A note on the Brill-Noether loci of small codimension in moduli space of stable bundles

Abstract

Let $X$ be a smooth projective curve of genus $g$ over the field $\mathbb{C}$. Let $M_X(2,L)$ denote the moduli space of stable rank $2$ vector bundles on $X$ with fixed determinant $L$ of degree $2g-1$. Consider the Brill-Noether subvariety $W_X^1(2,L)$ of $M_X(2,L)$ which parametrises stable vector bundles having at least two linearly independent global sections. In this article, for generic $X$ and $L$, we show that $W_X^1(2,L)$ is stably-rational when $g=3$, unirational when $g=4$, and rationally chain connected by Hecke curves, when $g\ge 5$. We also show triviality of low dimensional rational Chow groups of an associated Brill-Noether hypersurface.

11 pages, To appear in Mathematische Nachrichten