Measurement of the Alignment-Positron-Momentum Correlation inN12→C12(g.s.) by the Spontaneous-Level-Mixing Technique: Improved Evidence against a Second-Class Axial Current

The coefficient ${\alpha }_{+}$ of the alignment-positron-momentum correlation in the decay ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ → ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(g.s.) has been determined using the technique of spontaneous level mixing. We find ${\alpha }_{+}=-2.73\mathrm{}\left(39\right)/\mathrm{GeV}$; this agrees with the revised result of Sugimoto et al. The two coefficients ${\alpha }_{\pm }$ jointly yield [assuming strong conservation of vector current (CVC)] ${F_E}^{\mathrm{}\left(2\right)\mathrm{}}$ $F_A=-\frac{0.13\left(48\right)\mathrm{}}{2M}$, i.e., provide strong evidence against a second-class axial current in a model-independent way. Conversely, assuming ${F_E}^{\mathrm{}\left(2\right)\mathrm{}}\equiv 0$ yields $\frac{F_M}{F_A}=\frac{3.93\left(48\right)\mathrm{}}{2M}$, in excellent agreement with the CVC prediction.