arXiv2026
We study a minimal type-I seesaw framework in which a first-order phase transition (FOPT), driven by a singlet scalar, produces right-handed neutrinos (RHNs) through bubble collisions, realizing a cosmic-scale collider that probes ultra-high energy scales. The resulting RHN distribution sources novel low-frequency gravitational-waves (GWs) in addition to the standard bubble-collision contribution. A stable lightest RHN can account for the observed dark matter (DM) relic abundance for masses as low as $M_{1} \equiv m_{\rm DM} \gtrsim 10^{6}\,\mathrm{GeV}$, with the associated novel GW signal accessible in LISA, ET and upcoming LVK detectors. If the RHNs are unstable, their CP-violating decays generate the observed baryon asymmetry via leptogenesis for $M_{1} \gtrsim 10^{11}\,\mathrm{GeV}$ and phase transition temperatures $T_* \gtrsim 10^{6}\,\mathrm{GeV}$, for which the novel GW spectrum is detectable in ET, BBO and upcoming LVK. If RHN decays also populate a dark-sector fermion with mass $m_χ \in [10^{-4},10^{4}],\mathrm{GeV}$, successful co-genesis of baryons and asymmetric dark matter occurs for $T_* \gtrsim 10^{7}\,\mathrm{GeV}$ and $M_{1} \gtrsim 10^{9}\,\mathrm{GeV}$, naturally explaining $Ω_{\rm DM} \simeq 5Ω_{\rm B}$. The corresponding GW signals are testable with LISA, ET, and BBO. Finally, we analyze a UV-complete multi-Majoron model, based on a global $U(1)_N \times U(1)_{\rm B-L}$ extension, motivated from the hierarchy of lepton masses, which we dub as Mojaron collider. The corresponding FOPT in this model leaves a distinctive GW signature arising from RHN production during $U(1)_N$ symmetry breaking detectable by BBO, ET and upcoming LVK. Successful leptogenesis is realized for heaviest RHN mass $M_3 \sim 10^{10}\,\mathrm{GeV}$ and a $U(1)_N$ breaking vev $v_2 \sim \mathcal{O}(\mathrm{TeV})$, which sets the seesaw scale.