Variational problem for the Frenkel and the Bargmann-Michel-Telegdi (BMT) equations    [PDF]

A. A. Deriglazov
We propose Lagrangian formulation for the particle with value of spin fixed within the classical theory. The Lagrangian turns out to be invariant under non-abelian group of local symmetries. As the gauge-invariant variables for description of spin we can take either the Frenkel tensor or the BMT vector. Fixation of spin within the classical theory implies \$O(\hbar)\$\,-corrections to the corresponding equations of motion.
View original: http://arxiv.org/abs/1204.2494