Exercise Sheet 3: Scattering amplitudes in gauge theories Discussion on Wednesday 15.05, NEW 15 2102, Prof. Dr. Jan Plefka
Exercise 1.6 – MHV 4-gluon amplitude
i) Show that the four gluon color-ordered amplitude A(1−,2+,3+,4+) is given by the compact formula
i A(1−,2−,3+,4+) = h12i4 h12i h23i h34i h41i by employing the color-ordered Feynman rules.
ii) Check that the helicities of the four legs are correct upon acting with the helicity operator
hi = 12(−λαi ∂
∂λαi + ˜λαi˙ ∂
∂λ˜αi˙) on the above result.
iii) Use the U(1) decoupling theorem (or the results of excercise 1.5) to show that i A(1−,2+,3−,4+) = h13i4
h12i h23i h34i h41i.
Exercise 1.7 – MHV and anti-MHV 4-gluon amplitude
Show that the 4-gluon MHV amplitude ist identical to the 4-gluon MHV amplitude as it should be by parity, i.e. show that
h12i4
h12ih23ih34ih41i = [34]4
[12][23][34][41], with
4
X
i=1
pi = 0
holds.
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