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The figure shows the lepton-proton structure
function for diffractive events as a function of the virtuality Q2 of the
photon. The observable beta denotes the momentum of the scattered quark
measured relative to a colour neutral quark- gluon state which was emitted
by the proton. At small fractional energy of this colour neutral state
relative to the proton beam energy this state is called pomeron. The measured
Q2 dependence of the structure function is consistent with flat. At the
large quark momentum beta = 0. 4 the structure function looks different
from that of a hadron, for example that of the proton (see below). |
The dependence of a structure function on the virtuality Q2 of the
probing photon shows the contributions of quarks and gluons and is explained
by the QCD evolution equations:
d F_2/d ln Q^2 ~ P_qq f_q + P_qg f_g + P_qgamma
The terms P_ij f_j denote the convolution of a splitting function P_ij
with a parton density f_j. For example P_qg gives the probability of a
gluon splitting into a quark-antiquark pair, and f_g denotes the gluon
density. The third term of the equation is relevant for the case of the
photon structure function only.
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The proton structure function at x=0.4
decreases as Q2 increases. Above the average momentum x about 0.1 of the
valence quarks, the parton density gets small with increasing Q2. The scaling
violations are caused by tails of the valence quark distributions and correspond
to the first term on the right hand side of the above equation. |
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The photon structure function at x=0.4
increases with Q2: the third term P_qgamma in the above equation denotes
the splitting of the photon into a quark-antiquark pair. This splitting
function is to first order independent of Q2 and generates the characteristic
logarithmic increase of the photon structure function with Q2. |
The flat shape and the magnitude of the structure function for diffractive
processes implies that the exchanged object is neither a hadron consisting
of valence quarks, nor a photon. A consistent interpretation of the data
is given by a gluon dominated object. While in the case of the photon structure
function the splitting of a photon into a quark-antiquark pair is analysed,
the measurement of the structure function for diffractive exchange gives
information on the gluon splitting into a quark-antiquark pair and therefore
gives information on the structure of the carrier of strong interactions
(second term on the right hand side of the above equation).
The large gluon density of the exchanged object of diffractive processes
was confirmed in numerous measurements of the hadronic final state.
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