ar X iv : h ep - p h / 96 07 23 1 v 1 3 J ul 1 99 6 1 String Unification and Leptophobic Z ′ in Flipped SU ( 5 ) Jorge

Flipped SU(5) enthusiasts keep discovering hidden treasures, even after 10 years from its birth [1]. As is well known, the model attains its highest relevance in strings: efforts by several groups using different approaches have not (yet?) yielded appealing “string GUTs” [SO(10)]. Among levelone Kac-Moody models, only flipped SU(5) unifies SU(3) and SU(2), providing an explanation for the “LEP scale” [10GeV]. The discrepancy between “observed” and predicted unification scales – MLEP ∼ 10 GeV versus Mstring ∼ 10GeV – seems to have only way out: extra intermediate-scale states [2]. This solution was realized early on in stringy flipped SU(5) [3]. Here we summarize how this scenario may be achieved in practice [4], including the prediction for αs(MZ) [5], and also discuss the latest “flipped” goodie: a leptophobic Z ′ [6].