Fine-tuning favors mixed axion/axino cold dark matter over neutralinos in the minimal supergravity model

Over almost all of minimal supergravity (mSUGRA or CMSSM) model parameter space, there is a large overabundance of neutralino cold dark matter (CDM). We find that the regions of mSUGRA parameter space which match the measured abundance of CDM in the universe are highly fine-tuned, unless tanβ is very large. If instead we invoke the Peccei-Quinn-Weinberg-Wilczek solution to the strong CP problem, then the SUSY CDM may consist of an axion/axino admixture with an axino mass of order the MeV scale, and where mixed axion/axino or mainly axion CDM seems preferred. In this case, fine-tuning of the relic density is very low, showing that axion/axino CDM (aãCDM) is to be preferred in the paradigm model for SUSY phenomenology. For mSUGRA with aãCDM, quite different regions of parameter space are now DM-favored as compared to the case of neutralino DM. Thus, rather different SUSY signatures are expected at the LHC in the case of mSUGRA with aãCDM, as compared to mSUGRA with neutralino CDM.