A "Domain Wall" Scenario for the AdS/QCD

We propose a scenario for bottom-up gravity dual picture of QCD-like theories, which consists of two near-AdS$_5$ domains separated by the ``domain wall'' at which the effective coupling relatively rapidly switches from small perturbative value at its UV side to strong at its IR side. Its consequence are corresponding jumps in 5-d masses of the bulk fields, related to different anomalous dimensions of the corresponding operators. Inclusion of both weakly and strongly coupled parts of bulk wave functions allows for consistent inclusion of QCD hard processes. We further discuss how transitions from strong to weak coupling in hard observables should look like, exemplified by pion diffractive dissociation, which seems to show this transition experimentally. Then we propose a dynamical mechanism for this jump related to instantons, which are point-like bulk objects located at/near the domain wall. We further argue that in the limit of large number of colors $N_c\to\infty$ the "domain wall" is becoming a true singularity. Instanton-induced contributions to correlators and partition function of the instanton ensemble is reformulated in the $AdS_5$ language. Among other applications are lowest mesons as collective vibrations of the ``domain wall''.