Breaking a global or local $U(1{)}_{\mathrm{B}-\mathrm{L}}$ symmetry at high scales simultaneously generates Majorana masses for heavy righthanded neutrinos and produces a network of cosmic strings. The evolution and decay of these strings source a stochastic gravitational-wave background that may be probed by current and future gravitational-wave experiments, while the decays of the resulting massive righthanded neutrinos can generate the baryon asymmetry of the Universe via leptogenesis. We derive analytical bounds for successful leptogenesis with a global and local $U(1{)}_{B-L}$ symmetry, separately finding an absolute lower bound on the lightest righthanded neutrino mass $M_1>1.74\times {10}^8\mathrm{GeV}$ for thermal initial conditions and $M_1>\mathcal{O}({10}^6)\mathrm{GeV}$ for nonthermal initial conditions. Allowing for near-resonant leptogenesis relaxes these bounds to TeV scale in both cases, making it a viable target at collider searches complementing the GW signals. Full flavor effects are included, and crucially, we determine the region where successful leptogenesis can be probed through gravitational-wave observations in upcoming experiments such as LISA and Einstein Telescope. Importantly, we find that flavor effects rescue regions of the parameter space that are ruled out due to current CMB or gravitational-wave measurements.

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• Baryogenesis & leptogenesis
• Cosmology
• Dark matter
• Particle dark matter
• Particle production
• Phenomenology