String Phenomenology From a Worldsheet Perspective

<!--HTML-->I argue that the ten dimensional non--supersymmetric tachyonic superstrings may serve as good starting points for the construction of viable phenomenological vacua. Thus, enlarging the space of possible solutions that may address some of the outstanding problems in string phenomenology. A tachyon free six generation Standard--like Model is presented, which can be regarded as an orbifold of the $SO(16)\times E_8$ heterotic--string in ten dimensions. I conjecture that any $(2,0)$ heterotic--string in four dimensions can be connected to a $(2,2)$ one via an orbifold or by interpolations and provide some evidence for this conjecture. It implies that any Effective Field Theory (EFT) model that cannot be connected to a $(2,2)$ theory is necessarily in the swampland, and will simplifies the analysis of the moduli spaces of $(2,0)$. I argue that at present level of understanding of string theory construction of vacua in which all the moduli are stable, including the cosmological constant, is futile. An example of a quasi-realistic model in which all moduli are fixed, aside from the dilaton is presented.