Say $J^{\mu} = \bar{\psi} \gamma^{\mu} \psi$ is the QED current. While it is clear to me how to compute something like

$$\langle 0 |T\{ \bar \psi(x) \psi(0)\} |0\rangle$$

using a Feynman diagram expansion, it is not clear to me how I would go about computing

$$\langle 0| T\{J^{\mu}(x) J^{\nu}(0)\}|0\rangle.$$

How do I read off the Feynman rules? This is an important step for deep-inelastic scattering analysis, for instance (c.f. Peskin 18.5). And how about for a general local operator $$\langle 0| T\{\mathcal{O}_1^{\mu_i}(x) \mathcal{O}^{\nu_i}_2(0)\}|0\rangle?$$

Knowing how to compute this in perturbation theory would be useful, for instance, if I wanted to compute its OPE. If there is any physical interpretation, that would be helpful to know as well.


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