Geometry of Spin and Spin^c structures in the M-theory partition function

TL;DR

This paper investigates how multiple Spin and Spin^c structures influence the M-theory partition function, revealing anomalies, geometric obstructions, and connections to K-theory, scalar curvature, and the Gromov-Lawson-Rosenberg conjecture.

Contribution

It extends the analysis of Spin and Spin^c structures to M-theory, characterizing anomalies and phase factors using eta-invariants, KO-theory, and index theorems, with applications to string theory and geometry.

Findings

Identification of anomalies related to Spin structures in M-theory

Extension of phase and quantization conditions to Spin^c cases

Connection between KO-theory invariants and geometric obstructions

Abstract

We study the effects of having multiple Spin structures on the partition function of the spacetime fields in M-theory. This leads to a potential anomaly which appears in the eta-invariants upon variation of the Spin structure. The main source of such spaces are manifolds with nontrivial fundamental group, which are also important in realistic models. We extend the discussion to the Spin^c case and find the phase of the partition function, and revisit the quantization condition for the C-field in this case. In type IIA string theory in ten dimensions, the mod 2 index of the Dirac operator is the obstruction to having a well-defined partition function. We geometrically characterize manifolds with and without such an anomaly and extend to the case of nontrivial fundamental group. The lift to KO-theory gives the alpha-invariant, which in general depends on the Spin structure. This reveals many interesting connection to positive scalar curvature manifolds and constructions related to the Gromov-Lawson-Rosenberg conjecture. In the twelve-dimensional theory bounding M-theory, we study similar geometric questions, including choices of metrics and obtaining elements of K-theory in ten dimensions by pushforward in K-theory on the disk fiber. We interpret the latter in terms of the families index theorem for Dirac operators on the M-theory circle and disk. This involves superconnections, eta-forms, and infinite-dimensional bundles, and gives elements in Deligne cohomology in lower dimensions. We illustrate our discussion with many examples throughout.