Holographic Pomeron and the Schwinger mechanism

Abstract

We revisit the problem of dipole-dipole scattering via exchanges of soft Pomerons in the context of holographic QCD. We show that a single closed string exchange contribution to the eikonalized dipole-dipole scattering amplitude yields a Regge behavior of the elastic amplitude; the corresponding slope and intercept are different from previous results obtained by a variational analysis of semiclassical surfaces. We provide a physical interpretation of the semiclassical worldsheets driving the Regge behavior for $(\ensuremath{-}t)>0$ in terms of worldsheet instantons. The latter describe the Schwinger mechanism for string pair creation by an electric field, where the longitudinal electric field ${E}_{L}={\ensuremath{\sigma}}_{T}\mathrm{tanh}(\ensuremath{\chi}/2)$ at the origin of this nonperturbative mechanism is induced by the relative rapidity $\ensuremath{\chi}$ of the scattering dipoles. Our analysis naturally explains the diffusion in the impact parameter space encoded in the Pomeron exchange; in our picture, it is due to the Unruh temperature of accelerated strings under the electric field. We also argue for the existence of a ``micro-fireball'' in the middle of the transverse space due to the soft Pomeron exchange, which may be at the origin of the thermal character of multiparticle production in ep/pp collisions. After summing over uncorrelated multi-Pomeron exchanges, we find that the total dipole-dipole cross section obeys the Froissart unitarity bound.