- #1

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I am trying to understand what the parameter ##k## represents. For a while, I thought that this ##k## was counting the number of particles with negative helicities but that it seems to be incorrect. Can someone help? Thanks.

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- Thread starter nrqed
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- #1

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- 295

I am trying to understand what the parameter ##k## represents. For a while, I thought that this ##k## was counting the number of particles with negative helicities but that it seems to be incorrect. Can someone help? Thanks.

- #2

Gold Member

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https://en.wikipedia.org/wiki/Grassmannian

otherwise, I don't know.

- #3

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Thanks for your feedback.

- #4

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Originally, ##k## is the R-charge of the ##SU(4)## symmetry of the amplitude, so physically it represents the number of negative helicity gluons. Note, however, that the authors later (already in "Into the Amplituhedron" paper, for example) redefine this label according to ##k \equiv K = k-2##. This is why the 4 particle case they consider corresponds to ##n=4##, ##k=0## (and an arbitrary number of loops ##L##). Here ##k## is the new ##k##, i.e. the number of negative helicity gluons minus two. For ##n=4## this is the only non-trivial amplitude to consider since all the other helicity assignments correspond to amplitudes with vanishing kinematical support.I am trying to understand what the parameter ##k## represents

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