Physics – High Energy Physics – High Energy Physics - Theory
Scientific paper
2003-04-02
J.Phys. A36 (2003) L449
Physics
High Energy Physics
High Energy Physics - Theory
4 pages revtex
Scientific paper
10.1088/0305-4470/36/29/101
The ``little group'' for massless particles (namely, the Lorentz transformations $\Lambda$ that leave a null vector invariant) is isomorphic to the Euclidean group E2: translations and rotations in a plane. We show how to obtain explicitly the rotation angle of E2 as a function of $\Lambda$ and we relate that angle to Berry's topological phase. Some particles admit both signs of helicity, and it is then possible to define a reduced density matrix for their polarization. However, that density matrix is physically meaningless, because it has no transformation law under the Lorentz group, even under ordinary rotations.
Lindner Netanel H.
Peres Asher
Terno Daniel R.
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