Waterfall phase in supersymmetric hybrid inflation

We explore a class of realistic supersymmetric hybrid inflation models with a predicted scalar spectral index $n_s \approx 0.97-0.978$, which is in good agreement with the recent Atacama Cosmology Telescope (ACT) measurement. The waterfall field responsible for the gauge symmetry breaking in this scenario experiences some $e$-foldings during the inflationary epoch. The scalar perturbations associated with the waterfall field during this phase induce a stochastic gravitational wave spectrum that will be tested in the ongoing Pulsar Timing Array (PTA) measurements and in future experiments. In an $SU(5)$ setting an observable number density of the superheavy GUT monopole linked to the waterfall field can be realized.