Substructure dependence of jet cross sections at HERA and determination of alphas

TL;DR

This study uses ZEUS data from HERA to explore jet substructure in both photoproduction and NC DIS, employing jet shape and subjet multiplicity to statistically tag quark- and gluon-initiated jets. By measuring inclusive and dijet cross sections across jet kinematics and analyzing angular and mass distributions, the work probes underlying parton dynamics and differentiates direct versus resolved photon processes. The analysis demonstrates good agreement with leading-log parton-shower MC models in photoproduction and with NLO QCD in DIS, and it provides a precise DIS-based extraction of $\alpha_s(M_Z)=0.1176$ with a dominant theoretical uncertainty. Overall, the results confirm pQCD predictions for jet substructure, illustrate the power of quark/gluon tagging, and contribute to the global precision on the strong coupling constant.

Abstract

Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic ep scattering regimes have been measured with the ZEUS detector at HERA using an integrated luminosity of 82.2 pb-1. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies Et(jet) > 17 GeV. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of Et(jet), jet pseudorapidity, the jet-jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of alphas(Mz) of alphas(Mz) = 0.1176 +-0.0009(stat.) -0.0026 +0.0009 (exp.) -0.0072 +0.0091 (th.) was extracted from the measurements of jet shapes in deep inelastic scattering.

Figures (17)

• Figure 1: Measured mean integrated jet shape corrected to the hadron level (dots), $\langle\psi(r)\rangle$, for jets in photoproduction with $E_T^{\rm jet}>17$ GeV in different $\eta^{\rm jet}$ regions. The error bars, which are typically smaller than the dots, show the statistical and systematic uncertainties added in quadrature. For comparison, the predictions of Pythia including resolved plus direct processes for quark (dot-dashed lines), gluon (dashed lines) and all (solid lines) jets are shown. The open circles show the fractional difference of the data to the predictions of Pythia for all jets.
• Figure 2: Measured mean integrated jet shape corrected to the hadron level (dots), $\langle\psi(r)\rangle$, for jets in photoproduction with $E_T^{\rm jet}>17$ GeV in different $\eta^{\rm jet}$ regions. For comparison, the predictions of Herwig (dashed lines) and Pythia MI (dot-dashed lines) including resolved plus direct processes are shown. The open circles show the fractional difference of the data to the predictions of Herwig. Other details are as in the caption to Fig. \ref{['fig1']}.
• Figure 3: Measured mean integrated jet shape corrected to the hadron level (dots), $\langle\psi(r)\rangle$, for jets in photoproduction in the range $-1<\eta^{\rm jet}<2.5$ in different $E_T^{\rm jet}$ regions. For comparison, the predictions of Pythia including resolved (dashed lines), direct (dot-dashed lines) and resolved plus direct processes (solid lines) are shown. Other details are as in the caption to Fig. \ref{['fig1']}.
• Figure 4: Measured mean integrated jet shape in photoproduction corrected to the hadron level at a fixed value of $r=0.5$ (dots), $\langle\psi(r=0.5)\rangle$, as a function of (a) $\eta^{\rm jet}$ with $E_T^{\rm jet}>17$ GeV and (b) $E_T^{\rm jet}$ with $-1<\eta^{\rm jet}<2.5$. Other details are as in the captions to Figs. \ref{['fig1']} and \ref{['fig3']}.
• Figure 5: Measured mean integrated jet shape corrected to the hadron level and for electroweak radiative effects (squares), $\langle\psi(r)\rangle$, for jets in DIS with $E_T^{\rm jet}>17$ GeV in different $\eta^{\rm jet}$ regions. For comparison, NLO predictions corrected for hadronisation and $Z^0$-exchange effects (solid lines) are shown. Other details are as in the caption to Fig. \ref{['fig1']}.