The Mass, Normalization and Late Time behavior of the Tachyon Field
  We study the dynamics of the tachyon field $T$. We derive the mass of the tachyon as the pole of the propagator which does not coincide with the standard mass given in the literature in terms of the second derivative of $V(T)$ or $Log[V(T)]$. We determine the transformation of the tachyon in order to have a canonical scalar field $\phi$. This transformation reduces to the one obtained for small $\dot T$ but it is also valid for large values of $\dot T$. This is specially interesting for the study of dark energy where $\dot T\simeq 1$. We also show that the normalized tachyon field $\phi$ is constrained to the interval $T_2\leq T \leq T_1$ where $T_1,T_2$ are zeros of the original potential $V(T)$. This results shows that the field $\phi$ does not know of the unboundedness of $V(T)$, as suggested for bosonic open string tachyons. Finally we study the late time behavior of tachyon field using the L'H\^{o}pital rule. 
A. de la Macorra, U. Filobello, G. German