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{\bf EPS-HEP99} \\
{\bf Abstract \# 000} \\
{\bf Parallel sessions: 1,2} \\
{\bf Plenary sessions: 1,2}
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\abstractheader{ {\bf
Measurement of Dijet Cross-Sections
at Low ${\rm{\bf Q^2}}$ in $ep$ Collisions and the
Extraction of an Effective Parton Density for the Virtual Photon } \\ \\
H1 Collaboration, \\
DESY, Notkestr.85, D-22607 Hamburg, Germany
}
\abstractbody{
The triple-differential dijet cross-section, ${\rm d}^3\sigma_{ep}/{\rm d}Q^2{\rm d}\overline{E_t}^2{\rm d}x^{jets}_{\gamma}$,
is measured with the H1 detector at HERA
as a function of the photon virtuality $Q^2$, the
fraction of the photon's momentum carried by the parton entering the
hard scattering, $x^{jets}_{\gamma}$, and the
square of the mean transverse energy, $\overline{E_t}^2$, of the two highest $E_t$ jets.
Jets are
found using a longitudinal boost-invariant $k_T$ clustering
algorithm in the $\gamma^* p$ center of mass frame.
The measurements cover the ranges $1.6 < Q^2 < 80$\, GeV$^2$
in virtuality and $0.1 < y < 0.7$ in inelasticity $y$.
The results are well described by
leading order QCD models which include
the effects of a resolved component to the virtual photon. Models which
treat the photon as point-like fail to describe the data.
An effective leading order parton density for the virtual photon
is extracted as a function of the photon virtuality, the
probing scale and the parton momentum fraction. The
$x_{\gamma}$ and probing scale dependences of the parton density
show characteristic features of
photon structure, and a suppression of this structure with increasing $Q^2$ is seen.
}
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{\bf References }\\
1. H1 Collaboration, C.~Adloff {\it et al},
DESY-98-205 / submitted to Eur. Phys. J. C \\
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