Qcd-improved Factorization in Nonleptonic B Decays

Nonleptonic decays ofB mesons have attracted a lot of attention recently since they were observed experimentally in CLEO, BaBar and BELLE. These decays are important in extracting the information on the Cabibbo-KobayashiMaskawa (CKM) matrix elements and CP violation. On the theoretical side, it is the least understood area. Since nonleptonic decays involve nonperturbative effects such as final-state interactions, it is difficult to obtain a quantitative theoretical prediction. However, it has been recently found that nonperturbative effects such as the nonfactorizable contribution in nonleptonic decays could be systematically understood in the heavy quark limit with mb → ∞. Here I will review the current status of understanding on nonleptonic B decays very schematically. I will focus on the underlying ideas omitting technical complication. I hope that this talk will give a clear sketch of what is being studied currently. It has been found that nonfactorizable contributions in nonleptonic B decays into two light mesons could be calculated using perturbation theory in the heavy quark limit mb → ∞. I will explain in detail how we treat nonleptonic B decays in the heavy quark limit and discuss some phenomenological aspects. The theoretical framework for B decays is to use the effective weak Hamiltonian at the renormalization scale μ ≈ mb, which is schematically given as