|  Superselection Rules | 
In General
  > s.a. canonical quantum mechanics; theta sectors;
  Superposition Principle.
  * Idea: A statement that
    singles out some unit rays in a Hilbert space as not physically realizable,
    in operator quantum mechanics; Mathematically, they "are" the
    (non-trivial) center of the Lie algebra of observables.
  * Remark: If they commute
    (e.g., when there is a complete set of commuting observables) they imply
    that the Hilbert space can be decomposed into a direct sum of
    superselection sectors, on which the other observables act irreducibly;
    A physical state cannot be a superposition of states belonging to different
    sectors (and the projection operators on these are not observables).
  * Types: "Hard",
    kinematically rooted; "Soft", induced by dynamical decoherence.
  > Online resources:
    see Wikipedia page.
References
  > s.a. quantum field theory / measurement in quantum theory.
  @ General: Wick et al PR(52),
    PRD(70);
    Wightman NCB(95) [review];
    Kupsch Pra(98)qp/96;
    Cisneros et al EJP(98)qp [and the superposition principle];
    Giulini LNP(00)qp [hard and soft];
    Ojima mp/02,
    mp/02 [general framework];
    Hall JPA(04)qp [types, from constraints];
    Giulini a0710-in [rev].
  @ Symmetries, decoherence: Giulini et al PLA(95)gq/94;
    Giulini LNP(00)qp/99.
  @ In curved spacetime:
    Ashtekar & Sen JMP(80) [due to spacetime topology];
    Guido et al RVMP(01)mp/99;
    Brunetti & Ruzzi CMP(07)gq/05 [and general covariance],
    CMP(09)-a0801
      [generalized representatons of nets of C*-algebras and new topological superselection sectors];
    Vasselli CMP(15)-a1211
      [as categories of sections of presheaves of symmetric tensor categories].
  @ Special types: Aharonov & Susskind PR(67) [charge superselection rule];
    Baumgaertel & Grundling JMP(05)mp/04 [for theories with constraints];
    Ciolli RVMP(09) [2D massless free scalar field];
    Zych & Greenberger a1906 [mass superselection rule, resolution of puzzle].
  @ Special approaches to quantum mechanics: Colin et al JPA(06)qp/05 [pilot-wave theory].
  @ Related topics: Hernández-Coronado FP(12)-a1108 [and quantum Newtonian spacetime].
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