10 - H1 - Desy

1.2 Basics of electron - proton scattering 15
1.2.7 Photon structure
In the photoproduction regime two event classes depicted in figure 1.10 may be considered.
If the photon interacts directly with a parton in a proton, it is called direct photoproduction.
Besides, it is possible that the photon fluctuates into q¯q state before the interaction,
which is then called a resolved photoproduction. In resolved photoproduction usually two
contributions are distinguished. In the first case, the q¯q pair may form a vector meson
which carries the quantum numbers of a photon and then interacts with the proton like
a hadron. This case is modelled by the Vector Meson Dominance Model (VDM). The
photon may also fluctuate into an unbound quark pair which is subject to gluon radiation
and gluon splitting. The photon then acts as a source of partons and reveals a
hadronic structure. Similar to the case of the proton, the calculation of the latter can
be approximated by ascribing parton densities to the photon. Taking into account only
QED contributions, the probability f q/γ of finding a quark within the photon carrying a
momentum fraction x γ of the initial photon momentum can be written as [30]:
f q/γ (x γ , µ 2 γ) = e 2 q · α
2π (x2 γ + (1 − x γ ) 2 ) ln
( µ
2
γ (1 − x γ )
m 2 q x γ
)
. (1.30)
Here, e q denotes the electric charges of quark q and m q its mass and µ γ is the scale probing
the photon fluctuation.
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