  In this paper we consider the Newton polygons of $L$-functions coming from
additive exponential sums associated to a polynomial over a finite field
$\F_q$. These polygons define a stratification of the space of polynomials of
fixed degree. We determine the open stratum: we give the generic Newton polygon
for polynomials of degree $d\geq 2$ when the characteristic $p$ is greater than
3d, and the Hasse polynomial, i.e. the equation defining the hypersurface
complementary to the open stratum.
