We discuss and generalize to arbitrary spin the kind of single-particle spin states which have appeared naturally in field theories in the infinite-momentum frame. These states transform simply under the Galilean symmetry group which is important in the infinite-momentum frame, rather than under the rotation group. We also find that the spinors $U(P,\lambda )$ representing these states are very simple.

• Received 5 January 1972

DOI:https://doi.org/10.1103/PhysRevD.5.1956