Half-BPS M2-brane orbifolds

TL;DR

This paper extends the classification of half-BPS M2-brane backgrounds in eleven-dimensional supergravity to include orbifolds of the 7-sphere, revealing many new orbifold solutions while confirming smoothness for backgrounds with more than half supersymmetry.

Contribution

It generalizes previous classifications by allowing orbifold singularities, providing a comprehensive list of new half-BPS orbifold solutions in M-theory.

Findings

Many new half-BPS orbifolds identified

Background smoothness linked to supersymmetry level

Orbifolds described as iterated cyclic quotients

Abstract

Smooth Freund-Rubin backgrounds of eleven-dimensional supergravity of the form AdS_4 x X^7 and preserving at least half of the supersymmetry have been recently classified. Requiring that amount of supersymmetry forces X to be a spherical space form, whence isometric to the quotient of the round 7-sphere by a freely-acting finite subgroup of SO(8). The classification is given in terms of ADE subgroups of the quaternions embedded in SO(8) as the graph of an automorphism. In this paper we extend this classification by dropping the requirement that the background be smooth, so that X is now allowed to be an orbifold of the round 7-sphere. We find that if the background preserves more than half of the supersymmetry, then it is automatically smooth in accordance with the homogeneity conjecture, but that there are many half-BPS orbifolds, most of them new. The classification is now given in terms of pairs of ADE subgroups of quaternions fibred over the same finite group. We classify such subgroups and then describe the resulting orbifolds in terms of iterated quotients. In most cases the resulting orbifold can be described as a sequence of cyclic quotients.