Measurement of the Rate of Muon Capture in Hydrogen Gas and Determination of the Proton's Pseudoscalar Coupling g P

The rate of nuclear muon capture by the proton has been measured using a new experimental technique based on a time projection chamber operating in ultra-clean, deuterium-depleted hydrogen gas at 1 MPa pressure. The capture rate was obtained from the dierence between the measured disappearance rate in hydrogen and the world average for the + decay rate. 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 hyperne singlet ground state of the p atom is measured to be S = 725:0 17:4 s 1 , from which the induced pseudoscalar coupling of the nucleon, g P (q 2 = 0:88m 2 ) = 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.