We examine the problem of constructing three generation free fermionic stringmodels with grand unified gauge groups. We attempt the construction of $G\timesG$ models, where $G$ is a grand unified group realized at level 1. Thisstructure allows those Higgs representations to appear which are necessary tobreak the symmetry down to the standard model gauge group. For $G=SO(10)$, wefind only models with an even number of generations. However, for $G=SU(5)$ wefind a number of 3 generation models.
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机译：我们研究了构建具有统一大尺度组的三代自由费米子弦模型的问题。我们尝试构建$ G \ timesG $模型，其中$ G $是在1级实现的一个统一大组。此结构允许出现将对称性分解为标准模型规格组所必需的希格斯表示。对于$ G = SO（10）$，我们仅查找具有偶数代的模型。但是，对于$ G = SU（5）$，我们定义了许多3代模型。
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