Resolved photon processes in DIS and small x dynamics

TL;DR

The paper investigates forward jets and other small-x observables in DIS, where conventional LO/DGLAP and NLO QCD fail to describe data. By incorporating virtual resolved photon processes within the RAPGAP framework and exploring two DGLAP ladders toward the proton and photon, the authors show that resolved photons can reproduce HERA data across multiple observables. They find that the dominant resolved contribution arises from q_γ g_p → q g, and that the approach can mimic certain BFKL-like effects without invoking new dynamics. The work suggests resolved photon dynamics as a plausible alternative explanation to BFKL dynamics for small-x phenomena, while acknowledging that distinguishing between these mechanisms remains challenging and that NLO effects already cover part of the resolved-photon behavior.

Abstract

It has been found that recent results on forward jet production from deep inelastic scattering can neither be reproduced by models which are based on leading order alpha_s QCD matrix elements and parton showers nor by next-to-leading order calculations. The measurement of forward jet cross sections has been suggested as a promising probe of new small x dynamics and the question is whether these data provide an indication of this. The same question arises for other experimental data in deep inelastic scattering at small x which can not be described by conventional models for deep inelastic scattering. In this paper the influence of resolved photon processes has been investigated and it has been studied to what extent such processes are able to reproduce the data. It is shown that two DGLAP evolution chains from the hard scattering process towards the proton and the photon, respectively, are sufficient to describe effects, observed in the HERA data, which have been attributed to BFKL dynamics.

Figures (7)

• Figure 1: Deep inelastic scattering with a resolved virtual photon and the $q_{\gamma} g_p \to q g$ partonic subprocess.
• Figure 2: The forward jet cross section as a function of $x$ using the cuts specified in the text. The data points are preliminary H1 data H1_fjets_data. In $a.$ are shown the RAPGAP predictions for the sum of direct and resolved processes using again the scales $\mu^2=Q^2+p_T^2$ (solid line) and $\mu^2=4p_T^2$ (dotted line). Also included are the predictions for direct DIS processes with the scales $\mu^2=Q^2+p_T^2$ (dashed line) and $\mu^2=4p_T^2$ (dash-dotted line), respectively. The parton distribution of the virtual photon was parameterized according to SaS 2 DIS. In $b.$ the sum of direct and resolved processes (solid line) and the direct (dashed line) are presented using the DG + GRV description of the parton density in the photon and with the scale $\mu^2=Q^2+p_T^2$. In all cases the parton density of the proton was described by CTEQ4M.
• Figure 3: The ratio of cross sections for the production of two and one forward jets versus $x$, as predicted by the RAPGAP Monte Carlo for the sum of direct and resolved processes (solid line). The cuts applied are specified in the text. The prediction from the analytical calculation of BFKL_dijets is shown with the dashed line.
• Figure 4: The di - jet ratio $R_{2+1}$ as a function of $Q^2$ ($a.$ and $c.$) and $x$ ($b.$ and $d.$). The points are preliminary H1 data H1_2+1jets_data. In $a.$ and $b.$ are shown the RAPGAP predictions for the sum of direct and resolved processes using the scales $\mu^2=Q^2+p_T^2$ (solid line) and $\mu^2=4p_T^2$ (dotted line), as well as the predictions for direct processes with the scales $\mu^2=Q^2+p_T^2$ (dashed line) and $\mu^2=4p_T^2$ (dashed-dotted line). The SaS 2 DIS parameterization was used to describe the parton density in the virtual photon. In $c.$ and $d.$ the sum of direct and resolved processes (solid line) and the direct (dashed line) are presented using the DG description of the parton density in the photon with the scale $\mu^2=Q^2+p_T^2$. The parton density of the proton is given in all cases by the CTEQ4M parameterization.
• Figure 5: The inclusive jet cross section as a function of $Q^2$ for different regions of the jet $E_T$. The points are published H1 data H1_incl_jets, the curves represent the RAPGAP predictions for the sum of direct and resolved processes (solid line) and for direct processes only (dashed line).