Inclusive-jet photoproduction at HERA and determination of alphas

TL;DR

This study measures inclusive-jet photoproduction cross sections in ep collisions at HERA using three jet algorithms (k_T, anti-k_T, SIScone) and compares the results to NLO QCD predictions. It investigates non-perturbative effects and photon/proton PDFs, and extracts the strong coupling constant α_s(M_Z) along with its energy-scale dependence, finding agreement with the world average and the expected QCD running. The analysis validates the robustness of different jet algorithms in a photoproduction environment and demonstrates the potential of these measurements to constrain PDFs in regions of phase space sensitive to gluon densities. The work thus provides precise tests of pQCD and valuable inputs for PDF determinations in future global fits.

Abstract

Inclusive-jet cross sections have been measured in the reaction ep->e+jet+X for photon virtuality Q2 < 1 GeV2 and gamma-p centre-of-mass energies in the region 142 < W(gamma-p) < 293 GeV with the ZEUS detector at HERA using an integrated luminosity of 300 pb-1. Jets were identified using the kT, anti-kT or SIScone jet algorithms in the laboratory frame. Single-differential cross sections are presented as functions of the jet transverse energy, ETjet, and pseudorapidity, etajet, for jets with ETjet > 17 GeV and -1 < etajet < 2.5. In addition, measurements of double-differential inclusive-jet cross sections are presented as functions of ETjet in different regions of etajet. Next-to-leading-order QCD calculations give a good description of the measurements, except for jets with low ETjet and high etajet. The influence of non-perturbative effects not related to hadronisation was studied. Measurements of the ratios of cross sections using different jet algorithms are also presented; the measured ratios are well described by calculations including up to O(alphas2) terms. Values of alphas(Mz) were extracted from the measurements and the energy-scale dependence of the coupling was determined. The value of alphas(Mz) extracted from the measurements based on the kT jet algorithm is alphas(Mz) = 0.1206 +0.0023 -0.0022 (exp.) +0.0042 -0.0035 (th.); the results from the anti-kT and SIScone algorithms are compatible with this value and have a similar precision.

Figures (17)

• Figure 1: Overview of the relative theoretical uncertainties for inclusive-jet cross sections in photoproduction in the kinematic region of the measurements as functions of (a,b) $E_T^{\rm jet}$ and (c,d) $\eta^{\rm jet}$ for the $k_T$, anti-$k_T$ and SIScone jet algorithms. Shown are the relative uncertainties induced by the terms beyond NLO, the proton PDFs, the value of $\alpha_s(M_Z)$, the modelling of the QCD cascade and hadronisation and the photon PDFs.
• Figure 2: (a) The measured differential cross-section $d\sigma/dE_T^{\rm jet}$ based on the $k_T$ jet algorithm for inclusive-jet photoproduction with $-1<\eta^{\rm jet}<2.5$ (dots) in the kinematic region given by $Q^2<1$ GeV$^2$ and $142<W_{\gamma p}<293$ GeV. The NLO QCD calculation (solid line), corrected to include hadronisation effects and using the ZEUS-S (GRV-HO) parameterisations of the proton (photon) PDFs, is also shown. (b) The relative difference between the measured $d\sigma/dE_T^{\rm jet}$ and the NLO QCD calculation (dots). In both figures, the inner error bars represent the statistical uncertainties; the outer error bars show the statistical and systematic uncertainties not associated with the uncertainty in the absolute energy scale of the jets, added in quadrature; the shaded band displays the uncertainty due to the absolute energy scale of the jets and the hatched band displays the total theoretical uncertainty. In some bins, the error bars on the data points are smaller than the marker size and are therefore not visible.
• Figure 3: (a) The measured differential cross-section $d\sigma/d\eta^{\rm jet}$ based on the $k_T$ jet algorithm for inclusive-jet photoproduction with $E_T^{\rm jet}>17$ GeV (dots) in the kinematic region given by $Q^2<1$ GeV$^2$ and $142<W_{\gamma p}<293$ GeV. (b) The relative difference between the measured $d\sigma/d\eta^{\rm jet}$ and the NLO QCD calculation (dots). Other details as in the caption to Fig. \ref{['fig2']}.
• Figure 4: The measured differential cross-sections (a) $d\sigma/dE_T^{\rm jet}$ and (b) $d\sigma/d\eta^{\rm jet}$ based on the $k_T$ jet algorithm for inclusive-jet photoproduction with $E_T^{\rm jet}>17$ GeV and $-1<\eta^{\rm jet}<2.5$ (dots) in the kinematic region given by $Q^2<1$ GeV$^2$ and $142<W_{\gamma p}<293$ GeV. For comparison, the NLO QCD calculations including an estimation of non-perturbative effects (see text) are also shown. Other details as in the caption to Fig. \ref{['fig2']}.
• Figure 5: The measured differential cross-section $d\sigma/d\eta^{\rm jet}$ based on the $k_T$ jet algorithm for inclusive-jet photoproduction with $E_T^{\rm jet}>21$ GeV (dots) in the kinematic region given by $Q^2<1$ GeV$^2$ and $142<W_{\gamma p}<293$ GeV. In (b), the NLO QCD calculations including an estimation of non-perturbative effects (see text) are also shown. Other details as in the caption to Fig. \ref{['fig2']}.