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In mathematics and theoretical physics, Wigner's classification is a classification of the nonnegative (E ? 0) energy irreducible unitary representations of the Poincaré group which have sharp[when defined as?] mass eigenvalues. (Since this group is noncompact, these unitary representations are infinite-dimensional.) It was introduced by Eugene Wigner, to classify particles and fields in physics—see the article particle physics and representation theory. It relies on the stabilizer subgroups of that group, dubbed the Wigner little groups of various mass states.
The Casimir invariants of the Poincaré group are C1 = P?P?, where P is the 4-momentum operator, and C2 = W?W?, where W is the Pauli–Lubanski pseudovector. The eigenvalues of these operators serve to label the representations. The first is associated with mass-squared and the second with helicity or spin.
The physically relevant representations may thus be classified according to whether m > 0 ; m = 0 but P0 > 0; and m = 0 with P? = 0. Wigner found that massless particles are fundamentally different from massive particles.
For the first case, note that the eigenspace (see generalized eigenspaces of unbounded operators) associated with P =(m,0,0,0) is a representation of SO(3). In the ray interpretation, one can go over to Spin(3) instead. So, massive states are classified by an irreducible Spin(3) unitary representation that characterizes their spin, and a positive mass, m.
For the second case, look at the stabilizer of P =(k,0,0,-k). This is the double cover of SE(2) (see unit ray representation). We have two cases, one where irreps are described by an integral multiple of 1/2 called the helicity, and the other called the "continuous spin" representation.
The last case describes the vacuum. The only finite-dimensional unitary solution is the trivial representation called the vacuum.
