A unique Z_4^R symmetry for the MSSM

Understand

We consider the possible anomaly free Abelian discrete symmetries of the MSSM that forbid the mu-term at perturbative order.

• Allowing for anomaly cancellation via the Green-Schwarz mechanism we identify discrete R-symmetries as the only possibility and prove that there is a unique Z_4^R symmetry that commutes with SO(10).
• We argue that non-perturbative effects will generate a mu-term of electroweak order thus solving the mu-problem.
• The non-perturbative effects break the Z_4^R symmetry leaving an exact Z_2 matter parity.