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Дата изменения: Tue Aug 15 14:35:48 2000
Дата индексирования: Mon Oct 1 22:39:40 2012
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Weizsaecker-Williams approximation

Weizsaecker-Williams approximation is used to describe processes of electro-production in the case of small angle of charged particle scattering. In this case the virtual photon emitted by the scattering particle appears near to the mass shell (see Fig.). It gives a possibility to reduce the process of electro-production to the photo-production one with an appropriate photon spectrum:

$$f(x) = (q^2\,\alpha/(2 \pi)) (\log((1-x)/(x^2 \delta)) (1+(1-x)^2)/x -2(1-x-\delta \, x^2)/x),$$

where $\alpha$ is the fine structure constant, $q$ is a charge of incoming particle, $m$ is its mass, $\delta = (m/Q_{max})^2.$ $Q_{max}$ sets out the region of photon virtuality $(P^2 >-Q_{max}^2)$ which contributes to the process. It is assumed that region of large virtuality can be taken into account by direct calculation of electro-production. As a rule this contribution is small enough.

Parameters $q$, $m$, and $Q_{max}$ are defined by the user. See [Budnev-1975] for the further explanations. In the case of CompHEP  the Weizsaecker-Williams photon spectrum is available for charged leptons only.