What are all charge-like quantum numbers? In particle physics there are often things (like charge conjugation) that apply to all charge like quantum numbers. Everytime I read something about charge-like quantum numbers I only see a couple of examples but never a complete list of these numbers.

What are all the charge-like quantum numbers?


Defining precisely what are all the quantum numbers is a difficult question because it depends highly on the model under consideration, even for the standard model.

In particular any U(1) symmetry leads to a quantum number, and similarly some U(1) subgroup of non-abelian groups that commute with all other interactions can also be associated to quantum numbers (such as the weak isospin). So the more your model has, the more numbers you will have.

Now even if you ask the question for the "real" world described by the Standard Model it is not clear how to count these numbers: this depends highly on the precise domain you are studying. For example if you are looking just as the quarks u, d, and s, then the "strangeness" is a good quantum number, but it is not anymore when you add weak interaction (like other flavour numbers). Similarly the conservation of the individual lepton numbers hold well and is useful for many purposes, but it is not exact due to neutrino oscillations. Finally our current classification of quantum numbers could change if new interactions were discovered: for example some unification theories predict that the proton can decay and this would yield a violation of the baryon number.

So here is the list of the quantum numbers, in the context of the Standard model, according to the current status:

  • true charges: electric charge (related to the weak hypercharge and isospin), baryon and lepton numbers (the latter including subcases;
  • flavour charges (violated by weak interactions): one for each quark (strange, charm, bottomn, top), isospin;
  • leptonic charges (violated by neutrino oscillations): electronic, muonic and tauonic lepton numbers.
  • $\begingroup$ Are there quantum numbers that are never charge-like? I could imagine that this would apply to Spin but I don't know if other numbers also fall into this category. $\endgroup$ – Thomas Elliot Apr 18 '16 at 4:54
  • $\begingroup$ Indeed there are other quantum numbers which correspond to Casimir operators of the spacetime symmetry group. For the Poincaré group you find the mass and the spin. If you consider the super-Poincaré group (Poincaré with supersymmetry) then you have again the mass and some kind of spin (the definition is a bit more tricky). Finally if you consider the conformal group then other numbers appear (such as the scaling dimension). $\endgroup$ – Harold Apr 18 '16 at 7:15
  • $\begingroup$ To summarize I would say that quantum numbers correspond to: Casimir operators of spacetime symmetries, U(1) charges of internal symmetries and U(1) subgroup of other internal groups that commute with everything. $\endgroup$ – Harold Apr 18 '16 at 7:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.