  Quantum integrability emerging near a quantum critical point (QCP) is
manifested by exotic excitation spectrum that is organized by the associated
algebraic structure. A well known example is the emergent $E_8$ integrability
near the QCP of a transverse field Ising chain (TFIC), which was long predicted
theoretically and initially proposed to be realized in the
quasi-one-dimensional (q1D) quantum magnet CoNb$_2$O$_6$. However, later
measurements on the spin excitation spectrum of this material revealed a series
of satellite peaks that cannot be described by the $E_8$ Lie algebra. Motivated
by these experimental progresses, we hereby revisit the spin excitations of
CoNb$_2$O$_6$ by combining numerical calculation and analytical analysis. We
show that, as effects of strong interchain fluctuations, the spectrum of the
system near the 1D QCP is characterized by the $D_{8}^{(1)}$ Lie algebra with
robust topological soliton excitation. We further show that the $D_{8}^{(1)}$
spectrum can be realized in a broad class of interacting quantum systems. Our
results advance the exploration of integrability and manipulation of
topological excitations in quantum critical systems.
