earth.