Contents

cohomology

# Contents

## Definition

### Topological

For $n \in \mathbb{N}$ the Lie group $Spin^c(n)$ is a central extension

$U(1) \to Spin^c(n) \to SO(n)$

of the special orthogonal group by the circle group. This comes with a long fiber sequence

$\cdots \to B U(1) \to B Spin^c(n) \to B SO(n) \stackrel{W_3}{\to} B^2 U(1) \,,$

where $W_3$ is the third integral Stiefel-Whitney class .

By the definition at twisted cohomology, for a given class $[c] \in H^3(X, \mathbb{Z})$, a $c$-twisted $spin^c$-structure is a choice of homotopy

$\eta : c \stackrel{\simeq}{\to} W_3(T X) \,.$

The space/∞-groupoid of all twisted $Spin^c$-structures on $X$ is the homotopy fiber $W_3 Struc_{tw}(T X)$ in the pasting diagram of homotopy pullbacks

$\array{ W_3 Struc_{tw}(T X) &\to& W_3 Struc_{tw}(X) &\stackrel{tw}{\to}& H^3(X, \mathbb{Z}) \\ \downarrow && \downarrow && \downarrow \\ * &\stackrel{T X}{\to}& Top(X, B SO(n)) &\stackrel{W_3}{\to}& Top(X, B^2 U(1)) } \,,$

where the right vertical morphism is the canonical effective epimorphism that picks one point in each connected component.

### Smooth

Since $U(1) \to Spin^c \to SO$ is a sequence of Lie groups, the above may be lifted from the (∞,1)-topos Top $\simeq$ ∞Grpd to Smooth∞Grpd.

More precisely, by the discussion at Lie group cohomology (and smooth ∞-groupoid – structures) the characteristic map $W_3 : B SO \to B^2 U(1)$ in $\infty Grpd$ has, up to equivalence, a unique lift

$\mathbf{W}_3 : \mathbf{B} SO \to \mathbf{B}^2 U(1)$

to Smooth∞Grpd, where on the right we have the delooping of the smooth circle 2-group.

By the general definition at twisted differential c-structure , the 2-groupoid of smooth twisted $spin^c$-structures $\mathbf{W}_3 Struc_{tw}(X)$ is the joint (∞,1)-pullback

$\array{ \mathbf{W}_3 Struc_{tw}(T X) &\to& \mathbf{W}_3 Struc_{tw}(X) &\stackrel{tw}{\to}& H_{smooth}^2(X, U(1)) \\ \downarrow && \downarrow && \downarrow \\ * &\stackrel{T X}{\to}& Smooth \infty Grpd(X, \mathbf{B} SO(n)) &\stackrel{\mathbf{W}_3}{\to}& Smooth \infty Grpd(X, \mathbf{B}^2 U(1) } \,.$

## Applications

### Anomaly cancellation in physics

The existence of an ordinary spin structure on a space $X$ is, as discussed there, the condition for $X$ to serve as the target space for the spinning particle sigma-model, in that the existence of this structure is precisely the condition that the corresponding fermionic quantum anomaly on the worldline vanishes.

Twisted $spin^c$-structures appear similarly as the conditions for the analogous quantum anomaly cancellation, but now of the open type II superstring ending on a D-brane. This is also called the Freed-Witten anomaly cancellation.

More precisely, in these applications the class of $W_3(TX) - H$ need not vanish, it only needs to be $n$-torsion if there is moreover a twisted bundle of rank $n$ on the $D$-brane.

See the references below for details.

## References

### General

The notion of twisted $Spin^c$-structures as such were apparently first discussed in section 5 of

More discussion appears in section 3 of

The refinement to smooth twisted structures is discussed in section 4.1 of

### In physics

The need for twisted $Spin^c$-structures as Freed-Witten anomaly cancellation condition on the worldvolume of D-branes in string theory was first discussed in

More details are in

A clean formulation and review is provided in

• Loriano Bonora, Fabio Ferrari Ruffino, Raffaele Savelli, Classifying A-field and B-field configurations in the presence of D-branes (arXiv:0810.4291)

• Fabio Ferrari Ruffino, Classifying A-field and B-field configurations in the presence of D-branes - Part II: Stacks of D-branes (arXiv:1104.2798)

• Fabio Ferrari Ruffino, Topics on topology and superstring theory (arXiv:0910.4524)

and

• Kim Laine, Geometric and topological aspects of Type IIB D-branes (arXiv:0912.0460)

In (Laine) the discussion of FW-anomaly cancellation with finite-rank gauge bundles is towards the very end, culminating in equation (3.41).

Last revised on August 12, 2012 at 22:11:26. See the history of this page for a list of all contributions to it.