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{\large\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
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{H1 Collaboration}\\                            
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{\small \em DESY, Hamburg, Germany}\\[6mm]
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 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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