= Little group
The <little group> of a fixed <four-momentum> $p$ is its <stabilizer subgroup> in the <Lorentz group>: $G_p=\{\Lambda:\Lambda p=p\}$. It changes internal states while leaving that momentum unchanged. In four spacetime dimensions, a massive rest momentum has spatial rotations as its <little group>; the corresponding <Spin group> is <SU(2)>, permitting half-integer <spin>. A nonzero future-directed massless <four-momentum> instead has the planar Euclidean little-group structure. Its finite-helicity quantum states have trivial translation action. These <group representations>, combined with the momentum orbit, underlie <Wigner's classification>.<String Theory by David Tong>[Section 2.3.2, discussion of Wigner classification and massive versus massless states, printed p. 42]{ref}
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