Physics Asked on July 19, 2021
Let us work in dimension $D = 4-2epsilon$.
In 4-dimension, we can write $text{Tr}[A B]$, where $A$ and $B$ are string of gamma matrices, as
$sum_m text{Tr}[A~Gamma^m]text{Tr}[B~Gamma^m]$, where $Gamma^m = {1,gamma_5,gamma^mu,gamma_5gamma^mu,sigma_{munu}}$ are complete set of gamma matrices spanning the dirac space in 4-dim.
As it is well-known, generalizing this to non-integer $D$ dimension causes difficulties since $gamma_5$ (defined as $gamma_5= igamma^0gamma^1gamma^2gamma^3$ in 4-dim.) cannot be well-defined.
One often does not need to work with the explicit form of $gamma_5$, but uses the two relations to evaluate the trace:
i)$~{gamma_5,gamma^mu}=0,,$
ii)$~text{Tr}[gamma_5gamma^{mu}gamma^{nu}gamma^{rho}gamma^{sigma}]=-4iepsilon_{munurhosigma},.$
However, in $D$ dimension, the two relations cannot be satisfied simultaneously; and people use different $gamma_5$-schemes to treat $gamma_5$ in $D$-dimension.
However, when you evaluate both of the traces $text{Tr}[A B]$ and $sum_m text{Tr}[A~Gamma^m]text{Tr}[B~Gamma^m]$ in $D$-dimension by using different $gamma_5$-schemes they do not necessarily agree.
As an example,
take $A = (gammacdot p_1)gamma^alpha(gammacdot p_2)$ and $B =gamma^beta(gammacdot p_1)(gammacdot p_2)$.
Then evaluation of $sum_m text{Tr}[A~Gamma^m]text{Tr}[B~Gamma^m]$ requires $gamma_5$ scheme choice.
Then
$text{Tr}[A B] = -4~(D-2)~(2~(p_1cdot p_2)^2 – p_1^2~ p_2^2)$
$left(sum_m text{Tr}[A~Gamma^m]text{Tr}[B~Gamma^m]right)_{text{t’Hooft-Veltman}} = -4~(D-2)~left((D-2)~(p_1cdot p_2)^2 – (D-3)p_1^2~ p_2^2right)$
$left(sum_m text{Tr}[A~Gamma^m]text{Tr}[B~Gamma^m]right)_{text{NDR}} = -4D~(p_1cdot p_2)^2 + 8p_1^2~ p_2^2$
where ‘NDR’ is naive dimensional regularization scheme and ‘t’Hooft-Veltman’ is t’Hooft-Veltman-scheme.
All three results agree when $D$ is taken to be 4, but do not agree in $epsilon$ terms. Is there a way to ensure agreement down to $epsilon$ piece?
The answer to your question is laid out here:
I have a question though: given that you sound privy to the essence of the issue, do you happen to be the author of one of the above papers?
Answered by MadMax on July 19, 2021
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