Late decaying axino as CDM and its lifetime bound

Abstract

The axino with mass in the GeV region can be cold dark matter(CDM) in the galactic halo. However, if R-parity is broken, for example by the bilinear terms $\mu_\alpha$, then axino($\tilde a$) can decay to $\nu+\gamma$. In this case, the most stringent bound on the axino lifetime comes from the diffuse photon background and we obtain that the axino lifetime should be greater than $3.9\times 10^{24}\Omega_{\tilde a}h$ s which amounts to a very small bilinear R-parity violation, i.e. $\mu_\alpha<1$ keV. This invalidates the atmospheric neutrino mass generation through bilinear R-violating terms within the context of axino CDM.