Damping of Ripplons in SuperfluidHe4atT=0

The damping coefficient (imaginary part of the frequency) for ripplons in superfluid ${\mathrm{He}}^4$ at zero temperature is found to be ${\Gamma }_q=0.162\mathrm{} \hslash \frac{q^5}{{\rho }_0}$ owing to a process where one ripplon decays into two.