  The parity-transfer $({}^{16}{\rm O},{}^{16}{\rm F}(0^-,{\rm g.s.}))$
reaction is presented as a new probe for investigating isovector $0^-$ states
in nuclei. The properties of $0^-$ states provide a stringent test of the
threshold density for pion condensation in nuclear matter. Utilizing a $0^+
\rightarrow 0^-$ transition in the projectile, the parity-transfer reaction
transfers an internal parity to a target nucleus, resulting in a unique
sensitivity to unnatural-parity states. Consequently, the selectivity for $0^-$
states is higher than in other reactions employed to date. The probe was
applied to a study of the $0^-$ states in ${}^{12}{\rm B}$ via the ${}^{12}{\rm
C}({}^{16}{\rm O},{}^{16}{\rm F}(0^-,{\rm g.s.}))$ reaction at $247~{\rm
MeV/u}$. The excitation energy spectra were deduced by detecting the
${}^{15}{\rm O}+p$ pair produced in the decay of the ${}^{16}{\rm F}$ ejectile.
A known $0^-$ state at $E_x = 9.3~{\rm MeV}$ was observed with an
unprecedentedly high signal-to-noise ratio. The data also revealed new
candidates of $0^-$ states at $E_x=6.6 \pm 0.4$ and $14.8 \pm 0.3~{\rm MeV}$.
The results demonstrate the high efficiency of $0^-$ state detection by the
parity-transfer reaction.
