Role of the two-roton bound state in neutron scattering on superfluid4He

It is shown that due to the hybridisation between the one-particle excitation and the two-roton states in superfluid 4 He the interaction between the rotons is characterised by an affective roton-roton coupling g 4eff instead of the bare roton-roton coupling. The magnitude of this interaction, estimated from the neutron scattering measurements at momentum transfer between 2.4 AA -1 and 3.5 AA -1 , is g 4eff approximately=-1*10 -38 erg cm 3 and therefore the neutron scattering measurements show the existence of the two-roton bound state even at large momentum values.