Impact of flavour coupling on $SO(10)$-inspired leptogenesis

TL;DR

This work analyzes how flavour coupling alters predictions in $SO(10)$-inspired leptogenesis ($SO10INLEP$), focusing on a strongly hierarchical RH neutrino spectrum where $N_2$-leptogenesis and $N_1$ washout are crucial. It shows that including flavour coupling relaxes the lower bound on the lightest neutrino mass from $m_1 \gtrsim 1$ meV to about $0.65$ meV and introduces new muon- and electron-dominated solutions, while preserving the overall viability of strong thermal $SO(10)$-inspired leptogenesis (ST-SO10INLEP) within a slightly expanded parameter region. The analysis links low-energy parameters ($m_1$, $\theta_{23}$, $\delta$, Majorana phases) to the baryon asymmetry, and demonstrates that ST-SO10INLEP remains compatible with recent oscillation hints and cosmological bounds; importantly, the predicted $0\nu\beta\beta$ mass $m_{ee}$ enters a range accessible to KamLAND-Zen. Overall, flavour coupling solidifies the predictive structure of $SO10INLEP$ and provides a robust framework for confronting high-scale leptogenesis with upcoming experimental data.

Abstract

We discuss the impact of flavour coupling on the predictions of low energy neutrino parameters from $SO(10)$-inspired leptogenesis (SO10INLEP). The right-handed (RH) neutrino mass spectrum is strongly hierarchical and successful leptogenesis relies on generating the asymmetry from next-to-lightest RH neutrino decays ($N_2$-leptogenesis) and circumventing the lightest RH neutrino washout. These two conditions yield distinctive predictions such as a lower bound on the lightest neutrino mass $m_1 \gtrsim 1\,{\rm meV}$. We first review the status of SO10INLEP, noticing how cosmological observations are now testing a particular neutrino mass window, $m_1 \simeq (10$--$30)\,{\rm meV}$, where only the first octant is allowed and a large range of values for the Dirac phase is excluded. Including flavour coupling, we find that the lower bound relaxes to $m_1 \gtrsim 0.65\,{\rm meV}$. Moreover, new muon-dominated solutions appear slightly relaxing the upper bound on the atmospheric mixing angle. We also study the impact on strong thermal SO10INLEP (ST-SO10INLEP) scenario where, in addition to successful leptogenesis, one can washout a large pre-existing asymmetry. Contrarily to naive expectations, for which flavour coupling could jeopardise the scenario, allowing a large pre-existing asymmetry to survive unconditionally, we show, and explain analytically, that ST-SO10INLEP is still viable within almost the same allowed region of parameters. There is even a slight relaxation of the $m_1$ viable window from (9--30)meV to (4--40)meV for a $10^{-3}$ pre-existing asymmetry. The new results from atmospheric neutrinos, mildly favouring normal ordering and first octant, are now in nice agreement with the predictions of ST-SO10INLEP. Intriguingly, the predicted $0νββ$ signal is starting to be within the reach of KamLAND-Zen.

Figures (11)

• Figure 1: Scatter plot of the solutions obtained imposing successful SO10INLEP neglecting flavour coupling (left panel) and accounting for flavour coupling (right panel). The three grey areas correspond to the excluded regions by the three upper bounds on the absolute neutrino mass scale: Eq. (\ref{['upperbm1']}) from cosmological observations, Eq. (\ref{['upperbmee']}) from $0\nu\beta\beta$ and Eq. (\ref{['upperbmnue']}) from tritium beta decay. The three planes in light blue simply help understanding the 3-dim shape. Colour code: tauonic, muonic and strong thermal solutions are denoted by yellow, green and blue points, respectively.
• Figure 2: Relevant quantities for three benchmark solutions, one for each of the three types discussed in the main text: $\tau_A$ solution (left panels), $\tau_B$ solution (central panels), $\mu$ solution (right panel). The values of the parameters are shown in Table 1.
• Figure 3: Values of the lower bound on $m_1$ in the $\theta_{23}-\delta$ plane (isocontour lines) without flavour coupling (left) and with flavour coupling (right). The white dashed lines are the $1\sigma$, $2\sigma$and $3\sigma$ experimental constraints from nufit24. The orange area is the area excluded by the cosmological upper bound (\ref{['upperbm1']}).
• Figure 4: Two-dimensional projection on the plane $m_1$-$\theta_{23}$ of the scatter plot of the solutions obtained imposing successful SO10INLEP, neglecting flavour coupling effects (left panel) and accounting for flavour coupling effects (right panel). Colour code as in Fig. 1. In the left (right) panel the 3 (2) stars (triangles) denote the 3 (2) benchmark solutions in Table 1 (2).
• Figure 5: Two-dimensional projection on the plane $m_1$-$\theta_{23}$ of the scatter plot of the solutions obtained imposing successful SO10INLEP, neglecting flavour coupling (left) and accounting for flavour coupling (right). As in Fig. 1, the vertical grey areas denote the excluded regions by the three upper bounds on $m_1$: Eq. (\ref{['upperbm1']}) from cosmological observations, Eq. (\ref{['upperbmee']}) from $0\nu\beta\beta$ and Eq. (\ref{['upperbmnue']}) from tritium beta decay. The horizontal grey area is the $3\sigma$ excluded $\theta_{23}$ range of values from Eq. (\ref{['expranges']}). Same conventions as in Fig. 4.