are the standard Dirac $4 \times 4$ matrices, satisfying the relation $\{ \gamma_{\mu}, \gamma_{\nu} \} = 2 g_{\mu \nu} I_4$ for $\mu, \nu \in \{ 0,1,2,3 \}$. In addition, $\gamma_0^\dagger = \gamma_0$ and $\gamma_\mu^\dagger = \gamma_0 \gamma_\mu \gamma_0 = -\gamma_\mu$ for $\mu \in \{1,2,3\}$.

-   The matrix $\gamma_5 \coloneqq i \gamma_0 \gamma_1 \gamma_2 \gamma_3$ enters the two projection operators to fermion states of definite chirality, i.e., the operators projecting a Dirac field onto its left- and right-handed components. The matrix $\gamma_5$ satisfies the relations: $\gamma_5^\dagger=\gamma_5$, $\gamma_5^2 = I_4$, and $\{ \gamma_{\mu}, \gamma_5 \} = 0$ for $\mu \in \{0,1,2,3\}$.

-   The quantity $\epsilon_{i_0 i_1 \dots i_n}$ is the Levi-Civita symbol, defined as follows: $$\epsilon_{i_0 i_1 \dots i_n} \coloneqq \left\{
    \begin{array}{rl}
    +1 & \text{if $(i_0, i_1, \dots, i_n)$ is an even permutation of $(0, 1, \dots, n)$}\\
    -1 & \text{if $(i_0, i_1, \dots, i_n)$ is an odd permutation of $(0, 1, \dots, n)$}\\
    0 & \text{otherwise}
    \end{array} \right.$$

-   $s$, $u$, and $t$ are the standard Mandelstam variables. The quantity $s$ is equal to the square of the CM energy; $t$ is equal to the square of the $4$-momentum transfer; $u$ is equal to the square of the $4$-momentum transfer for 