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The photon here does not interact with the nucleus; it just needs the nucleus for momentum conservation. Why does the screening effect then effect pair production cross section? Also, why does the cross section increase with atomic number?

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  • $\begingroup$ You have to give a specific reaction, a link to a paper, to make an intelligible question. for example, this does not help in understanding your question en.wikipedia.org/wiki/Screening_effect . The photon interacts with the fringe fields of the whole ATOM, not with the nucleus, and the whole atom takes the momentum balance. $\endgroup$ – anna v Oct 1 '16 at 17:48
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I think that the curvature of the field is larger for larger atomic number $Z$. Therefore it is easier in the vicinity of a larger nucleus. Then the screening effect takes away the field strength again.

Something has to interact with the nucleus, otherwise it would not be able to absorb the momentum. So a virtual photon has to couple to the new particles and the nucleus. If there are more particles in the nucleus to couple to, this should increase the cross section.

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  • $\begingroup$ What happens in the screening effect though? shoudln't a higher Z also mean more screening effect since there would be greater no of electrons shielding the nucleus? $\endgroup$ – Sabeeka Nazeer Oct 3 '16 at 14:55

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