Direct measurements of phonon–phonon scattering in liquid4He

We have measured the scattering between two phonon-beams, at different angles, in HeII. When the angle between their propagation directions is 30°, we detect scattering between the low-energy phonons ( / k B ∼ 1 K) in the two beams. No scattering is seen between low-energy phonon beams at 40° due to the low three phonon scattering rate at large angles. At 30° and 40°, we detect scattering between the high-energy phonons ( / k B > 10 K) in the two beams for the first time. This is due to four phonon scattering. There is no detectable scattering between the low- and high-energy phonons at these small angles. When beams collide nearly head-on, there is scattering between the low- and high-energy phonons, and also scattering between the high-energy phonons and R + -rotons. The power and pulse length dependencies of these interactions are presented and discussed. The results are analysed to obtain the lifetime of the probe phonons in the scattering beam of phonons.