Breakdown of sound in superfluid helium

As elementary particles carry energy and momentum in the Universe, quasiparticles are the elementary carriers of energy and momentum quanta in condensed matter. And, as elementary particles, under certain conditions quasiparticles can be unstable and decay, emitting pairs of less energetic ones. Pitaevskii [Sov. Phys. JETP 9, 830 (1959)] proposed that such processes exist in superfluid helium, a quantum fluid where the very concept of quasiparticles was borne by Landau and which presented the first notable success of that concept. Pitaevskii's decays have important consequences, including the possible breakdown of a quasiparticle [M. B. Stone et al., Nature (London) 440, 187 (2006)]. Here, we present neutron scattering experiments, which provide evidence that such decays explain the collapsing lifetime (strong damping) of higher-energy phonon-roton sound-wave quasiparticles in superfluid helium. This damping develops when helium is pressurized towards crystallization, or warmed towards approaching the superfluid transition. Our results resolve a number of puzzles raised by previous experiments and reveal the ubiquity of quasiparticle decays and their importance for understanding quantum matter.