Possible Origins of R-parity
A very attractive way to motivate R-parity is with a B-L continuous gauge symmetry which is spontaneously broken at a scale inaccessible to current experiments. A continuous forbids renormalizable terms which violate B and L. If is only broken by scalar vacuum expectation values (or other order parameters) that carry even integer values of 3(B-L), then there exist an exactly conserved discrete remnant subgroup at our scale which has the desired properties.
This phenomenon can arise as an automatic symmetry in SO(10) grand unified theories. This natural occurrence of R-parity is possible because in SO(10) the Standard Model fermions arise from the 16-dimensional spinor representation, while the Higgs arises from a 10 dimensional vector representation. In order to make an SO(10) invariant coupling, one must have an even number of spinor fields (i.e. there is a spinor parity). After GUT symmetry breaking, this spinor parity descends into R-parity so long as no spinor fields were used to break the GUT symmetry.
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