Measurement of dijet cross-sections at low Q"2 and the extraction of an effective parton density for the virtual photon

The triple-differential dijet cross-section, d"3#sigma#_e_p/dQ"2d anti E_t"2dx_#gamma#"j"e"t"s, 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_#gamma#"j"e"t"s, and the square of the mean transverse energy, anti 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. (orig.)33 refs.Available from TIB Hannover: RA 2999(98-205) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman