For example, when we write down the operator definition of quark fragmentation matrix element:(adsbygoogle = window.adsbygoogle || []).push({});

##\Phi_{ij} = \sum_X \int d^4 x e^{ikx}\langle 0|\psi_i(x)|P,X\rangle\langle P,X|\bar{\psi}_j(0)|0\rangle##.

Can we rewrite is as:

##\Phi_{ij} = \sum_X \int d^4 x e^{ikx}\langle P,X|\bar{\psi}_j(0)|0\rangle\langle 0|\psi_i(x)|P,X\rangle = \sum_X \int d^4 x e^{ikx}\langle P,X|\bar{\psi}_j(0)\psi_i(x)|P,X\rangle##?

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# Can we sum out the vacuum state ##|0\rangle\langle 0|## ?

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