String phenomenology is a branch of theoretical physics that attempts to construct realistic or semi-realistic models of particle physics based on string theory. The term 'realistic' is usually taken to mean that the low energy limit of string theory yields a model which bears a resemblance to the Minimal Supersymmetric Standard Model (MSSM). String phenomenology is a branch of theoretical physics that attempts to construct realistic or semi-realistic models of particle physics based on string theory. The term 'realistic' is usually taken to mean that the low energy limit of string theory yields a model which bears a resemblance to the Minimal Supersymmetric Standard Model (MSSM).