Constraints on neutrino masses from leptogenesis models

TL;DR

This work revisits the CP-violation bounds relevant for leptogenesis, showing that the Davidson-Ibarra limit is not universal when right-handed neutrinos are only moderately hierarchical and that resonant or nontrivial hierarchy effects can enhance the CP asymmetry. By combining a refined thermal leptogenesis dynamics calculation with analyses of hierarchical and quasi-degenerate right-handed neutrino spectra, the authors derive revised bounds on neutrino masses, finding $m_3 \lesssim 0.15$ eV at 3σ under strong hierarchy, while milder hierarchies or quasi-degeneracies can relax this bound and even allow $m_3$ above 1 eV in some scenarios. They also extend the framework to alternative minimal models with fermion SU(2) triplets, showing that leptogenesis can proceed despite gauge interactions, with TeV-scale possibilities for quasi-degenerate triplets that are potentially testable at colliders. Overall, the paper broadens the viable neutrino-mass landscape compatible with leptogenesis, highlighting how the spectrum and flavor structure of new physics crucially shape the bounds and experimental prospects.

Abstract

Upper bounds on the CP asymmetry relevant for leptogenesis are reexamined and found weaker than in previous literature, both for hierarchical and for quasi-degenerate right-handed neutrinos. Successful leptogenesis implies the usual lower bound on right-handed neutrino masses, and an upper bound on left-handed neutrino masses (which we obtain to be 0.15eV at 3sigma) only if right-handed neutrinos are assumed to be much more hierarchical than left-handed neutrinos. Otherwise both bounds can be considerably relaxed. The constraint on light neutrino masses varies assuming different interpretations of why neutrinos should be quasi-degenerate. With conservative assumptions, we find that a mild quasi-degeneracy allows neutrinos heavier than an eV compatibly with leptogenesis. We also extend computations of thermal leptogenesis to an alternative model of neutrino mass mediated by fermion triplets which was never considered so far for leptogenesis. Leptogenesis can be successful despite the effect of gauge interactions, resulting in only slightly stronger constraints on neutrino masses.

Figures (6)

• Figure 1: The Davidson-Ibarra bound on the CP asymmetry $\varepsilon_1$ was derived and holds for $M_{2,3}/M_1 = \infty$. The random samplings of the parameter space (performed assuming different finite hierarchies among right-handed neutrinos) confirms that $|\varepsilon_1|$ can be above the DI bound. For all points $|\lambda_{ij}|<\sqrt{4\pi}$, $\Delta m^2_{\rm atm} = 2~10^{-3}\,{\rm eV}^2$, $M_1=10^8\,{\rm GeV}$. As usual, the density of points depends on arbitrary details of the sampling procedure and carries no information about the likelihood of different regions.
• Figure 2: Maximal value of $\varepsilon_1$ assuming a large hierarchy at fixed $\tilde{m}_1$. The dashed line shows the attempt in epsP. The random sampling was performed as an additional check that the analytical bound of eq. (\ref{['eq:epsDeg']}) is correct.
• Figure 3: Leptogenesis constraint on neutrino masses assuming $M_{2,3}\gg M_1$. The plot shows the measured baryon asymmetry (horizontal line) compared with the maximal leptogenesis value as function of the heaviest neutrino mass $m_3$, renormalized at low energy. Error bars are at $3\sigma$.
• Figure 4: In fig. \ref{['fig:mdeg']}a we estimate how much the maximal value of neutrino mass compatible with thermal leptogenesis increases when right-handed neutrinos are allowed to be quasi-degenerate. The dashed line includes only the resonant enhancement of CP-violation, eq. (\ref{['eq:epsmax3deg']}), while the continuous line includes all effects. A numerical sampling confirms that these constraints can be saturated and even slightly exceeded. Fig. \ref{['fig:mdeg']}b holds in models where everything is as degenerate as neutrinos, see eq.s (\ref{['sys:splits']}). The parameter $n$ quantifies how much $\tilde{m}_i$ are assumed to be close to neutrino masses $m_i$. As $n$ increases our assumptions get relaxed, and therefore the constraint on $m_3$ becomes weaker.
• Figure 5: Fermion triplet leptogenesis. Feynman diagrams that give the new interaction rate $\gamma_A$.