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Skyrmion
In theoretical physics, a skyrmion, named for Tony Skyrme , is a homotopically nontrivial classical solution of a nonlinear sigma model with a non-trivial target manifold topology i.e. a particular case of a topological soliton. It arises, for example, in chiral models of mesons where the target manifold is a homogeneous space of 
(the structure group),
where
- SU(N)L and SU(N)R are the left and right copies respectively
- SU(N)diag is the diagonal subgroup
If spacetime has the topology S3×R (for space and time respectively), then classical configurations are classified by an integral winding number because the third homotopy group, 
(the congruence sign here refers to homeomorphism, not isomorphism).
It is possible to add a topological term to the chiral lagrangian whose integral only depends upon the homotopy class. This results in superselection sectors in the quantized model.
Skyrmions have been used to model baryons.
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