In particle physics, a particle is a fundamental unit of matter or energy. It refers to the smallest identifiable unit that retains the properties and characteristics of a specific type of particle. Particles can be either elementary or composite.
Elementary particles: These are particles that are considered to be indivisible and are not composed of smaller constituents. They are the building blocks of matter and cannot be broken down into smaller particles. Elementary particles can be classified into two categories:
a. Fermions: These particles have half-integer values of spin (such as 1/2, 3/2, etc.) and obey the Pauli exclusion principle, which states that no two identical fermions can occupy the same quantum state simultaneously. Examples of fermions include quarks (which make up protons and neutrons) and leptons (such as electrons and neutrinos).
b. Bosons: These particles have integer values of spin (such as 0, 1, 2, etc.) and do not obey the Pauli exclusion principle. Bosons act as force carriers or mediators of fundamental interactions. For example, the photon is a boson that mediates the electromagnetic force, while the W and Z bosons mediate the weak nuclear force.
Composite particles: These particles are composed of combinations of elementary particles. Composite particles are made up of quarks and/or antiquarks held together by the strong nuclear force. The most familiar composite particles are protons and neutrons, which are composed of three quarks each.
Particle physics aims to understand the properties, behavior, and interactions of these particles by studying their properties such as mass, charge, spin, and lifetime. Particle accelerators and detectors are used to create and observe particles in high-energy collisions, providing insights into the fundamental forces and the structure of matter.