The relevant particles were collective excitations
Landau’s 1941 theory organized low-energy motion in liquid helium through quasiparticles. Phonon-like excitations and a roton branch represented motions of the strongly interacting liquid, rather than independent helium atoms.
The 1962 award recognized a much broader body of condensed-matter theory, especially liquid helium. The helium account illustrates how an effective description can make quantitative thermodynamic predictions.
Sources: [2]
Energy and momentum constrain possible dissipation
A moving fluid can lose energy by creating an excitation only when both energy and momentum allow it. Minimizing excitation energy divided by momentum gives the Landau threshold for that mechanism. It is a criterion for excitation creation, not a guaranteed engineering flow speed.
Sources: [2]
Heat and neutron scattering tested different predictions
The two-fluid description connects an entropy-carrying normal component to a superfluid component. Second sound probes coupled thermal motion. Neutron scattering later tested excitation energies more directly, providing information different from a narrow-channel flow measurement.
Sources: [2]
Dilute-gas occupation is useful but incomplete context
The linked occupation model explains quantum statistics, and the crossover model discusses pairing. Dense helium needs interactions and collective excitations beyond either simple picture. Condensate fraction and superfluid density answer different questions and should not be identified.