Probing superheavy dark matter with gravitational waves

We study the superheavy dark matter (DM) scenario in an extended B−L model, where one generation of right-handed neutrino ν_R is the DM candidate. If there is a new lighter sterile neutrino that co-annihilate with the DM candidate, then the annihilation rate is exponentially enhanced, allowing a DM mass much heavier than the Griest-Kamionkowski bound (∼10⁵ GeV). We demonstrate that a DM mass M_νR ≳ 10¹³ GeV can be achieved. Although beyond the scale of any traditional DM searching strategy, this scenario is testable via gravitational waves (GWs) emitted by the cosmic strings from the U(1)_B−L breaking. Quantitative calculations show that the DM mass O(10⁹−10¹³ GeV) can be probed by future GW detectors.