There are
natural line bundles
over
(or
); the fibre at
of
is the
cotangent space
, where
is the
marked point
in
. The Witten class
is then the first Chern class of
. These live naturally in the cohomology of either the
compactified or non-compactified moduli space.
Jeffrey Herschel Giansiracusa
2005-06-27