Measurement of dijet cross-sections in low ${\rm Q}^2$ ep collisions and the extraction of an effective parton density for the virtual photon

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.