Jet photoproduction at HERA

TL;DR

This paper delivers a complete next-to-leading order QCD analysis of jet photoproduction at HERA using the subtraction method to cancel infrared singularities. It examines the reliability of perturbative predictions across one- and two-jet observables, highlighting moderate scale uncertainties but identifying phase-space regions where fixed-order results fail and all-order resummation is required. It discusses the operational separation of pointlike (direct) and hadronic (resolved) photon components, showing that NLO corrections mix these contributions and that simple x_gamma cuts are not clean discriminants. It provides phenomenological predictions under HERA-like conditions, finding generally stable predictions for ET around 10 GeV and offering observables that can serve as stringent tests of perturbative QCD and photon structure.

Abstract

We compute various kinematical distributions for one-jet and two-jet inclusive photoproduction at HERA. Our results are accurate to next-to-leading order in QCD. We use the subtraction method for the cancellation of infrared singularities. We perform a thorough study of the reliability of QCD predictions; in particular, we consider the scale dependence of our results and discuss the cases when the perturbative expansion might break down. We also deal with the problem of the experimental definition of the pointlike and hadronic components of the incident photon, and briefly discuss the sensitivity of QCD predictions upon the input parameters of the calculation, like $α_S$ and the parton densities.

Figures (15)

• Figure 1: Comparison between monochromatic photon-proton and electron-proton (Weizsäcker-Williams approximation) cross sections, for various jet distributions.
• Figure 2: Scale dependence of the single-inclusive jet pseudorapidity and two-jet invariant mass distributions, for different values of the minimum allowed transverse energy of the observed jets.
• Figure 3: Scale dependence of the azimuthal distance distribution in two-jet events, for different values of the minimum allowed transverse energy of the two hardest jets ($E_{1 T}^{cut}\neq E_{2 T}^{cut}$).
• Figure 4: Inclusive two-jet total cross section for $E_{1 T}>E^{cut}_{ T}$ and $E_{2 T}>E^{cut}_{ T}+\Delta$, as a function of $\Delta$, for two different values of $E^{cut}_{ T}$.
• Figure 5: Scale dependence of the azimuthal distance distribution in two-jet events, for different values of the minimum allowed transverse energy of the two hardest jets ($E_{1 T}^{cut}=E_{2 T}^{cut}=E^{cut}_{ T}$).