Adjoint Dirac contribution to the Yang-Mills beta function (${\beta }_{\mathrm{Dirac},\mathrm{ad}}(g)=4C_A{g}^3/[3(4\pi {)}^2]$)

One adjoint Dirac spinor contributes the displayed screening term to the four-dimensional renormalization-group beta function in the modified minimal subtraction scheme. The fermion contribution to the gluon self-energy insertion has pole $-8g^2{C}_A(k^2{\delta }_{\mu \nu }-k_{\mu }{k}_{\nu })/(3ϵ(4\pi {)}^2)$. The corresponding background gauge-field counterterm is $\delta Z_A=-8g^2{C}_A/(3ϵ(4\pi {)}^2)$; the background gauge-field renormalization identity $Z_g=Z_A^{-1/2}$ gives $g_0={\mu }^{ϵ/2}(g+4C_A{g}^3/[3ϵ(4\pi {)}^2])$. Differentiation at fixed $g_0$ yields the screening coefficient. The mass-independent subtraction includes massive fields until effective-theory threshold matching is performed.

For a non-Abelian simple gauge factor with $C_A>0$, pure Yang-Mills theory contributes $-11C_A{g}^3/[3(4\pi {)}^2]$. Adding $N_D$ adjoint Dirac spinors changes this to $-C_A(11-4N_D)g^3/[3(4\pi {)}^2]$. Vanishing would require $N_D=11/4$, which is not a nonnegative integer. This excludes cancellation by these fermions alone, apart from the trivial value $g=0$; other matter representations or fields change the coefficient.