Measurement of the Formation Rate of Muonic Hydrogen Molecules

Background: The rate lambda_ppmu characterizes the formation of ppmu molecules in collisions of muonic pmu 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_ppmu is required for a correct interpretation of these experiments. Purpose: Recently the MuCap experiment has measured the capture rate Lambda_S from the singlet pmu atom, employing a low density active target to suppress ppmu formation (PRL 110, 12504 (2013)). Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in lambda_ppmu 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 ppmu formation was introduced, which allowed the extraction of lambda_ppmu from the observed time distribution of decay electrons. Results: The ppmu formation rate was measured as lambda_ppmu = (2.01 +- 0.06(stat) +- 0.03(sys)) 10^6 s^-1. This result updates the lambda_ppmu value used in the above mentioned MuCap publication. Conclusions: The 2.5x 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 lambda_ppmu to a minor contribution to the overall uncertainty of Lambda_S and g_P, as determined in MuCap. Our final value for lambda_ppmu shifts Lambda_S and g_P by less than one tenth of their respective uncertainties compared to our results published earlier.