Mainly axion cold dark matter in the minimal supergravity model

We examine the minimal supergravity (mSUGRA) model under the assumption that the strong CP problem is solved by the Peccei-Quinn mechanism. In this case, the relic dark matter (DM) abundance consists of three components: {\it i}). cold axions, {\it ii}). warm axinos from neutralino decay, and {\it iii}). cold or warm thermally produced axinos. To sustain a high enough re-heat temperature (T_R\agt 10^6 GeV) for many baryogenesis mechanisms to function, we find that the bulk of DM should consist of cold axions, while the admixture of cold and warm axinos should be rather slight, with a very light axino of mass \sim 100 keV. For mSUGRA with mainly axion cold DM (CDM), the most DM-preferred parameter space regions are precisely those which are least preferred in the case of neutralino DM. Thus, rather different SUSY signatures are expected at the LHC in the case of mSUGRA with mainly axion CDM, as compared to mSUGRA with neutralino CDM.