Abstract
Consistent heterotic free fermionic string models are classified in terms of their number of spacetime supersymmetries, N. For each of the six distinct choices of gravitino sector, we determine what number of supersymmetries can survive additional GSO projections. We prove by exhaustive search that only three of the six can yield N = 1, in addition to the N = 4, 2, or 0 that five of the six can yield. One choice of gravitino sector can only produce N = 4 or 0. Relatedly, we find that only Z 2, Z 4, and Z 8 twists of the internal fermions with worldsheet supersymmetry are consistent with N = 1 in free fermionic models. Any other twists obviate N = 1.
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