 Solution

ID: past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2017/6/b/solution

Top decay occurs before a colour-bound hadron can form. Using representative scales

Γ_{t} \sim 1

–

1.5 GeV

and

Λ_{QCD} \sim 0.2 GeV

τ_{t} \sim \frac{ℏ}{Γ _{t}} \sim 5 \times 1 0^{- 25} s < τ_{had} \sim \frac{ℏ}{Λ _{QCD}} \sim 3 \times 1 0^{- 24} s .

Thus top quarks do not live long enough to form ordinary top-containing mesons. The decay products still participate in QCD radiation and hadronization, so “free decay” here means decay before binding, not a stable observable free coloured asymptotic particle.

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