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Photon Beams

A resolved photon is similar in spirit to a meson. A VMD photon is associated with either $\rho^0$, $\omega$, $\phi$ or $\mathrm{J}/\psi $, and so corresponds to a well-defined valence flavour content. Since the $\rho^0$ and $\omega$ are supposed to add coherently, the $\u\overline{\mathrm{u}}: \d\overline{\mathrm{d}}$ mixing is in the ratio $4:1$. Similarly a GVMD state is characterized by its $\mathrm{q}\overline{\mathrm{q}}$ classification, in rates according to $e_{\mathrm{q}}^2$ times a mass suppression for heavier quarks.

In the older photon physics options, where a quark content inside an electron is obtained by a numerical convolution, one does not have to make the distinction between valence and sea flavour. Thus any quark (antiquark) initiator leaves behind the matching antiquark (quark), and a gluon leaves behind a quark + antiquark pair. The relative quark flavour composition in the latter case is assumed proportional to $e_{\mathrm{q}}^2$ among light flavours, i.e. $2/3$ into $\u + \overline{\mathrm{u}}$, $1/6$ into $\d + \overline{\mathrm{d}}$, and $1/6$ into $\mathrm{s}+ \overline{\mathrm{s}}$. If one wanted to, one could also have chosen to represent the remnant by a single gluon.

Stephen_Mrenna 2012-10-24