  From heavy fermions to cuprates and iron pnictides, a spin resonance is a
staple of nearly magnetic superconductors. Possible explanations include a
two-particle bound state or loss of magnon damping in the superconductor. While
both scenarios suggest a central role for magnetic fluctuations, distinguishing
them is important to identify the right theoretical framework and to guide the
search for higher temperature superconductors. Using inelastic neutron
scattering technique, we show that the spin resonance in optimally doped
Fe(Se,Te) splits into three peaks in a high magnetic field, a signature of a
two-particle triplet bound state
