Abstract
A recent $D$-brane model designed to accommodate a phenomenologically acceptable fourth generation of chiral fermions was noted to produce an unexpected additional unbroken nonanomalous U(1) gauge group at the string scale. We show that the corresponding charges acting on minimal supersymmetric standard model fields count baryon and lepton numbers. If broken spontaneously at lower scales, these $\mathrm{U}(1{)}_{B}$ and $\mathrm{U}(1{)}_{L}$ symmetries provide potential avenues for preserving baryogenesis while nonetheless explaining the suppression of proton decay (without the need for $R$ parity), as well as the smallness of right-handed neutrino Majorana masses compared to the string scale.
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