RADIATIVE MUON-CAPTURE ON HYDROGEN AND THE INDUCED PSEUDOSCALAR COUPLING-CONSTANT

The first observation of the elementary process mu(-)p --> v(mu)n gamm a is reported. The muons were stopped in a purified liquid hydrogen ta rget and captured mainly on the ortho p mu p molecule. The photons wer e converted to e(+)e(-) pairs which were analyzed in a medium-resoluti on high-acceptance magnetic spectrometer. This reaction is particularl y sensitive to the pseudoscalar coupling constant, g(p), which was fou nd to be: g(p)(q(2) = -0.88m(mu)(2)) = (10.0 +/- 0.9 +/- 0.3) . g(a)(0 ). This value of g(p), determined from the photon energy spectrum usin g a perturbation theory calculation based on tree-level Feynman diagra ms, is similar to 1.5 times the theoretical value, (6.70 +/- 0.18) . g (a)(0).