Editing Feynman slash notation (section)

==With four-momentum==
This section uses the {{math|(+ − − −)}} [[metric signature]]. Often, when using the [[Dirac equation]] and solving for cross sections, one finds the slash notation used on [[four-momentum]]: using the [[Dirac basis]] for the gamma matrices,
:<math>\gamma^0 = \begin{pmatrix} I & 0 \\ 0 & -I \end{pmatrix},\quad \gamma^i = \begin{pmatrix} 0 & \sigma^i \\ -\sigma^i & 0 \end{pmatrix} \,</math>

as well as the definition of contravariant four-momentum in [[natural units]],
:<math> p^\mu = \left(E, p_x, p_y, p_z \right) \,</math>

we see explicitly that
:<math>\begin{align}
  {p\!\!/} &= \gamma^\mu p_\mu = \gamma^0 p^0 - \gamma^i p^i \\
           &= \begin{bmatrix} p^0 & 0 \\ 0 & -p^0 \end{bmatrix} - \begin{bmatrix} 0 & \sigma^i p^i \\ -\sigma^i p^i & 0 \end{bmatrix} \\
           &= \begin{bmatrix} E & -\vec{\sigma} \cdot \vec{p} \\ \vec{\sigma} \cdot \vec{p} & -E \end{bmatrix}.
\end{align}</math>

Similar results hold in other bases, such as the [[Weyl basis]].