Simulation of the synchro-betatron sideband instability caused by electron clouds at KEKB

Electron clouds cause a fast head-tail instability above a threshold density. Experiments at KEKB revealed the presence of a synchro-betatron sideband above the beam blow-up threshold, which indicates the presence of the head-tail instability. The sideband appears near nu_y + knu_s, where 1 < k < 2, which differs from ordinary instability seen near nu_y - nu_s. We study the origin of the sideband using a computer simulation.