An obstruction to generalised discrete torsion in string theory

Abstract

We reconsider the possibility of turning on generalised discrete torsion in orbifold compactifications, whereby different discrete torsion phases are attached to different fixed points of a given twisted sector. By an explicit inspection of the genus one and genus two vacuum amplitudes for the $T^6/\mathbb{Z}_2^2$ and $T^7/\mathbb{Z}_2^3$ cases, we show that this is not possible in String Theory. In fact, this generalised discrete torsion, although modular invariant at genus one, leads to an inconsistent interpretation of the spectrum which, in turn, implies a loss of modular invariance on genus-two Riemann surfaces. Our finding is in agreement with a recent classification of allowed topological deformations in equivariant cohomology, which does not include this possibility.