Measurements of High-Frequency Sound Propagation in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$

Measurements of the attenuation and velocity of pulsed high-frequency sound have been performed up to 133 MHz in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$. A new collective mode of the order parameter was discovered at a frequency extrapolated to $T_c$ of $\omega =(1.165\mathrm{}\pm 0.05){\Delta }_{\mathrm{BCS}}\left(T_c\right)$, where ${\Delta }_{\mathrm{BCS}}\mathrm{}\left(T\right)\mathrm{}$ is the energy gap in the weak-coupling BCS theory. The group velocity has been observed to decrease by as much as $\frac{2}{3}$ of the zero-sound velocity.