Dimensions of Fano schemes of lines on hypersurfaces

Abstract

Let $X\subset\mathbb{P}^n_\mathbb{C}$ be a smooth hypersurface of degree $d\ge2$, with $n\ge3$. We prove that an $m$-dimensional integral projective family of lines sweeping a variety of dimension $y$ satisfies $m\le\max\{2y-d-1,n-3\}$. This proves the de Jong-Debarre conjecture for $d\le n$ and the Beheshti-Starr bound for lines on smooth hypersurfaces for $d\ge n+1$.