What is a particle?

A particle in our sense is a thing that acts locally in space and time (while it exists, it has a position associated with it at any given time). Each type of particle is characterised by a set of properties called quantum numbers and a mass. The quantum numbers are discrete properties, meaning that they can only take some specific values. Examples of quantum numbers are electric charge, colour charge, spin, parity, etc.

Particles with integer spin (0, 1, 2, …) are called bosons, particles with half-integer spin (1/2, 3/2, …) are called fermions.

Some particles are massless, which just means that their mass is zero.

An unstable particle's average lifetime can be computed if its mass and relevant quantum numbers are known in addition to the parameters that govern the interactions between parameters, described later.