Dark radiation from the axino solution of the gravitino problem

TL;DR

The paper addresses the puzzle of extra radiation suggested by CMB observations by connecting it to a SUSY framework where a light axino solves the gravitino problem. Gravitino decays after BBN produce dark radiation in the form of relativistic axions and axinos, giving a predicted $\Delta N_\text{eff} = O(1)$ and yielding a new upper bound $T_R \lesssim 10^{11}$ GeV, largely independent of PQ parameters. This bound is compatible with successful thermal leptogenesis and implies that DM can be constituted by axions and axinos without conflicting with BBN or late decays. Planck's improved measurements of $N_\text{eff}$, together with potential LHC measurements of gluino masses, offer a testable cross-check of this scenario, linking early-universe radiation, reheating, and collider phenomenology in a single cohesive framework.

Abstract

Current observations of the cosmic microwave background could confirm an increase in the radiation energy density after primordial nucleosynthesis but before photon decoupling. We show that, if the gravitino problem is solved by a light axino, dark (decoupled) radiation emerges naturally in this period leading to a new upper bound on the reheating temperature T_R < 10^{11} GeV. In turn, successful thermal leptogenesis might predict such an increase. The Large Hadron Collider could endorse this opportunity. At the same time, axion and axino can naturally form the observed dark matter.

Figures (1)

• Figure 1: The solid lines represent upper bounds on the reheating temperature $T_\text{R}$ using \ref{['ourdeltaneff1']} and \ref{['tightneffbound']}as function of the gravitino mass $m_{3/2}$ for three different values of the gluino mass $m_{\widetilde{g}}= 550 \text{ GeV}, 10^4 \text{ GeV}$ and $10^5 \text{ GeV}$. We restrict $m_{3/2} < 2 \,m_{\widetilde{g}}$. The shaded regions show the corresponding parameter space consistent with our scenario. Indicated are bounds from late and early enough gravitino decay \ref{['lifetimerange']}. As discussed successful thermal leptogenesis requires $T_\text{R}$ sufficiently larger than $2 \times 10^9 \text{ GeV}$. The dotted lines correspond to values of $\Delta N_\text{eff}=0.52$. This is the possible Planck 2-$\sigma$ exclusion limit, if the observed central value coincides with the standard model expectation. The star corresponds to parameter values as appearing in \ref{['ourdeltaneff1']}.