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We study how to measure the current structure of the process that $B$ mesondecays into two unstable fermions $\bar f_a$ and $f_b$ in model independentway. We use the momentum distributions of subsequent decay products affected by$\bar f_a f_b$ spin correlation. We have found the following: (1) We canextract the absolute values of two effective coupling constants from theopening angle between the particles decayed from $\bar f_a$ and $f_b$ (2) Wecan extract the real part of the interference from the energy distribution ofone of the decayed particles from $\bar f_a$ or $f_b$. (3) No new informationcan be obtained from the energy distribution of two decayed particles from$\bar f_a$ and $f_b$. (4) The imaginary part of interference is extracted fromthe azimuthal angle asymmetry of final-state decay products. (5) If only one oftwo fermions is unstable, we can extract the real part of interference fromeach of the energy distribution and opening angle distribution. We show severalsimple examples.

General Analysis of $B$ Meson Decay into Two Fermions