Large-n excitations in the ferromagnetic Ising field theory in a small magnetic field: mass spectrum and decay widths

In presence of a small magnetic field h, the elementary excitations in the scaling two-dimensional Ising model are studied perturbatively in h in the ferromagnetic phase. For excitations with large numbers n, the mass spectrum is obtained in the first order in h. The decay widths of excitations with energies above the stability threshold are calculated in the leading h^3-order.