Gauge and gravitational anomalies in D=4 N=1 orientifolds

TL;DR

This work demonstrates that mixed $U(1)$-gauge and $U(1)$-gravitational anomalies in 4D $N=1$ orientifolds are canceled by a Green-Schwarz mechanism mediated solely by twisted Ramond-Ramond axions. By analyzing the RR odd-spin one-loop partition function and factorizing it, the authors derive explicit Wess-Zumino couplings of D9/D5 branes and orbifold fixed-points, showing a universal structure in $N=1$ sectors and sector-specific behavior in $N=2$ sectors, with a notable $3/2$ gravitational rescaling from fixed points. While $N=1$ sectors admit a clean unified GS description, certain $N=2$ sectors introduce ambiguities requiring additional inputs, and $N=4$ sectors exhibit vanishing inflow though nontrivial RR couplings persist. The results inform the low-energy action, including Fayet-Iliopoulos terms and possible tree-level gauge-coupling corrections, with implications for constructing phenomenologically viable string vacua.

Abstract

The cancellation of U(1)-gauge and U(1)-gravitational anomalies in certain D=4 N=1 Type IIB orientifolds is analyzed in detail, from a string theory point of view. We verify the proposal that these anomalies are cancelled by a Green-Schwarz mechanism involving only twisted Ramond-Ramond fields. By factorizing one-loop partition functions, we also get the anomalous couplings of D-branes, O-planes and orbifold fixed-points to these twisted fields. Twisted sectors with fixed-planes participate to the inflow mechanism in a peculiar way.