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 $1.6 < Q^2 < 80$ 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.