Cosmic Collider Gravitational Waves sourced by Right-handed Neutrino production from Bubbles: Testing Seesaw, Leptogenesis and Dark Matter

TL;DR

The work proposes a Cosmic Collider scenario where a first-order phase transition driven by a singlet scalar non-thermally produces RHNs during bubble collisions, generating a new low-frequency GW component in addition to the conventional bubble-collision background. RHN production and subsequent decays (or stability) can realize DM, leptogenesis, or asymmetric DM, with the GW spectra from particle production offering a distinctive signature that complements the standard PT signal and ties high-scale neutrino physics to observables in LISA, ET, BBO, and LVK. A UV-complete multi-Majoron model with U(1)_N × U(1)_{B-L} symmetry is developed, yielding a two-step FOPT and a Mojaron collider signature that can probe the seesaw scale M_N ~ 10^9–10^15 GeV and the associated leptogenesis dynamics. Overall, the paper demonstrates that high-scale beyond-Standard-Model physics can be tested through correlated gravitational-wave observables across multiple detectors, providing a promising route to test seesaw, leptogenesis, and dark matter in the early universe.

Abstract

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.

Figures (16)

• Figure 1: Schematic diagram for Cosmic Collider Shakya:2025qpi during runaway FOPTs showing the correlation between RHN production, Dark matter and Leptogenesis along with the resulting cosmological signals - the well established bubble collision GWs as well as the novel RHN production GWs as discussed in this analysis.
• Figure 2: GW spectrum plotted against power-law integrated sensitivity curves for various present and upcoming GW detectors for $\alpha = 10$, $\beta/H = 150$. The solid lines corresponds to GW spectrum from particle production and the dashed lines are from bubble collision. The three sets of curves correspond to phase transitions at temperatures - BP1 ($T_* = 10^{-5}$ TeV), BP2 ($T_* = 1$ TeV) and BP3 ($T_* =10^6$ TeV) (orange, red, indigo), respectively. In calculating particle production contribution we have taken $\kappa \sim 1$ as discussed.
• Figure 3: The parameter reach of the present and future GW experiment network. In each shaded region, corresponding experiment (see labels) detects the GW signal coming from a FOPT with SNR $\geq 10$. On top of this plane, the three benchmark points BP1, BP2 and BP3, taken in Fig \ref{['fig:1']} is shown. We have used $\alpha = 10$ and $\kappa = 1$. The subscript 'bc' stands for 'bubble collision' and 'pp' for 'particle production'. The BBN bound (horizontal gray band) rules out the region $\beta/H < 1.1$. The hatched gray and orange regions have been ruled out from LVK $\mathcal{O}(3)$ data.
• Figure 4: For PT parameter BP-2 (a) DM Relic vs RHN mass : plotted for three different benchmark points of the coupling strength (b) DM relic vs RHN coupling : again plotted for three different benchmark points of the RHN mass. In both cases $\lambda = \mathcal{O}(10^{-1})$ is taken.
• Figure 5: DM relic density in the $(m_{\rm DM}, y_{\rm DM})$ plane for two phase transition benchmarks with $\mathcal{O}(1)$ values of $\lambda$. Left: BP2, where bubble-collision GWs are detectable in LISA and BBO with $\mathrm{SNR}>10$ for the entire plane. Right:$\alpha=10$, $\beta/H=150$, and $T_*=500~\mathrm{TeV}$, with bubble-collision GWs detectable in BBO for the entire plane. Upper triangular regions are excluded due to over-closure of the universe. Vertical dotted lines indicate $\mathrm{SNR}=10$ for particle-production GWs for different detectors (see label); regions to the right of these lines yield $\mathrm{SNR}>10$. Values of $\kappa$ for the RHN production GWs is given by Eq. \ref{['fiftyone']}.