Does any one know what would be tr(t^at^bt^ct^d), where ta etc are Gell-Mann matrices? This came about when analyzing the color factor for the compton effect for QCD. So, must be pretty common, but I could not find a proper reference. In general is there any reference for trace of arbitrary number of Gell Mann matrices?

Garrett Sheppard

Garrett Sheppard

Open question

2022-08-17

Does any one know what would be t r ( t a t b t c t d ), where t a etc are Gell-Mann matrices? This came about when analyzing the color factor for the compton effect for QCD. So, must be pretty common, but I could not find a proper reference. In general is there any reference for trace of arbitrary number of Gell Mann matrices?

Answer & Explanation

Brogan Navarro

Brogan Navarro

Beginner2022-08-18Added 24 answers

I take the SU(N) generators in the fundamental representation normalized such that
Tr [ T a T b ] = 1 2 δ a b
The commutator of two generators define the structure constants f a b c
[ T a , T b ] = i f a b c T c
The anticommutator of two generators is
{ T a , T b } = 1 N δ a b 1 + d a b c T c
I mean the identity matrix and 1 I mean the identity matrix and d a b c are the "d-symbol" defined as
d a b c = 2 Tr [ { T a , T b } T c ]
Then, there is a useful identity
Tr [ T a T b T c T d ] = 1 4 N δ a b δ c d + 1 8 ( d a b e d c d e f a b e f c d e + i f a b e d c d e + i f c d e d a b e )

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