Implications of mixed axion/neutralino dark matter for the Cosmic Frontier: a Snowmass whitepaper

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If one simultaneously invokes the SUSY solution to the gauge hierarchy problem and the PQ solution to the strong CP problem, then one might expect mixed axion/neutralino dark matter (DM), i.e. two dark matter particles. In this case, dark matter production involves oscillation production of axions, thermal and non-thermal neutralino production and thermal and non-thermal production of axinos and saxions. The latter particles may feed additional neutralinos into the cosmic soup, or dilute all relics via additional entropy production, or feed dark radiation into the cosmos. Depending on the SUSY and PQ model, the PQ scale can easily exist from 10⁹-10¹⁶ GeV. Axions may lie well beyond the range currently being probed by experiment. In the case of relic WIMPs, theoretical predictions must be rescaled for the possibility that WIMPs only comprise a fraction of the local DM abundance.