A lifting gerbe is a cocycle/gerbe – or equivalently the principal infinity-bundle that it classifies – arising as the obstruction to the lift of a -principal bundle or more generally of any -principal infinity-bundle, through a shifted central extension
of , for some abelian group .
Such an extension is classified (or rather its delooping is) by a cocycle
in group cohomology.
In the literature the following simplest case of the general situation is usually considered exclusively:
for an ordinary group and
In the literature this is often discussed in terms of the model for gerbes given by -principal bundle gerbes: let be the total space of the bundle classified by . Regard this as the surjective submersion that is part of the data of the bundle gerbe to be constructed. By principality of , there is a canonical morphism
that sends two elements in the fiber of the bundle to the unique group element that takes the first to the second. Along this morphism, form the pullback of the extension to get in
This is the line bundle ingredient in the lifting bundle gerbe. There is canocically a multiplication map that completes the definition.
See projective unitary group for details.
The archetypical example of a lifting 2-gerbe is a Chern-Simons gerbe.
A review is for instance in section 2.2 of