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Dear developers:

SUNSimplify[SUNTrace[SUNT[i, j, i, j]], Explicit -> True] just gives -2/3;

while SUNSimplify[SUNT[i, j, i, j], Explicit -> True] gives -2/9.

why these two differ by a factor "3"?

In what conditions should I have to use SUNTrace when evaluating QCD processes?

For example, when calculating an scalar QCD process(gluon gluon-->q0,q0bar),I just evaluate the color factor at last, before that I just have the squared matrix element equals (\!\(

\*SubsuperscriptBox[\(g\), \(s\), \(4\)]\ \((

\*SuperscriptBox[\(t\), \(2\)]\

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] + t\ u\

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] + t\ u\

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] +

\*SuperscriptBox[\(u\), \(2\)]\

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`j\)\)] .

\*SubscriptBox[\(T\), \(TraditionalForm\`\(TraditionalForm\`i\)\)])\)\))/(32 s^2)

At this momente, I just wonder whether I have to use SUNTrace before SUNSimplify. The answer is yes, only by that can the result match the correct one. But I don't know why I have to?

Thanks for the help!

Best Regards!

Lingxiao Xu

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