Jet-radius dependence of inclusive-jet cross sections in deep inelastic scattering at HERA

TL;DR

This study measures inclusive-jet production in neutral-current deep inelastic scattering at HERA with the ZEUS detector, examining the dependence on jet radius $R$ using the $k_T$ algorithm in the Breit frame. The authors compare cross sections $d\sigma/dE_{T,B}^{\rm jet}$ and $d\sigma/dQ^2$ to NLO QCD predictions, finding good agreement for $R$ in the range 0.5–1 and demonstrating a linear rise of the ratio of jet cross sections with $R$. They extract the strong coupling constant $\alpha_s(M_Z)$ from $d\sigma/dQ^2$ with $R=1$ at $Q^2>500$ GeV$^2$, obtaining $\alpha_s(M_Z)=0.1207^{+0.0014}_{-0.0014}\,\text{(stat.)}^{+0.0035}_{-0.0033}\,\text{(exp.)}^{+0.0022}_{-0.0023}\,\text{(th.)}$, and verify the energy-scale dependence of $\alpha_s$ against two-loop QCD predictions. The results reinforce the precision of NLO QCD for jet production in DIS and provide a robust test of $\alpha_s$ running over a broad momentum range.

Abstract

Differential inclusive-jet cross sections have been measured for different jet radii in neutral current deep inelastic ep scattering for boson virtualities Q^2 > 125 GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 81.7 pb^-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally inclusive mode for different values of the jet radius R. Differential cross sections are presented as functions of Q^2 and the jet transverse energy, E_T,B^jet. The dependence on R of the inclusive-jet cross section has been measured for Q^2 > 125 and 500 GeV^2 and found to be linear with R in the range studied. Next-to-leading-order QCD calculations give a good description of the measurements for 0.5 <= R <= 1. A value of alpha_s(M_Z) has been extracted from the measurements of the inclusive-jet cross-section dsigma/dQ^2 with R=1 for Q^2 > 500 GeV^2: alpha_s(M_Z) = 0.1207 +- 0.0014 (stat.) -0.0033 +0.0035 (exp.) -0.0023 +0.0022 (th.). The variation of alpha_s with E_T,B^jet is in good agreement with the running of alpha_s as predicted by QCD.

Figures (4)

• Figure 1: (a) The measured differential cross-section $d\sigma/dE^{\rm jet}_{T,{\rm B}}$ for inclusive-jet production with $-2<\eta^{\rm jet}_{\rm B}<1.5$ (dots) for different jet radii, in the kinematic range given by $Q^2>125$ GeV$^2$ and $|\cos\gamma_h|<0.65$. The NLO QCD calculations with $\mu_R=E^{\rm jet}_{T,{\rm B}}$ (solid lines), corrected to include hadronisation and $Z^0$ effects and using the ZEUS-S parameterisations of the proton PDFs, are also shown. Each cross section has been multiplied by the scale factor indicated in brackets to aid visibility. (b) The fractional differences between the measured $d\sigma/dE^{\rm jet}_{T,{\rm B}}$ and the NLO QCD calculations (dots); the hatched bands display the total theoretical uncertainty. The inner error bars represent the statistical uncertainty. 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 bands display the uncertainty due to the absolute energy scale of the jets.
• Figure 2: The measured differential cross-section $d\sigma/dQ^2$ for inclusive-jet production with $E^{\rm jet}_{T,{\rm B}}>8$ GeV and $-2<\eta^{\rm jet}_{\rm B}<1.5$ (dots) for different jet radii, in the kinematic range given by $|\cos\gamma_h|<0.65$. Other details as in the caption to Fig. \ref{['fig1']}.
• Figure 3: The measured cross-section $\sigma_{\rm jets}$ as a function of the jet radius for inclusive-jet production with $E^{\rm jet}_{T,{\rm B}}>8$ GeV and $-2<\eta^{\rm jet}_{\rm B}<1.5$ (dots), in the kinematic range given by $|\cos\gamma_h|<0.65$ and (a) $Q^2>125$ GeV$^2$ and (b) $Q^2>500$ GeV$^2$. The insets show the LO (dot-dashed lines) and NLO (dashed lines) QCD calculations. The NLO QCD calculations corrected to include hadronisation and $Z^0$ effects are shown as solid lines. Other details as in the caption to Fig. \ref{['fig1']}.
• Figure 4: The $\alpha_s$ values determined from the measured $d\sigma/dE^{\rm jet}_{T,{\rm B}}$ with $R=1$ as a function of $E^{\rm jet}_{T,{\rm B}}$ (dots). The dashed line indicates the renormalisation group prediction at two loops obtained from the $\alpha_s(M_Z)$ value determined in this analysis and the shaded area represents its uncertainty. The inner error bars represent the statistical uncertainties of the data. The outer error bars show the statistical and systematic uncertainties added in quadrature. The dotted vertical bars, shifted to aid visibility, represent the theoretical uncertainties.