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16 Theoretical framework
n c and flavours n f results in a photon structure function F γ 2
F γ 2 (x γ , µ 2 γ ) = 2x ∑
γ
n c,n f
e 2 q f q/γ(x γ , µ 2 γ )
= 3 ∑ e 4 α
q
2π x γ(x 2 γ + (1 − x γ) 2 ) ln
n f
( µ
2
)
γ (1 − x γ )
.
m 2 qx γ
(1.32)
Unlike the proton structure F 2 , the quark charge contribute to the fourth power in F γ 2 .
In addition, it depends directly on the scale µ 2 γ, while for F 2 the scale enters only via the
QCD evolution equations. In figure 1.11 the distribution x γ ·f q/γ is shown for the example
of µ 2 γ = 220 GeV2 , only QED effects are considered. The contribution of the bottom quark
is very small due to its large mass. The probability of finding a charm quark however
is much larger. Charm contribution even exceeds lighter d quark contribution due to