Nuclear polarization of 12B in muon capture reaction

The average and longitudinal polarizations of the recoil 12B are studied in detail in the reaction, 12C + μ- → 12B + ν sub(μ). The contributions from excited states are properly taken into account. They cannot be neglected even if we adopt the low branching ratios for the excited states obtained in the recent experimental data. The effects of core polarization and exchange currents are considerably large. The core polarization enhances the average polarization, while the exchange currents reduce it. Contrary to this, both effects reduce the partial muon capture rate for the ground state of 12B. The latest experimental data on the ratio of the average polarization to the longitudinal polarization gives the coupling constant ratio, g sub(P) / g sub(A) = 9.1 +- 1.8, with the 0p-shell wave function of Hauge and Maripuu and the Sussex interaction for the core polarization. Practically the same value of g sub(P) / g sub(A) is obtained with the Cohen-Kurath type wave function. (author)