In 1985, Dixon, Harvey, Vafa and Witten considered string compactification on Calabi-Yau orbifolds (arising as global quotients X/G by a finite group G) for the purpose of symmetry breaking [DHVW]. Later on, orbifold string theory became an important part of string theory. For example, orbifolds provide some of the simplest nontrivial models in string theory. Until very recently, only physical argument for mirror symmetry had been given for orbifold models [GP]. In fact, orbifolds are such a… CONTINUE READING