A complete description of the cohomological invariants of even genus hyperelliptic curves

Abstract

When the genus $g$ is even, we extend the computation of the cohomological invariants of $\mathcal{H}_g$ to non algebraically closed fields, we give an explicit functorial description of the invariants and we completely describe their multiplicative structure. In the appendix, we use our methods to give a very short proof of a result by Cornalba on the Picard group of the compactification $\overline{\mathcal{H}}_g$ and extend it to positive characteristic.