Primordial black holes and second-order gravitational waves in axionlike hybrid inflation

We investigate the possibility that primordial black holes (PBHs) can be formed from large curvature perturbations generated during the waterfall phase transition in a hybrid inflation model driven by an axionlike particle (ALP) $\ensuremath{\phi}$. The model predicts a spectral index ${n}_{s}\ensuremath{\simeq}0.964$ and a tensor-to-scalar ratio $r\ensuremath{\simeq}0.003$, in agreement with Planck data and potentially testable by next-generation cosmic microwave background (CMB) experiments such as CMB-S4 and LiteBIRD. We find that the PBH mass and the peak of the associated scalar-induced gravitational wave spectrum are correlated with the ALP mass. In particular, PBHs in the mass range ${10}^{\ensuremath{-}13}\ensuremath{-}{10}^{\ensuremath{-}12}{M}_{\ensuremath{\bigodot}}$ can constitute either the entire dark matter content of the Universe or a significant fraction of it. The predicted second-order gravitational waves from this mechanism are within the sensitivity reach of future observatories like Laser Interferometer Space Antenna and Einstein Telescope. The typical reheating temperature in the model is around ${10}^{6}--{10}^{7}\text{ }\text{ }\mathrm{GeV}$ is consistent with big bang nucleosynthesis constraints.