  The search for new elementary particles is one of the most basic pursuits in
physics, spanning from subatomic physics to quantum materials. Magnons are the
ubiquitous elementary quasiparticle to describe the excitations of
fully-ordered magnetic systems. But other possibilities exist, including
fractional and multipolar excitations. Here, we demonstrate that strong quantum
interactions exist between three flavors of elementary quasiparticles in the
uniaxial spin-one magnet FeI2. Using neutron scattering in an applied magnetic
field, we observe spontaneous decay between conventional and heavy magnons and
the recombination of these quasiparticles into a super-heavy bound-state. Akin
to other contemporary problems in quantum materials, the microscopic origin for
new physics in FeI2 is the quasi-flat nature of excitation bands and the
presence of Kitaev anisotropic magnetic exchange interactions.
