The MuCap experiment at the Paul Scherrer Institute has measured the rate ΛS 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. ΛS 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 ± 5.4stat ± 5.1syst) s −1 and derive the proton's pseudoscalar coupling g P (q 2 0 = −0.88 m 2 µ ) = 8.06 ± 0.55.

Background: The rate λ ppµ characterizes the formation of ppµ molecules in collisions of muonic pµ 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 λ ppµ is required for a correct interpretation of these experiments. Purpose:Recently the MuCap experiment has measured the capture rate Λ S from the singlet pµ atom, employing a low density active target to suppress ppµ formation (PRL 110, 12504 (2013)). Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in λ ppµ had to be improved. Method:The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultra-pure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to ppµ formation was introduced, which allowed the extraction of λ ppµ from the observed time distribution of decay electrons. Results:The ppµ formation rate was measured as λ ppµ = (2.01 ± 0.06stat ± 0.03sys) × 10 6 s −1 . This result updates the λ ppµ value used in the above mentioned MuCap publication. Conclusions:The 2.5× higher precision compared to earlier experiments and the fact that the measurement was performed at nearly identical conditions to the main data taking, reduces the uncertainty induced by λ ppµ to a minor contribution to the overall uncertainty of Λ S and g P , as determined in MuCap. Our final value for λ ppµ shifts Λ S and g P by less than one tenth of their respective uncertainties compared to our results published earlier.

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.

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