Rare Exclusive Top Decays t , b M and Vector-Meson Helicity

Abstract

We present a leading-order study of the rare exclusive decays $t\to bM$, where $M$ denotes a pseudoscalar or vector meson. Starting from the tree-level transition $t\to b{W}^{\ast }\to b(q{\bar{q}}^{\prime })$ and using a factorization approximation, we derive compact expressions for the amplitudes and partial widths in which the long-distance QCD dynamics is encoded in the meson decay constants $f_M$. We provide numerical branching-ratio predictions for representative modes such as $t\to b{\pi }^{+}$, $t\to b{K}^{+}$, $t\to b{D}_s^{(\ast )}$, and $t\to b{B}_c^{(\ast )}$, finding $\mathrm{Br}\sim 10^{-8}$--$10^{-7}$. For vector final states, we derive helicity amplitudes and Standard-Model polarization fractions, and we outline the sensitivity of these observables to anomalous chiral $tWb$ couplings in the limit $m_b\to 0$. Our results provide an updated Standard Model (SM) baseline for rare exclusive top-quark decays and a starting point for EFT interpretations of non-standard charged-current interactions.