The MuCap experiment: A measurement of the muon capture rate in hydrogen gas

Abstract

We have recently measured the rate of nuclear muon capture by the proton, using a novel technique which involves a time projection chamber operating in ultraclean, deuterium‐depleted hydrogen gas. The target's low gas density of 1% compared to liquid hydrogen is key to avoiding uncertainties that arise from the formation of muonic molecules. The capture rate from the hyperfine singlet ground state of the μp atom was obtained from the difference between the μ− disappearance rate in hydrogen and the world average for the μ+ decay rate, yielding ΛS = 725.0±17.4 s−1, from which the induced pseudoscalar coupling of the nucleon, gP(q2 = 0.88mμ2) = 7.3±1.1, is extracted. This result is consistent with theoretical predictions for gP that are based on the approximate chiral symmetry of QCD.