Identical vertical scales emphasize variation in lineshape in the vicinity of hc. a–c, Solid lines, fits to single resonant mode (yellow shaded region) plus a higher energy continuum excitation (blue shaded region) convolved with the instrumental resolution function. For wavevectors h ≥ 0.2, higher energy excitations are well represented by a two-particle continuum of the form where Θ is a Heaviside step function and ɛ2(Q) is defined by the calculated upper boundary of the two-particle continuum (white line in Fig. 2a); ɛ1(Q) and A were refined by the least squares fitting. d–f, For h ≤ 0.15 this description fails and the spectrum is fitted by two superimposed damped harmonic oscillator spectra, each in the form of a difference of two lorentzians whose HWHM Γ parameterizes damping, (green shaded regions). The gaussian representing elastic incoherent nuclear scattering is also included at all wavevectors. Dashed lines and solid symbols in a–c show data on a one-fifth intensity (I) scale. Error bars show statistical uncertainty estimated as the square root of total neutron count measured at each point.