Structure of the virtual photon at HERA

The dependence of the structure of the photon on its virtuality, Q"2, and on the scale of the probing jets, is investigated by measuring the reaction e"+p #-># e"+ + jet + jet + X with the ZEUS detector at HERA. Samples of such events ranging from Q"2 #approx# 0 GeV"2 to Q"2 of 5000 GeV"2 have been selected from the data taken in 1996 and 1997, corresponding to an integrated luminosity of 38.1 pb"-"1. The measured inclusive differential dijet cross section d#sigma#/dx_#gamma#"o"b"s is presented as a function of Q"2 and the square of the transverse energy of the jets, E_T"2, where x_#gamma#"o"b"s is an estimator of the fraction of the photon's momentum participating in the hard dijet subprocess, and compared with the predictions of leading order Monte Carlo programs for various photon parton distribution functions. The ratio of dijet cross sections with x_#gamma#"o"b"s < 0.75 to x_#gamma#"o"b"s > 0.75 show the number of events exhibiting resolved photonic structure to decrease relative to those events without such structure as the photon virtuality increases, whilst the dependence on the scale of the probing jets is small. In comparing simulated Monte Carlo events with the data, it is found that events which simulate photonic structure are needed up to a virtuality of at least 10.5 GeV"2. (author)Available from British Library Document Supply Centre-DSC:DXN031190 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo