Leptogenesis in Parity Solutions to the Strong CP Problem and Standard Model Parameters

TL;DR

This paper explores theories with exact spacetime parity solving the strong CP problem and generating the baryon asymmetry, deriving bounds on the parity-breaking scale and linking it to observable parameters and future experiments.

Contribution

It provides the first detailed calculation of the parity-breaking scale in minimal parity solutions to the strong CP problem with leptogenesis, connecting it to Standard Model parameters and experimental prospects.

Findings

Lower bounds on right-handed neutrino masses and parity-breaking scale $v_R$

Consistency of $v_R$ bounds with current data at 1 sigma

Potential for future experiments to test the theory via neutron EDM measurements

Abstract

We study the simplest theories with exact spacetime parity that solve the strong CP problem and successfully generate the cosmological baryon asymmetry via decays of right-handed neutrinos. Lower bounds are derived for the masses of the right-handed neutrinos and for the scale of spontaneous parity breaking, $v_R$. For generic thermal leptogenesis, $v_R \gtrsim 10^{12}$ GeV, unless the small observed neutrino masses arise from fine-tuning. We compute $v_R$ in terms of the top quark mass, the QCD coupling, and the Higgs boson mass and find this bound is consistent with current data at $1 \sigma$. Future precision measurements of these parameters may provide support for the theory or, if $v_R$ is determined to be below $10^{12}$ GeV, force modifications. However, modified cosmologies do not easily allow reductions in $v_R$ -- no reduction is possible if leptogenesis occurs in the collisions of domain walls formed at parity breaking, and at most a factor 10 reduction is possible with non-thermal leptogenesis. Standard Model parameters that yield low values for $v_R$ can only be accommodated by having a high degree of degeneracy among the right-handed neutrinos involved in leptogenesis. If future precision measurements determine $v_R$ to be above $10^{12}$ GeV, it is likely that higher-dimensional operators of the theory will yield a neutron electric dipole moment accessible to ongoing experiments. This is especially true in a simple UV completion of the neutrino sector, involving gauge singlet fermions, where the bound from successful leptogenesis is strengthened to $v_R \gtrsim 10^{13}$ GeV.