Structure of real and virtual photons from ZEUS

Abstract

Measurements sensitive to the structure of both real and virtual photons are presented and compared to theoretical models with various photon parton distribution functions (PDFs). Measurements for real photons show a tendency for the available photon PDFs to be too small to describe the data. For virtual photons, the photon PDF is seen to decrease with increasing photon virtuality. In order to describe the data, resolved photon processes are required up to a photon virtuality of at least 4.5 GeV 2 .

Hard Photoproduction and the Structure of the Photon

“…The ZEUS collaboration has measured the dijet cross section dσ/dx OBS γ for jets of E jet T > 5.5 GeV in the three different photon virtuality ranges Q 2 ∼ 0 (Q 2 < 1 GeV 2 ), 0.1 < Q 2 < 0.55 GeV 2 and 1.5 < Q 2 < 4.5 GeV 2 [33]. This measurement is complementary to the H1 analysis in that the Q 2 ∼ 0 data are analyzed together with the deep inelastic data at 1.5 < Q 2 < 4.5 GeV 2 in a consistent way.…”

“…In order to make a precise statement concerning the evolution of the virtual photon parton densities with Q 2 , ZEUS has measured the ratio of the dijet cross section for x OBS γ < 0.75 to that for x OBS γ ≥ 0.75 in bins of Q 2 [33]. The result is shown in Figure 7.…”