Ultrasonic spectroscopy of the J${=1\mathrm{}}^{\mathrm{-}}$ collective mode in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$

Measurements of the attenuation of zero sound in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ show coupling to a collective mode in applied magnetic fields at frequencies ν∼2Δ(T), which is identified as the J${=1\mathrm{}}^{\mathrm{-}}$ mode. The attenuation as a function of reduced temperature in zero magnetic field is subtracted from that measured in magnetic fields up to 80 mT to reveal both the $J_z$=+1 and $J_z$=-1 components. The measured G factor is in good agreement with theory. The $J_z$=+1 resonance occurs in the pair excitation continuum and subtracts from the total attenuation, resulting in an antipeak.