Improved Perturbation Theory and Four-Dimensional Space-Time in IIB Matrix Model

We have analyzed IIB matrix model based on the improved mean field approximation (IMFA) and have obtained a clue that the four-dimensional space-time appears as its most stable vacuum. This method is a systematic way to give an improved perturbation series and was first applied to IIB matrix model by Nishimura and Sugino. In our previous paper we reformed this method and proposed a criterion for convergence of the improved series, that is, the appearance of the ``plateau''. In this paper, we perform higher order calculations, and find that our improved free energy tends to have a plateau, which shows that IMFA works well in IIB matrix model.