The axial charge in the weak nucleon axial vector current has been precisely determined by measuring the alignment correlation terms in the β-ray angular distributions of the purely spin aligned mirror pair nuclei 12B(Iπ= 1+,T1/2=20.2 ms) and 12N(Iπ= 1+,T1/2=11.0 ms). The axial charge was determined to be y=4.96 ±0.09 (stat.)±0.05 (syst.) at a 90% confidence level. The previously obtained data in the year 1996 was reanalyzed to be added to the present result. The combined result is y=4.90±0.10 at a 90% confidence level. The axial charge is enhanced as much as (72±4)% from the impulse approximation model calculated value yIA=2.85. Calculations using the impulse approximation with the soft-π contribution yth=4.15 explain the data quite well but not completely. If we introduce the in-medium mass renormalization the unexplained part of the experimental result corresponds to an in nuclear-medium nucleon mass reduction of (16±4)% at a 90% confidence level relative to the free nucleon mass, at the place where the decaying nucleon resides.
|ジャーナル||Physical Review C - Nuclear Physics|
|出版ステータス||Published - 2002 1月 1|
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