Experimental observation of quantum many-body excitations of E8 symmetry in the Ising chain ferromagnet CoNb2O6

Close to the quantum critical point of the transverse-field Ising spin-chain model, an exotic dynamic spectrum was predicted to emerge upon a perturbative longitudinal field. The dynamic spectrum consists of eight particles and is governed by the symmetry of the E-8 Lie algebra. Here we report on high-resolution terahertz spectroscopy of quantum spin dynamics in the ferromagnetic Ising-chain material CoNb2O6. At 0.25 K in the magnetically ordered phase we identify characteristics of the first six E-8 particles, m(1) to m(6), and the two-particle (m(1) + m(2)) continuum in an applied transverse magnetic field of B-c(1D) = 4.75 T, before the three-dimensional magnetic order is suppressed above B-c(3T )approximate to( )5.3 T. The observation of the higher-energy particles (m(3) to m(6)) above the low-energy two-particle continua features quantum many-body effects in the exotic dynamic spectrum.