Test of the rotational invariance of the weak interaction

We consider the possibility that the weak interaction might recognize a preferred direction in space. We report a search for anisotropy in the $\beta$ decay of unpolarized ${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}$ nuclei. Our result may be expressed in terms of an expansion of the angular distribution of the decay electron: $\Gamma \left(\theta \right)\propto 1+{\epsilon }_{1}cos\theta +{\epsilon }_2{cos}^2\theta +\cdots$, where $\theta$ is the angle between the direction of emission of the electron and any supposed asymmetry axis. We find for any axis that $\left|{\epsilon }_1\right|<\mathrm{}{10}^{-6.8\mathrm{}}$ and $\left|{\epsilon }_2\right|<\mathrm{}{10}^{-5.7\mathrm{}}$ (90% confidence limits).