Metastable strings and dumbbells in supersymmetric hybrid inflation

We study symmetry breaking and topological defects in a supersymmetric model with gauge group U(2), which can be identified with the right-handed part SU(2)$_{R}$× U(1)$_{B−L}$ of an extended electroweak symmetry of the Standard Model. The model has two phases of hybrid inflation terminated by tachyonic preheating where either monopoles and strings or, alternatively, dumbbells are formed. In the first case a stochastic gravitational wave background is predicted in the LIGO-Virgo band, possibly extending to the LISA frequency band and to nanohertz frequencies, which is generated by a metastable cosmic string network. In the second case no topological defects survive inflation and no stochastic gravitational wave background is produced