# Does the top quark decay into an on-shell W boson?

Considering that the top/truth quark is the only quark with higher mass than the massive bosons, is the W boson in its decay different than the off-shell bosons that mediate the weak decay of other particles?

My usual disclaimer: I use these questions as a way of learning for myself about different aspects of physics, so please view this as an attempt, rather than as an answer in itself.

The top quark is the only quark heavy enough to decay into an on-shell W boson, usually accompanied by a bottom quark. We observe a jet containing the bottom quark decay products, $e_{\nu}$, $\mu_{\nu}$, $\tau_{\nu}$.

Image source for all images displayed: T.Tait Talk.pdf

Decays into other possible candidate such as strange or down quarks are suppressed by the small Cabibbo–Kobayashi–Maskaw matrix elements $V_{ts}$ and $V_{td}$

As the decay into $W_b$ is found to occur on nearly every occasion, we can then chart the decay of the weak boson into a branching ratios are 1:1:1:6 (jets). This ratio is 11% each for the neutrinos listed above and 67% for jets.

Three notable aspects of the top quark decay process:

1. The CKM elements are almost diagonal.

2. The W coupling constants are identical.

3. The light fermion masses are almost all small compared to the $M_W$ (mass of the W boson).

Top decay is a left handed interaction, using Dirac /Gamma matrices

$$\gamma^\mu (I -\gamma_5)$$

In a charged lepton decay, the lepton exhibits a tendency to move on the direction of top polarisation.

• This might be implied by your original answer, but I am still curious. Do you know if the W in top decay behaves differently, that is, it decays in other ways than an off-shell W boson? My impression is that off-shell Ws' decays are dictated by the original particle, and bound by conservation of its properties. Whereas the on-shell W has its own physical properties and can decay correspondingly. What do you think? – Ketil Tunheim Oct 21 '16 at 17:02