Polarisation describes how the spin projections of particles are oriented relative to a chosen direction, such as the particle’s direction of motion. In the ATLAS Experiment, polarisation is a statistical property that indicates whether spins tend to point in one direction rather than in random directions.

For example, W bosons produced in proton-proton collisions can be polarised, meaning that across many events their spins show a preferred orientation. This polarisation affects the angular distribution of the charged lepton in W → ℓν decays, which can be used to infer the W boson’s polarisation.