Measurement of Dijet Cross-Sections at Low Q2 and the Extraction of an Effective Parton Density for the Virtual Photon
Abstract
The triple-differential dijet cross-section, d3 sigmaep/dQ2 dEt2 dxgjets, is measured with the H1 detector at HERA as a function of the photon virtuality Q2, the fraction of the photon's momentum carried by the parton entering the hard scattering, xgjets, and the square of the mean transverse energy, Et2, of the two highest Et jets. Jets are found using a longitudinal boost-invariant kT clustering algorithm in the gamma* p center of mass frame. The measurements cover the ranges 1.6 < Q2 < 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 xgamma and probing scale dependences of the parton density show characteristic features of photon structure, and a suppression of this structure with increasing Q2 is seen.
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