 Goldstone boson

Each broken continuous internal-symmetry generator normally produces a massless Goldstone mode; for a symmetry breaking pattern $G\to H$, their number is $\dim G-\dim H$ in the standard relativistic setting.

A Goldstone boson is a type of excitation that arises in quantum field theory as a result of spontaneous symmetry breaking. When a system exhibits symmetry in its underlying laws, but the ground state (or vacuum state) does not share that symmetry, Goldstone's theorem states that there will be massless scalar excitations called Goldstone bosons.