  The aim of this note is to give an alternative proof of a theorem of Koras
and Russell, that is, a characterization of smooth contractible affine
varieties endowed with a hyperbolic action of the group
$\mathbb{G}_{m}\simeq\mathbb{C}^{\text{*}}$, using the language of polyhedral
divisors developed by Altmann and Hausen as generalization of
$\mathbb{Q}$-divisors.
