Saxion Cosmology for Thermalized Gravitino Dark Matter

Understand

In all supersymmetric theories, gravitinos, with mass suppressed by the Planck scale, are an obvious candidate for dark matter; but if gravitinos ever reached thermal equilibrium, such dark matter is apparently either too abundant or too hot, and is excluded.

• However, in theories with an axion, a saxion condensate is generated during an early era of cosmological history and its late decay dilutes dark matter.
• We show that such dilution allows previously thermalized gravitinos to account for the observed dark matter over very wide ranges of gravitino mass, keV < $m_{3/2}$ < TeV, axion decay constant, $10^9$ GeV < $f_a$ < $10^{16}$ GeV, and saxion mass, 10 MeV < $m_s$ < 100 TeV.
• Constraints on this parameter space are studied from BBN, supersymmetry breaking, gravitino and axino production from freeze-in and saxion decay, and from axion production from both misalignment and parametric resonance mechanisms.