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

Abstract. 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$1.6 < Q^2 < 80$\end{document} GeV$$^2$$ in virtuality and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$0.1 < y < 0.7$\end{document} 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.