[QUE/QFT-06003]

\(\newcommand{\ket}[1]{|#1\rangle} \newcommand{\matrixelement}[3]{\langle#1|#2|#3\rangle}\)
Compute the matrix element  \[ \matrixelement{s,\vec{q}}{J_\mu(x)}{r,\vec{p}}\] where \(J_\mu(x)\) is the current for a Dirac particle and \( \ket{s,\vec{q}}\) and \(\ket{r,\vec{p}} \) denote  one particle states of spin half particle with spin and momenta as specified.