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twisted spin structure

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An ordinary spin structure on a special orthogonal group-principal bundle is a lift of the corresponding cocycle g:X→BSO through the spin group fibration BSpin→BSO. The obstruction for this to exist is a cohomology class w 2∈H 2(X,ℤ 2) – the second Stiefel-Whitney class: it exists precisely if this class is trivial, [w 2(g)]=0.

Conversely, one can ask for an SO-cocycle g with prescribed non-trivial obstruction [w 2(g)]=α∈H 2(X,ℤ 2). These may usefully be understood as α-twisted spin-structures, following the general logic of twisted cohomology.

Definition

Let

ℤ 2→Spin→SO→Bℤ 2\mathbb{Z}_2 \to Spin \to SO \to \mathbf{B} \mathbb{Z}_2

be the fiber sequence in H=ETop∞Grpd or H=Smooth∞Grpd given by the spin group extension of the special orthogonal group (regarded as a topological group or as a Lie group, respectibely). Its delooping defines the second Stiefel-Whitney class

w 2:BSO→B 2ℤ 2w_2 : \mathbf{B }SO \to \mathbf{B}^2 \mathbb{Z}_2

so that for any X∈X we have a characteristic class

w 2:H(X,BSO)→H(X,B 2ℤ 2)w_2 : \mathbf{H}(X,\mathbf{B}SO) \to \mathbf{H}(X, \mathbf{B}^2 \mathbb{Z}_2)
[w 2]:SOBund(X)→H 2(X,ℤ 2).[w_2] : SO Bund(X) \to H^2(X,\mathbb{Z}_2) \,.

For X a manifold define the groupoid of twisted spin-structures SpinStruc tw(X) to be the (∞,1)-pullback

SpinStruc tw(X) → H 2(X,ℤ 2) ↓ ↓ H(X,BSO) →w 2 H(X,B 2ℤ 2),\array{ SpinStruc_{tw}(X) &\to& H^2(X, \mathbb{Z}_2) \\ \downarrow && \downarrow \\ \mathbf{H}(X, \mathbf{B}SO) &\stackrel{w_2}{\to}& \mathbf{H}(X, \mathbf{B}^2 \mathbb{Z}_2) } \,,

where the right vertical morphism picks one cocycle representative in each cohomology class.

The cocycles in SpinStruc tw(X) are twisted Spin-bundles.

The obstruction cocycles in H(X,[B 2ℤ 2) are Bℤ 2-principal 2-bundles. These may be modeled by ℤ 2-bundle gerbes. In this incarnation the obstruction cocycles w 2(g) above have been discussed as spin gerbes in (MurraySinger).

References

A model of twisted spin structures by bundle gerbes is discussed in

  • Michael Murray, Michael Singer, Gerbes, Clifford modules and the index theorem Annals of Global Analysis and Geometry Volume 26, Number 4, 355-367, DOI: 10.1023/B:AGAG.0000047514.71785.96 (arXiv:math/0302096)
  • Atsushi Tomoda, A relation of spin-bundle gerbes and Mayer’s Dirac operator (pdf)

The general abstract discussion given above appears as an example in

Revised on May 28, 2012 09:18:17 by Urs Schreiber (82.113.121.232)
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