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Concept
Wigner's classification
Résumé
In mathematics and theoretical physics, Wigner's classification
is a classification of the nonnegative energy irreducible unitary representations of the Poincaré group which have either finite or zero 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 (Einstein notation) where P is the 4-momentum operator, and 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
but or whether
with
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 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
This is the double cover of SE(2) (see projective 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.
For the third case The only finite-dimensional unitary solution is the trivial representation called the vacuum.
As an example, let us visualize the irreducible unitary representation with and It corresponds to the space of massive scalar fields.
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