Measurement of the formation rate of muonic hydrogen molecules


V. A. Andreev, Petersburg Nuclear Physics Institute (PNPI)
T. I. Banks, University of California, Berkeley
R. M. Carey, Boston University
T. A. Case, University of California, Berkeley
S. M. Clayton, University of Illinois Urbana-Champaign
K. M. Crowe, University of California, Berkeley
J. Deutsch, Université Catholique de Louvain
J. Egger, Paul Scherrer Institut
S. J. Freedman, University of California, Berkeley
V. A. Ganzha, Petersburg Nuclear Physics Institute (PNPI)
T. Gorringe, University of Kentucky
F. E. Gray, University of California, Berkeley
D. W. Hertzog, University of Illinois Urbana-Champaign
M. Hildebrandt, Paul Scherrer Institut
P. Kammel, University of Illinois Urbana-Champaign
B. Kiburg, University of Illinois Urbana-Champaign
S. Knaack, University of Illinois Urbana-Champaign
P. A. Kravtsov, Petersburg Nuclear Physics Institute (PNPI)
A. G. Krivshich, Petersburg Nuclear Physics Institute (PNPI)
B. Lauss, Paul Scherrer Institut
K. R. Lynch, York College CUNY
E. M. Maev, Petersburg Nuclear Physics Institute (PNPI)
O. E. Maev, Petersburg Nuclear Physics Institute (PNPI)
F. Mulhauser, University of Illinois Urbana-Champaign
C. Petitjean, Paul Scherrer Institut
G. E. Petrov, Petersburg Nuclear Physics Institute (PNPI)
R. Prieels, Université Catholique de Louvain
G. N. Schapkin, Petersburg Nuclear Physics Institute (PNPI)
G. G. Semenchuk, Petersburg Nuclear Physics Institute (PNPI)
M. A. Soroka, Petersburg Nuclear Physics Institute (PNPI)
V. Tishchenko, University of Kentucky
A. A. Vasilyev, Petersburg Nuclear Physics Institute (PNPI)
A. A. Vorobyov, Petersburg Nuclear Physics Institute (PNPI)
M. E. Vznuzdaev, Petersburg Nuclear Physics Institute (PNPI)

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Background: The rate λppμ 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 gP, capture occurs from both atomic and molecular states. Thus knowledge of λppμ is required for a correct interpretation of these experiments. Purpose: Recently the MuCap experiment has measured the capture rate ΛS from the singlet pμ atom, employing a low-density active target to suppress ppμ formation [V. Andreev (MuCap Collaboration), Phys. Rev. Lett. 110, 012504 (2013)]PRLTAO0031-900710.1103/PhysRevLett.110.012504. Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in λppμ 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 ppμ formation was introduced, which allowed the extraction of λppμ from the observed time distribution of decay electrons. Results: The ppμ formation rate was measured as λppμ=(2.01±0.06stat±0.03sys)×106s-1. This result updates the λppμ value used in the abovementioned MuCap publication. Conclusions: The 2.5× 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 λppμ to a minor contribution to the overall uncertainty of ΛS and gP, as determined in the MuCap experiment. Our final value for λppμ shifts ΛS and gP by less than one-tenth of their respective uncertainties compared to our results published earlier.

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