As far as I know, the spectrum of any (?) String Theory is of the form $$ M^2\propto N $$ where $N$ is the number operator. The lightest known particle being the electron, I am led to think that we should observe particles with masses $m_e,2m_e,3m_e,\dots$
In more general terms, the spectrum of ST seems to be harmonic, as the operators are always essentially oscillators. Cf. the Veneziano amplitude, with poles at $s=4(n-1)/\alpha'$. This brings me to my question: how does the phenomenology of ST deal with the (obviously?) unobserved tower of particles, with masses in harmonic progression? does the geometry of the extra dimensions have anything to do with the apparently erratic behaviour of low-energy physics?