Measurements of the Muon-Capture Rate inHe3andHe4

Abstract

In order to test the universality of the $V-A$ Fermi interaction and the presence of the induced pseudoscalar term in the Hamiltonian, we have measured the rate of the reaction ${\mu }^{-}+{\mathrm{He}}^3\to {\mathrm{H}}^3+{\nu }_{\mu }$. This reaction is closely analogous to the fundamental muon-capture interaction ${\mu }^{-}+p\to n+{\nu }_{\mu }$. We also have measured the total muon-capture rates in ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$; that is, the rates for the reactions ${\mu }^{-}+{\mathrm{He}}^3\to \mathrm{all}\mathrm{final}\mathrm{states}$ and ${\mu }^{-}+{\mathrm{He}}^4\to \mathrm{all}\mathrm{final}\mathrm{states}$. Negative muons were brought to rest in a high-pressure helium-gas target. The capture processes all yield a charged particle, whose energy was measured by observation of scintillation in the helium gas. Captures into the ${\mathrm{H}}^3$ channel were recognized by the unique energy (1.9 MeV) of the triton recoil. The total capture rates obtained were: $\Lambda \left({\mathrm{He}}^3\right)={{2170}_{-430\mathrm{}}}^{+170\mathrm{}}$ ${\sec }^{-1\mathrm{}}$, and $\Lambda \left({\mathrm{He}}^4\right)={{375}_{-300\mathrm{}}}^{+30\mathrm{}}$ ${\sec }^{-1\mathrm{}}$. The partial capture rate to the triton ground state was measured as $\Lambda ({\mathrm{He}}^3\to {\mathrm{H}}^3)=1505\mathrm{}\pm 46$ ${\sec }^{-1\mathrm{}}$. These results are in good agreement with theoretical predictions based on a universal Fermi interaction, and on a conserved vector current. They also indicate that the induced-pseudoscalar-coupling coefficient is positive. Interpretation of these results in terms of the fundamental muon-capture process is somewhat ambiguous because of the uncertainty in the structure of the helium nucleus.