A hierarchy of ``meson" bound state excitations in the 1D ferromagnetic Ising chain CoNb$_2$O$_6$*

The quantum magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of the one-dimensional ferromagnetic Ising spin chain. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb$_2$O$_6$. A series of nine spin flip bound states is observed, whose energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr{\"o}dinger equation. Additionally, a novel bound state of excitations on neighboring chains is observed just below the onset of a two particle continuum.