In the ordinary formulation of quantum mechanics, physical observables are represented by
self-adjoint operators. But, not all of self-adjoint operators in the theory correspond to physi-
cal observables in the real world. The superselection rule is a necessary condition imposed on
a self-adjoint operator for being observable. Sometimes the superselection rule is regarded as a
reasonable hypothesis that cannot be derived from the other principles. I introduce notions of
covariant measurement and isolated conservation law. By using them I derived the superselec-
tion rules in a general scheme. As an application of this scheme I discuss why color-multiplet
quantities in QCD are invisible in the macroscopic world.

references
arXiv:1112.5701
http://www.phys.cs.is.nagoya-u.ac.jp/~tanimura/paper/kokyuroku1774-2012.pdf