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The little group is a subgroup of the Poincar6 group which leaves the four-momentum of a given particle invariant. The little groups for massive and massless particles are locally isomorphic to the three-dimensional rotation group and the two-dimensional euclidean group respectively. GAUGE TRANSFORMATIONS AS LORENTZ-BOOSTED ROTATIONS by D. HAN et. al.
A good explanation can be found at page 8 in https://indico.cern.ch/event/544849/contributions/2214530/attachments/1301154/1942518/yskim.pdf