Nuclear Muon Capture in Hydrogen Isotopes

Abstract

We present two precision experiments in nuclear muon capture which are performed at PSI’s 600 MeV proton accelerator. The muon capture rates in hydrogen and deuterium are measured using the lifetime method and time projection chambers as active muon stopping targets. The MuCap experiment—muon capture on the proton—is in its final analysis. We present a result of the singlet $\mu p$ capture rate $\varLambda _S$ which can be directly related to $g_P$, the pseudoscalar form factor in weak interactions, and which is predicted by low energy heavy baryon chiral perturbation theory. The MuSun experiment measures the doublet capture rate in the $\mu d$ atom using a new cryogenic time projection chamber at 34 K. By effective field theory a unique low energy constant LEC can be determined which calibrates the rate of the main $pp$ fusion reaction of the sun.