Spectrum and correlation functions of a quasi-one-dimensional quantum Ising model

For a model of weakly coupled quantum Ising chains we outline the phase diagram and establish that well below the transition line the system has a remarkably one-dimensional spectrum. We study the dynamical magnetic susceptibility and find a very rich spectrum with several modes corresponding to a fundamental particle and its bound states. The approach is based on the Bethe ansatz and the random phase approximation applied to the interchain exchange.