The Q^2 Dependence of Dijet Cross Sections in gamma p Interactions at HERA

TL;DR

This study investigates how the structure of the photon affects dijet production in gamma-proton interactions at HERA by measuring cross sections across different photon virtualities, revealing a decreasing resolved photon component with increasing Q^2.

Contribution

It provides the first detailed measurement of the Q^2 dependence of the photon structure through dijet cross sections at HERA, comparing data with theoretical models.

Findings

The ratio of dijet cross sections with x_gamma < 0.75 to x_gamma > 0.75 decreases with Q^2.

The resolved photon component diminishes as Q^2 increases but persists up to 4.5 GeV^2.

Current models do not fully describe the observed Q^2 dependence.

Abstract

The dependence of the photon structure on the photon virtuality, Q^2, is studied by measuring the reaction e^+p\to e^+ + {\rm jet} + {\rm jet} + {\rm X} at photon-proton centre-of-mass energies 134 < W < 223 GeV. Events have been selected in the Q^2 ranges \approx 0 GeV^2, 0.1-0.55 GeV^2, and 1.5-4.5 GeV^2, having two jets with transverse energy E_T^{jet} > 5.5 GeV in the final state. The dijet cross section has been measured as a function of the fractional momentum of the photon participating in the hard process, x_gamma. The ratio of the dijet cross section with x_gamma < 0.75 to that with x_gamma > 0.75 decreases as Q^2 increases. The data are compared with the predictions of NLO pQCD and leading-order Monte Carlo programs using various parton distribution functions of the photon. The measurements can be interpreted in terms of a resolved photon component that falls with Q^2 but remains present at values of Q^2 up to 4.5 GeV^2. However, none of the models considered gives a good description of the data.