Background: The rate lambda(pp mu) characterizes the formation of pp mu molecules in collisions of muonic p mu atoms with hydrogen. In measurements of the basic weak muon capture reaction on the proton to determine the pseudoscalar coupling g(P), capture occurs from both atomic and molecular states. Thus knowledge of lambda(pp mu) is required for a correct interpretation of these experiments.Purpose: Recently the MuCap experiment has measured the capture rate Lambda(S) from the singlet p mu atom, employing a low-density active target to suppress pp mu formation [V. Andreev et al. (MuCap Collaboration), Phys. Rev. Lett. 110, 012504 (2013)]. Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in lambda(pp mu) had to be improved.Method: The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultrapure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to pp mu formation was introduced, which allowed the extraction of pp mu from the observed time distribution of decay electrons.Results: The pp mu formation rate was measured as lambda(pp mu) = (2.01 +/- 0.06(stat) +/- 0.03(sys)) x 10(6) s(-1). This result updates the pp mu value used in the abovementioned MuCap publication.Conclusions: The 2.5x higher precision compared to earlier experiments, and the fact that the measurement was performed under nearly identical conditions as the main data taking, reduces the uncertainty induced by lambda(pp mu) to a minor contribution to the overall uncertainty of Lambda(S) and g(P), as determined in the MuCap experiment. Our final value for lambda(pp mu) shifts Lambda(S) and g(P) by less than one-tenth of their respective uncertainties compared to our results published earlier.

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ(S) of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ(S) is determined from the difference between the μ(-) disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 × 10(10) μ(-) decays, from which we extract the capture rate Λ(S) = (714.9 ± 5.4(stat) ± 5.1(syst)) s(-1) and derive the proton's pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

We propose to measure the rate for muon capture on the deuteron to better than 1.5 process on a nucleus that can both be calculated and measured to a high degree of precision. The measurement will provide a benchmark result, far more precise than any current experimental information on weak interaction processes in the two-nucleon system. Moreover, it can impact our understanding of fundamental reactions of astrophysical interest, like solar pp fusion and the ν+d reactions observed by the Sudbury Neutrino Observatory. Recent effective field theory calculations have demonstrated, that all these reactions are related by one axial two-body current term, parameterized by a single low-energy constant. Muon capture on the deuteron is a clean and accurate way to determine this constant. Once it is known, the above mentioned astrophysical, as well as other important two-nucleon reactions, will be determined in a model independent way at the same precision as the measured muon capture reaction.

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation. The capture rate from the hyperfine singlet ground state of the microp atom was obtained from the difference between the micro(-) disappearance rate in hydrogen and the world average for the micro(+) decay rate, yielding Lambda(S)=725.0+/-17.4 s(-1), from which the induced pseudoscalar coupling of the nucleon, g(P)(q(2)=-0.88m(2)(micro))=7.3+/-1.1, is extracted.

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation.The capture rate from the hyperfine singlet ground state of the p atom was obtained from the difference between the ÿ disappearance rate in hydrogen and the world average for the decay rate, yielding S 725:0 17:4 s ÿ1 , from which the induced pseudoscalar coupling of the nucleon, g P q 2 ÿ0:88m 2 7:3 1:1, is extracted.