Limitations on the superposition principle: superselection rules in   non-relativistic quantum mechanics
  The superposition principle is a very basic ingredient of quantum theory. What may come as a surprise to many students, and even to many practitioners of the quantum craft, is tha superposition has limitations imposed by certain requirements of the theory. The discussion of such limitations arising from the so-called superselection rules is the main purpose of this paper. Some of their principal consequences are also discussed. The univalence, mass and particle number superselection rules of non-relativistic quantum mechanics are also derived using rather simple methods. 
C. Cisneros, R. P. Mart\'inez-y-Romero, H. N. Nu\~nez-Yepez, A.L.   Salas-Brito