Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 55 / 1 / b

R-parity is the multiplicative ${\mathbb{Z}}_2$ quantum numberwhere $B,L,s$ are baryon number, lepton number and spin. Ordinary Standard Model particles have $R_p=+1$ and their superpartners have $R_p=-1$. Gauge invariance alone allows MSSM superpotential terms $LL{E}^c$, $LQ{D}^c$, $U^c{D}^c{D}^c$ and $L{H}_u$. Simultaneous baryon- and lepton-number violation can induce rapid proton decay. Exact R-parity forbids these renormalizable terms, since the associated matter parity is odd; it does not forbid every higher-dimensional proton-decay operator, for example $QQQL$.

Two consequences are superpartner production in pairs from ordinary-particle initial states and stability of the lightest supersymmetric particle: a single parity-odd particle cannot decay only into parity-even particles. Cascades therefore end in the lightest supersymmetric particle; if it is neutral and weakly interacting, it produces missing-energy signatures.

A continuous R-symmetry rotates $\theta$ and the supercharges. With $R(\theta )=1$, the superpotential has R-charge two and the fermion in a chiral superfield has one less charge than its scalar. R-parity is only a discrete restriction, represented by combining matter parity with $\theta \mapsto -\theta$, and need not preserve a full continuous R-symmetry. For example, a gaugino Majorana mass term violates an unbroken continuous R-symmetry but preserves R-parity. Thus proton protection by parity and continuous R-charge selection rules are distinct requirements.