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The heavy quark effective field theory (HQEFT) containing both effective`quark fields' and `antiquark fields' is investigated in detail. By integratingout (but not neglecting) the effective antiquark fields, we present a newformulation of effective theory which differs from the usual heavy quarkeffective theory (HQET) and exhibits valuable features because of the inclusionof the contributions from the antiquark fields. Matrix elements of vector andaxial vector heavy quark currents between pseudoscalar and vector mesonscontaining a heavy quark (b or c) are then evaluated systematically up to theorder of $1/m^2_Q$ and parameterized by a set of universal form factors. With aconsistent normalization condition between the effective heavy hadron states,the form factors at zero recoil are related to the ground state meson masses,which enables us to estimate the values of form factors at zero recoil. Inparticular, the Luke's theorem comes out automatically in the new formulationof HQEFT without the need of imposing the equation of motion $iv\cdot DQ^{+}_{v} =0$. Consequently, the differential decay rates of both $B\toD^{\ast}l\nu$ and $B\to D l\nu$ do not receive $1/m_{Q}$ order corrections atzero recoil, which is not the case in the usual HQET. Thus we quote that theCabibbo-Kobayashi-Maskawa matrix element $| V_{cb}|$ can nicely be extractedfrom either of these two exclusive semileptonic decays at the order of$1/m^2_Q$. Our estimates for $|V_{cb}|$ are presented.Comment: 28 pages, Revtex, 5 figures, 3 tables, minor changes with "dressed  heavy quark" being addressed, to be published in Int. J. Mod. Phys.

WEAK MATRIX ELEMENTS AND |Vcb| IN HQEFT

WEAK MATRIX ELEMENTS AND |V_cb| IN HQEFT