M. Schaad

Repolarization of Negative Muons by Polarized Bi-209 Nuclei

A large ..mu../sup -/ polarization was achieved in muonic Bi atoms with the help of the strong hyperfine field in a polarized nuclear target. Using /sup 209/Bi nuclei polarized to (59 +- 9)% in ferromagnetic BiMn, we observed a ..mu..-e decay asymmetry of (13.1 +- 3.9)%, which gives ..mu../sup -/ polarization per nuclear polarization equal to -1.07 +- 0.35. This value is almost consistent with -0.792 calculated for nuclei with spin I = (9/2) and a positive magnetic moment under the assumption that the hyperfine interaction becomes effective in the lowest muonic states.

New limits on generation mixing for massive neutrinos from π→ev decay

Improved upper limits of generation-mixing amplitudes /b U//sub ie/ are presented for the electron-flavour in the heavy neutrino mass range of 20-70 MeV//b c//sup 2/. In conjunction with astrophysical considerations the new limits tend to exclude together the existence of heavy neutrinos in the investigated mass range.

Experimental results on radiative muon capture in complex nuclei

Experimental results for the radiative muon capture branching ratio in several nuclei and the photon-muon spin asymmetry in40Ca are reported. For the measurements a two arm Nalspectrometer was used. Apart from26O and40Ca, where we confirm previous results of our group, new data on12C,26Fe,165Ho and209Bi are presented. No available theoretical models explain all the data consistently. The preliminary results for the asymmetry measurements, obtained using a stroboscopic method are in agreement with previous measurements and with theoretical calculations.

No pre-critical enhancement observed in 13C(π+, γ)13N(g.s.) at q ≅ 2mπ

The cross section and angular distribution for the reaction 13C(π+, γ)13N(g.s.) have been measured from 37 to 85° in the laboratory, at a pion energy of 115.5 MeV. The observed cross section ranges from 320 to 660 nbsr. These results do not show the large magnitude and wide-angle peaking expected if pre-critical effects due to nascent pion condensation were present. In addition, the observed cross section is less than one-half of the predictions of available theoretical calculations which do not include the pre-critical effect. Data on the reaction 1H(π−, γ)n at Tπ = 116.6 MeV were also obtained for calibration purposes. These data agree with expectations based on knowledge of the inverse reaction and previous measurements.

Radiative Muon Capture and the Induced Pseudoscalar Coupling in Nuclei

The experimental attempts to exploit the sensitivity of the radiative muon capture (RMC) reaction[1]

Search for massive neutrinos in. pi. r arrow e. nu. decay

{\mu^{ - }}N\left( {A,Z} \right) \to N * \left( {A,Z - 1} \right){\nu_{\mu }}\gamma

NEW LIMITS ON GENERATION MIXING FOR MASSIVE NEUTRINOS FROM PI ---> E NEUTRINO DECAY

to the induced pseudoscalar coupling g P until now are limited to the two nuclear targets 16 O[2] and 40 Ca [2,3,4]. The observables are the photon energy spectrum above a cutoff energy of 57 MeV, below which the muon decay electron induced bremsstrahlung prohibits the measurement, and the muon spin photon angular correlation. The latter quantity has only been measured with poor statistical accuracy for 40 Ca[3,5,6]. The data are scarce because the low rate (the radiative branch above 57 MeV contributes only 2 * 10-5 to the total muon absorption rate in 40 Ca), the modest resolution attainable for medium energy photons and the high background environment of fast neutrons severely trouble the measurement. But even this limited amount of data has lead to surprising results, when compared to theoretical calculations [7,8,9], which proceed in the impulse approximation with a Hamiltonian derived from the elementary μ - p → nν μ γ reaction and a nuclear response function, adapted to describe the results of other medium energy weak or electromagnetic processes. The interpretation of the integrated photon yield and the photon spectrum in terms of the pseudoscalar coupling yields g P = (4.5±1.5)g A for 40 Ca[4] and g P exceeding 12g A for 16 O [1,2,9].