  Order parameter collective modes are the fingerprint of a condensed phase.
The spectroscopy of these modes in superfluid $^3$He and unconventional
superconductors can provide key information on the symmetry of the condensate
as well as the microscopic pairing mechanism responsible for the ground state
and excitation energies. We report the discovery of a new collective mode in
superfluid $^3$He-B which we identify as an excited bound state of Cooper
pairs. We use interferometry within an acoustic cavity that is very sensitive
to changes in the velocity of transverse sound. Our measurements of sound
velocity and mode frequency, together with the observation of acoustic
birefringence indicate that this new mode is weakly bound with an excitation
energy within 1% of the pair-breaking edge of $2\Delta$. Based on the selection
rules for coupling of transverse sound to a collective mode in $^3$He-B,
combined with the observation of acoustic birefringence near the collective
mode frequency, we infer that the new mode is most likely a spin-triplet (S=1),
$f$-wave pair exciton (L=3) with total angular momentum, J=4. The existence of
a pair exciton with J=4 suggests an attractive, sub-dominant, $f$-wave pairing
interaction in liquid $^3$He.
