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Radiative Seesaw Model with Baryon Number Violation and Upper Limit on Neutron-anti-Neutron Transition Time

Rabindra N. Mohapatra, Nobuchika Okada

TL;DR

The paper confronts the challenge of generating the baryon asymmetry when the post-inflation reheating temperature $T_R$ is below the sphaleron threshold, by extending the scotogenic (radiative seesaw) model to include baryon-number violation via a dimension-6 operator that couples right-handed neutrinos to right-handed quarks. It analyzes non-thermal production of RHNs from inflaton decay, derives cosmological constraints from long-lived inert scalars, and connects the baryogenesis mechanism to observable neutron-antineutron oscillations, yielding an upper bound on the transition time $\tau_{n-\bar{n}}$ that lies within the sensitivity of the ESS HIBEAM/NNBAR project. A detailed discussion of washout avoidance and BBN safety accompanies the analysis, and a UV-complete renormalizable realization is presented to justify the effective operator and protect proton stability. The main result is that the model predicts $\tau_{n-\bar{n}}$ values in the range $10^8$–$10^{11}$ s, making upcoming $n-\bar{n}$ searches a decisive test; a null result would strongly constrain or exclude this baryogenesis pathway in the radiative seesaw framework.

Abstract

The minimal scotogenic model where small neutrino masses arise via radiative seesaw, is known to provide a unified framework for neutrino mass and origin of matter via leptogenesis. However if the inflation reheat temperature of the universe is below the sphaleron reheating temperature, then leptogenesis fails and one way to understand the origin of matter would be to add an effective interaction involving the right handed neutrino (RHN) $N$ of the form $\frac{1}{Λ^2}N u_Rd_Rd_R$. This model can lead to observable neutron-anti-neutron ($n-\bar{n}$) oscillation. We show that if RHNs are produced non-thermally, we can get a cosmological upper limit on the transition time $τ_{n-\bar{n}}$, which is within the reach of the planned ESS HIBEAM/NNBAR experiment. The proton stability is guaranteed by the scotogenic $Z_2$ invariance, which prevents the appearance of the Dirac mass term for the neutrino

Radiative Seesaw Model with Baryon Number Violation and Upper Limit on Neutron-anti-Neutron Transition Time

TL;DR

The paper confronts the challenge of generating the baryon asymmetry when the post-inflation reheating temperature is below the sphaleron threshold, by extending the scotogenic (radiative seesaw) model to include baryon-number violation via a dimension-6 operator that couples right-handed neutrinos to right-handed quarks. It analyzes non-thermal production of RHNs from inflaton decay, derives cosmological constraints from long-lived inert scalars, and connects the baryogenesis mechanism to observable neutron-antineutron oscillations, yielding an upper bound on the transition time that lies within the sensitivity of the ESS HIBEAM/NNBAR project. A detailed discussion of washout avoidance and BBN safety accompanies the analysis, and a UV-complete renormalizable realization is presented to justify the effective operator and protect proton stability. The main result is that the model predicts values in the range – s, making upcoming searches a decisive test; a null result would strongly constrain or exclude this baryogenesis pathway in the radiative seesaw framework.

Abstract

The minimal scotogenic model where small neutrino masses arise via radiative seesaw, is known to provide a unified framework for neutrino mass and origin of matter via leptogenesis. However if the inflation reheat temperature of the universe is below the sphaleron reheating temperature, then leptogenesis fails and one way to understand the origin of matter would be to add an effective interaction involving the right handed neutrino (RHN) of the form . This model can lead to observable neutron-anti-neutron () oscillation. We show that if RHNs are produced non-thermally, we can get a cosmological upper limit on the transition time , which is within the reach of the planned ESS HIBEAM/NNBAR experiment. The proton stability is guaranteed by the scotogenic invariance, which prevents the appearance of the Dirac mass term for the neutrino
Paper Structure (12 sections, 30 equations, 5 figures)

This paper contains 12 sections, 30 equations, 5 figures.

Table of Contents

  1. 1. Introduction
  2. 2. The scotogenic Lagrangian with $B$-violation
  3. 2.1 Phenomenology of the new physical scalars in the model
  4. 3. Cosmology of the model
  5. 3.1 Failure of non-thermal Leptogenesis
  6. 3.2 Baryogenesis via $N\to u_Rd_Rd_R$ decay
  7. 4. Theoretical limits on $\tau_{n-\bar{n}}$ in the model
  8. 4.1 Constraints from the decay time of $\eta$
  9. 4.2 Limits from dilution effects on $n_B$ from the decay of $\eta$
  10. 5. Avoiding baryon asymmetry wash-out
  11. 6. UV-complete model for the effective B-violating interaction
  12. 7. Conclusion

Figures (5)

  • Figure 1: Profile of the $T_d$ as a function of $\eta$ mass. We find that $m_\eta/T_d\simeq 20-27$.
  • Figure 2: The predicted free neutron-anti-neutron oscillation time $\tau_{n-\bar{n}}$ is shown as a function of $T_{decay}$ for several values of $|\lambda_5|$. The gray shaded region is excluded by the current experimental limit, $\tau_{n-\bar{n}} > 10^{8}\ \mathrm{sec.}$Baldo-Ceolin:1994hzwSuper-Kamiokande:2020bov and $T_{decay} > 1$ MeV from BBN.
  • Figure 3: Theoretical upper limit on $\tau_{n-\bar{n}}$ as a function of $\frac{\Lambda}{M_N}$ for $| \lambda_5| =10^{-3}$ and various values of $\epsilon_B$.
  • Figure 4: Theoretical upper limit on $\tau_{n-\bar{n}}$ as a function of $\frac{\Lambda}{M_N}$ for $| \lambda_5| =10^{-4}$ and various values of $\epsilon_B$
  • Figure 5: Theoretical upper limit on $\tau_{n-\bar{n}}$ as a function of $\frac{\Lambda}{M_N}$ for $\epsilon_B=0.1$ for different values of $| \lambda_5| =10^{-4}$.