Vector-like pairs and Brill-Noether theory

How likely is it that there are particles in a vector-like pair of representations in low-energy spectrum, when neither symmetry nor anomaly consideration motivates their presence? We address this question in the context of supersymmetric and geometric phase compactification of F-theory and Heterotic dual. Quantisation of the number of generations (or net chiralities in more general term) is also discussed along the way. Self-dual nature of the fourth cohomology of Calabi-Yau fourfolds is essential for the latter issue, while we employ Brill-Noether theory to set upper bounds on the number ℓ of vector-like pairs of chiral multiplets in the SU(5)GUT(5+5¯) representations. For typical topological choices of geometry for F-theory compactification for SU(5) unification, the range of 0≤ℓ≲4 for perturbative unification is not in immediate conflict with what is already understood about F-theory compactification at this moment.