= Two-body top-quark decay width
{title2=$\Gamma(t\to bW^+)=\frac{g_W^2|V_{tb}|^2m_t^3}{64\pi m_W^2}(1-x)^2(1+2x)$}
Here $x=m_W^2/m_t^2$, the <bottom quark> <mass> is neglected, and the <W boson> is on shell. The <weak charged current> gives the spin-averaged squared <scattering amplitude> $g_W^2|V_{tb}|^2(m_t^2-m_W^2)(m_t^2+2m_W^2)/(4m_W^2)$. The <two-body Lorentz-invariant phase space> has $p^*=(m_t^2-m_W^2)/(2m_t)$; multiplying by $p^*/(8\pi m_t^2)$ gives the displayed <decay width>. Its longitudinal-vector contribution explains the large-$m_t$ cubic growth, and its threshold factor makes the width vanish as $m_t\downarrow m_W$. The formula requires $m_t>m_W$ and receives finite-mass and <radiative corrections>.
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