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We investigated the partial-disorder state in the Shastry-Sutherland lattice BaNd2ZnS5 using time-of-flight inelastic neutron scattering spectrometer HYSPEC at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ORNL). By modeling the field evolution of low-energy spin excitations up to 4T, we revealed the disordered states are due to the detailed balance between external magnetic field and antiferromagnetic Ising inter-dimer interactions. At the critical field Hc ~ 1.9T, the external field cancels the antiferromagnetic inter-dimer interactions as mean field and produces isolated dimers. The intra-dimer interaction was found to be mostly Ising along spin direction while small mixing of J++ and J-- are allowed. Parametric measurements from 0 to 4T on the order parameters of field-polarized ferromagnetic and antiferromagnetic phase at 40mK were carried out using DEMAND diffractometer at High Flux Isotope Reactor (HFIR), ORNL. The order parameter exhibits features of a quantum critical point and can be modeled with a theory of SU(4) coherent state. The longitudinal mode with double-spin flip cannot be quantitively characterized but is required to explain the observed data. This is suggestive that the enigmatic metamagnetic transition is from the ordered antiferromagnetic state to the disordered quantum paramagnetic state.