Increasing the effective number of neutrinos with decaying particles

TL;DR

The paper tackles the tension between early-universe nucleosynthesis and late-time structure-formation measurements by proposing decaying-particle scenarios that increase the effective number of neutrinos, $N_\nu$, after BBN but before the onset of structure formation. It develops three SUSY-based mechanisms—saxion decay to axions, gravitino decay to axino and axion, and Dirac right-handed sneutrino decay to gravitino and right-handed neutrino—that can inject relativistic radiation with lifetimes in the 1–$10^8$ s window while preserving light-element abundances. By deriving the relations between particle abundance, lifetime, and $\Delta N_\nu$, the work identifies parameter regions that realize $\Delta N_\nu\sim1$ under BBN and CMB/large-scale-structure constraints, and discusses implications for small-scale power and the observed $\sigma_8$ discrepancy. The study suggests that late-time, free-streaming radiation from these decays could reconcile some measurements and provides testable predictions for future CMB and large-scale-structure observations.

Abstract

We present models of decaying particles for increasing the effective number of neutrinos N_nu after big bang nucleosynthesis but before the structure formation begins. We point out that our scenario not only solves the discrepancy between the constraints on N_nu from these two epochs, but also provides a possible answer to deeper inconsistency in the estimation of the matter power spectrum amplitude at small scales, represented by sigma_8, between the WMAP and some small scale matter power measurements such as the Lyman-alpha forest and weak lensing. We consider (a) saxion decay into two axions; (b) gravitino decay into axino and axion; (c) Dirac right-handed sneutrino decay into gravitino and right-handed neutrino.

Figures (2)

• Figure 1: Constraints on the parameter space $m_s$ and $F_a$ in the saxion decay scenario with $T_R = 10^6$ GeV. We have chosen $\delta s_i = F_a$. The lines labeled (a)--(f) are defined as follows. (a) $\Delta N_\nu =1$ on this line. (b) Lower and upper bounds on the PQ scale with $\theta \sim 0.1$. (c) Upper line corresponds to $\tau_s \sim 10^8$ sec, and lower line corresponds to $\tau_s \sim 1$ sec. (d) BBN bounds coming from radiative decay for $40$ MeV $\lesssim m_s \lesssim 1$ GeV and hadronic decay for $m_s \gtrsim 1$ GeV. For $m_s \lesssim 40$ MeV, the bound comes from the CMB. (e) Lower bound on $m_s$ from gravitino thermal production. (f) Lower bound from axino thermal production for $m_{\tilde{a}} = 0.01\,m_s$. For $m_{\tilde{a}} = m_s$, the constraint coincides with the line (a) accidentally.
• Figure 2: Same as Fig. \ref{['fig:TR1e6']}, except for $T_R=10$ MeV and $\delta s_i =10^{17}$ GeV.