Nucleon axial radius and muonic hydrogen - A new analysis and review

Richard J. Hill, Peter Kammel, William J. Marciano, Alberto Sirlin

Research output: Contribution to journalReview articlepeer-review

51 Scopus citations


Weak capture in muonic hydrogen (μH) as a probe of the chiral properties and nucleon structure predictions of quantum chromodynamics (QCD) is reviewed. A recent determination of the axial-vector charge radius squared,r2 A (z exp.)= 0.46(22) fm2, from a model independent z expansion analysis of neutrino-nucleon scattering data is employed in conjunction with the MuCap measurement of the singlet muonic hydrogen capture rate,λMuCap singlet = 715.6(7.4)s-1, to update the induced pseudoscalar nucleon coupling gpMuCap = 8.23(83) derived from experiment, and gptheory = 8.25(25) predicted by chiral perturbation theory. Accounting for correlated errors this implies gptheory/ gpMuCap = 1.00(8), confirming theory at the 8% level. If instead, the predicted expression for is employed as input, then the capture rate alone determines r2 A (μH) = 0.46(24)fm2, or r2 A(ave.)=0.46(16)fm2 together with the independent z expansion neutrino scattering result, a weighted average . Sources of theoretical uncertainty are critically examined and potential experimental improvements are described that can reduce the capture rate error by about a factor of 3. Muonic hydrogen can thus provide a precise and independent value which may be compared with other determinations, such as ongoing lattice gauge theory calculations. The importance of an improved determination for phenomenology is illustrated by considering the impact on critical neutrino-nucleus cross sections at neutrino oscillation experiments.

Original languageEnglish
Article number096301
JournalReports on Progress in Physics
Issue number9
StatePublished - Jul 30 2018

Bibliographical note

Publisher Copyright:
© 2018 IOP Publishing Ltd.


  • muon capture
  • neutrino oscillation
  • neutrino scattering
  • nucleon form factors
  • radiative corrections
  • weak axial current

ASJC Scopus subject areas

  • General Physics and Astronomy


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