Postsphaleron darkogenesis

TL;DR

The paper presents a post-sphaleron darkogenesis mechanism in a nearly conformal dark sector with a GeV-scale, supercooled phase transition that dilutes preexisting BAU and DM and enables late-time generation of both. A GeV-scale Majorana mother fermion psi is non-thermally produced via dilaton-oscillation–driven parametric resonance after the PT; its CP-violating decays generate a DS asymmetry that is transferred to the visible sector through a neutron portal, yielding an asymmetric DM chi_1 and the observed BAU. The scenario also accounts for nano-Hz gravitational waves seen in pulsar timing arrays and makes testable predictions for neutron-antineutron oscillations, with a phenomenology constrained by washout, inverse-decay, and BBN considerations and accessible to future SHiP, PandaX-xT, and ESS NNBAR experiments.

Abstract

A supercooled phase transition in a nearly conformal dark sector can provide a natural setting for darkogenesis via its out-of-equilibrium dynamics, where a particle-antiparticle number asymmetry in the dark sector can be reprocessed into the visible sector, yielding the observed baryon asymmetry and an asymmetric dark matter. We consider a scenario where the number asymmetry is generated from the decay of a mother particle produced via parametric resonance during the phase transition induced due to its coupling to the dilaton associated with spontaneous breaking of scale invariance. It is shown that the correct baryon asymmetry and dark matter abundance can be realized for a dark phase transition at $\mathcal{O}(1) \, \rm GeV$, which can also explain the nano-Hz gravitational wave signal reported by pulsar timing array experiments. The scenario will be tested further in neutron-antineutron oscillation experiments.

Figures (6)

• Figure 1: Schematic and timeline of the scenario: $t_c$ corresponds to the critical time when the true minimum emerges. Strong supercooling and substantial dilution of preexisting baryon asymmetry and dark matter number density persist until nucleation at $t_n$. At $t_p$, bubbles of the true vacuum collide, reflect, and arrange the dilaton VEV near the basin of attraction of the true minimum. As the system approaches the minimum at $t_o$ while oscillating around it, $\psi$ fields are produced via parametric resonance. At $t_{\rm F}$, the large occupation of $\psi$ back-reacts, causing the production to freeze. At $t_\psi$, the produced $\psi$ decays into $\chi_{1,2}$ and $\eta$. $\chi_2$ transfers its asymmetry to the visible sector via a neutron portal coupling, while $\chi_1$ acts as the asymmetric DM. Thereafter, at $t_\varphi$, the remnant coherent dilaton field decays perturbatively due to the Higgs-portal coupling of $\eta$, completing the reheating process. All the time scales relevant are smaller than the Hubble time $t_{\rm H}$ for the GeV scale PT.
• Figure 2: Two different snapshots of the occupation number distribution for the Majorana fermion $\psi_1$, namely when the number of dilaton oscillation $N_{\rm osc}=3$ (left) and at the time of freeze-out, when $N_{\rm osc}=N_{\rm FO}=10$ (right). We have taken $m_{\psi_1}=5 \, \rm GeV$, $m_\varphi=0.5 \, \rm GeV$, $b=-2$, $\xi_0=0.8$, and $r=300$.
• Figure 3: (a) Contour in the parameter space of the bulk mass parameter for $\psi_1$, denoted as $b$ and coupling $|y|$, where the observed baryon asymmetry is reproduced (green), while in the dark shaded regions, washout processes destroy the asymmetry generated as embodied in Eqs. \ref{['Eq:ymax2']}, \ref{['Eq:epd']}, or the particle production condition in Eq. \ref{['Eq:neccCondn']} is not satisfied. In (b), (c), similar contours are depicted in the $r$-vs-$b$ and $r$-vs-$y$ parameter space, where $r$ is the ratio of the effective UV and IR localized masses for $\psi_1$ as defined in Eq. \ref{['Eq:rdef']}. In all the subfigures, the chosen parameters are $\langle \phi \rangle=1$ GeV, $m_{\psi_1}=5$ GeV, $m_\varphi = 0.5$ GeV, $\xi_0=0.8$, $g_*=100$, and $c_V = 0.5$, while the other fixed parameter for each subfigure is mentioned in the plot label.
• Figure 4: (a) Feynman diagram for DM-nucleon interaction. (b) Constraints in the Higgs-portal coupling vs. $\mu_{11}$ parameter space from DM direct detection experiment (red solid), beam dump experiments, and BBN (gray) for $m_{\chi_1}=2$ GeV, and $m_\eta=0.24$ GeV. Future projected bounds from SHiP and PandaX-xT are portrayed as black dot-dashed and red dashed lines.
• Figure 5: (a, b) Constraint from $n$-$\bar{n}$ oscillations (red), and future projections by a factor of $2, 5,$ and $10$ above the current constraint (red dashed). Gray regions are constraints from successful baryogenesis and BBN, where we have chosen $\langle \phi \rangle=1$ GeV, $m_{\psi_1}/m_{\psi_2}=0.5$, $m_\varphi=0.5$ GeV, $b=-2$, $r=200$, $\xi_0=0.8$, and $c_V=0.5$ to determine the washout constraints for the left subfigure. All other relevant parameters are mentioned on the plot labels. (c) Feynman diagram for $n$-$\bar{n}$ oscillations generated by the Majorana mass for $\psi$.