Axion Isocurvature Perturbations in Low-Scale Models of Hybrid Inflation

The QCD axion solves the strong CP problem and represents an attractive particle candidate for cold dark matter (CDM). However, quantum fluctuations of the axion field during inflation easily result in large CDM isocurvature perturbations that are in conflict with observations of the cosmic microwave background (CMB). In this paper, we demonstrate how this problem can be solved in low-scale models of hybrid inflation that may emerge from supersymmetric grand unified theories. We consider both F-term hybrid inflation (FHI) and D-term hybrid inflation (DHI) in supergravity, explicitly taking into account the effect of hidden-sector supersymmetry breaking. We discuss the production of cosmic strings and show how the soft terms in the scalar potential readily allow to achieve the correct scalar spectral index. In both cases, we are able to identify large regions in parameter space that are consistent with all constraints. In particular, we find that evading the CDM isocurvature constraint always requires a small Yukawa or gauge coupling of O(10^-3) or smaller. This translates into upper bounds on the gravitino mass of O(10^5) GeV in FHI and O(10^9) GeV in DHI. Our results point to interesting scenarios in well-motivated parameter regions that will be tested in future axion and CMB experiments.