### Differential Characteristic Cocycles

#### Posted by Urs Schreiber

Domenico, Jim and myself are in the process of producing a new version of our article on differential characteristic maps. Since this is part of the bigger story of *differential cohomology in a cohesive topos* we had originally restricted attention to a particular construction. But reactions showed that this made readers tend to miss the impact. So now we have added a brief section with more indications of the applications, that are being described in more detail elsewhere.

And we have rewritten the extended abstract. That I want to hereby bounce off the $n$Café readership. For the latest pdf version and a hyperlinked version of the abstract see behind the link

Cech cocycles for differential characteristic classes.

Here is the

**extended Abstract**

What is called *secondary characteristic classes* in Chern-Weil theory is a refinement of ordinary characteristic classes of principal bundles from cohomology to differential cohomology: to bundles and higher gerbes with smooth connection. We consider the problem of refining the construction of secondary characteristic classes from cohomology sets to cocycle spaces; and from Lie groups to higher connected covers of Lie groups by smooth $\infty$-groups: by smooth groupal $A_\infty$-spaces. This allows us to study the homotopy fibers of the differential characteristic maps thus obtained and to show how these describe differential obstruction problems. This applies in particular to higher twisted differential spin structures called *twisted differential string structures* and *twisted differential fivebrane structures* .

To that end we define for every $L_\infty$-algebra $\mathfrak{g}$ a smooth $\infty$-group $G$ integrating it, and define smooth $G$-principal $\infty$-bundles with connection. For every $L_\infty$-algebra cocycle of suitable degree, we give a refined $\infty$-Chern-Weil homomorphism that sends these $\infty$-bundles to classes in differential cohomology that lift the corresponding curvature characteristic classes.

When applied to the canonical 3-cocycle of the Lie algebra of a simple and simply connected Lie group $G$ this construction gives a refinement of the secondary first fractional Pontryagin class of $G$-principal bundles to cocycle space. Its homotopy fiber is the 2-groupoid of smooth $\mathrm{String}(G)$-principal 2-bundles with 2-connection, where $\mathrm{String}(G)$ is a smooth 2-group refinement of the topological string group. Its homotopy fibers over non-trivial classes we identify with the 2-groupoid of
*twisted differential string structures* that appears in the Green-Schwarz anomaly cancellation
mechanism of heterotic string theory.

Finally, when our construction is applied to the canonical 7-cocycle on the Lie 2-algebra of the String-2-group, it produces a secondary characteristic map for $\mathrm{String}$-principal 2-bundles which refines the second fractional Pontryagin class. Its homotopy fiber is the 6-groupoid of principal 6-bundles with 6-connection over the *Fivebrane 6-group* . Its homotopy fibers over nontrivial classes are accordingly *twisted differential fivebrane structures* that have beeen argued to control the anomaly cancellation mechanism in magnetic dual heterotic string theory.

## Re: Differential Characteristic Cocycles

A couple of very trivial things from the paper.

On p. 4 you have $\langle F_A \rangle$ with no mention of $A$.

You have an ‘allow to’ and an ‘allows to’ which sound wrong, e.g.,

[note typo in ‘dergree’] and

‘Allow’ needs an object, so “allows us to…”, but maybe better

and