Towards the core of the cosmological constant problem

We apply a new self-tuning mechanism to the well-known Kachru-Kallosh-Linde-Trivedi (KKLT) model to address the cosmological constant problem. In this mechanism the cosmological constant $\lambda$ contains a supersymmetry breaking term ${\mathcal E}_{\rm SB}$ besides the usual scalar potential ${\mathcal V}_{\rm scalar}$ of the $N=1$ supergravity, which is distinguished from the usual theories where $\lambda$ is directly identified with ${\mathcal V}_{\rm scalar}$ alone. Also in this mechanism, whether $\lambda$ vanishes or not is basically determined by the tensor structure of the scalar potential density, not by the zero or nonzero values of the scalar potential itself. As a result of this application we find that the natural scenario for the vanishing $\lambda$ of the present universe is to take one of the AdS (rather than dS) vacua of KKLT as the background vacuum of our present universe. This AdS vacuum scenario has more nice properties as compared with dS vacua of the usual flux compctifications. The background vacuum is stable both classically and quantum mechanically (no tunneling instabilities), and the value $\lambda =0$ is also stable against quantum corrections because in this scenario the perturbative corrections of ${\mathcal V}_{\rm scalar}$ and quantum fluctuations $\delta_Q {\hat I}_{\rm brane}^{(NS)} + \delta_Q {\hat I}_{\rm brane}^{(R)}$ on the branes are all gauged away by an automatic cancelation between ${\mathcal V}_{\rm scalar} + \delta_Q {\hat I}_{\rm brane}^{(NS)} + \delta_Q {\hat I}_{\rm brane}^{(R)}$ and ${\mathcal E}_{\rm SB}$.