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Large neutrino asymmetry from forbidden decay of dark matter

Debasish Borah, Nayan Das, Indrajit Saha

Abstract

Dark matter (DM), in spite of being stable or long-lived on cosmological scales, can decay in the early Universe due to finite-temperature effects. In particular, a first order phase transition (FOPT) in the early Universe can provide a finite window for such decay, guaranteeing DM stability at lower temperatures, consistent with observations. The FOPT can lead to the generation of stochastic gravitational waves (GW) with peak frequencies correlated with DM mass. On the other hand, early DM decay into neutrinos can create a large neutrino asymmetry which can have interesting cosmological consequences in terms of enhanced effective relativistic degrees of freedom $N_{\rm eff}$, providing a solution to the recently observed Helium anomaly among others. Allowing DM decay to occur below sphaleron decoupling temperature, thereby avoiding overproduction of baryon asymmetry, forces the FOPT to occur at sub-electroweak scale. This leaves the stochastic GW within range of experiments like LISA, $μ$ARES, NANOGrav etc.

Large neutrino asymmetry from forbidden decay of dark matter

Abstract

Dark matter (DM), in spite of being stable or long-lived on cosmological scales, can decay in the early Universe due to finite-temperature effects. In particular, a first order phase transition (FOPT) in the early Universe can provide a finite window for such decay, guaranteeing DM stability at lower temperatures, consistent with observations. The FOPT can lead to the generation of stochastic gravitational waves (GW) with peak frequencies correlated with DM mass. On the other hand, early DM decay into neutrinos can create a large neutrino asymmetry which can have interesting cosmological consequences in terms of enhanced effective relativistic degrees of freedom , providing a solution to the recently observed Helium anomaly among others. Allowing DM decay to occur below sphaleron decoupling temperature, thereby avoiding overproduction of baryon asymmetry, forces the FOPT to occur at sub-electroweak scale. This leaves the stochastic GW within range of experiments like LISA, ARES, NANOGrav etc.
Paper Structure (10 sections, 57 equations, 10 figures, 4 tables)

This paper contains 10 sections, 57 equations, 10 figures, 4 tables.

Table of Contents

  1. Introduction
  2. The framework
  3. Co-genesis of Large neutrino asymmetry and Dark Matter
  4. Large neutrino asymmetry and $N_{\rm eff}$
  5. Results and Discussion
  6. Possible UV completion
  7. Conclusion
  8. Details of FOPT and Gravitational Waves
  9. Direct detection of light DM
  10. Efficient conversion of $\Phi_2 \rightarrow \Phi^\dagger_2$

Figures (10)

  • Figure 1: The schematic diagram of co-genesis
  • Figure 2: Top panel: Finite temperature masses of $\chi$ and $\Phi_2$ for BP1 (left) and BP6 (right) mentioned in table \ref{['tab1']}, \ref{['tab2']} with $m_{\chi}$=15.2$\times10^{-3}$ GeV (left) and $m_{\chi}$=81 GeV (right). Bottom panel: Evolution of comoving number densities for BP1 (left) and BP6 (right) with $y_1=7.34 \times10^{-10}$ (left) and $y_1=1.05\times10^{-7}$ (right).
  • Figure 3: GW spectra for BP1 to BP6 of table \ref{['tab1a']}, \ref{['tab2a']}.
  • Figure 4: Left panel: Dark matter mass versus effective Yukawa coupling $y_1$ for different residual neutrino asymmetry in case of BP1 given in table \ref{['tab1']}. Right panel: Parameter space in $m_\chi$-$\Delta N_{\rm eff}$ plane with color code indicating SNR greater than 5 for future experiments and greater than 1 for ongoing experiments.
  • Figure 5: Parameter space in $m_\chi$-$m_{\Phi_2}$ plane with colour code indicating SNR greater than 5 for future experiments and greater than 1 for ongoing experiments.
  • ...and 5 more figures