$\pi -{}_{}{}^4{}_{}{}^{}\mathrm{He}$ inelastic cross sections have been measured at $T_{\pi }=180\mathrm{}$ MeV. In the region of the $J^{\pi }=1^{-}$ states $E_x\simeq 23 \mathrm{to} 30$ MeV) the $\frac{{\pi }^{+}}{{\pi }^{-}}$ cross-section ratio $R_{\pi }$ was found to be 1.05 ± 0.08 at ${\theta }_{\mathrm{lab}}=30\mathrm{}°$. Such a small deviation from 1.0 implies that isospin mixing between the $T=0\mathrm{}$ and $T=1\mathrm{}$ states in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is quite weak, in striking contrast to the strong isospin mixing deduced from the large ratio of photonucleon cross sections, $R_{\gamma }=\frac{\sigma (\gamma ,p)}{\sigma (\gamma ,n)}\simeq 1.7$. Thus the conclusion drawn from the photonucleon work, that there is a large charge-symmetry-breaking nuclear force in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, needs to be reexamined.

• Received 10 April 1985

DOI:https://doi.org/10.1103/PhysRevLett.57.543