A ZEUS next-to-leading-order QCD analysis of data on deep inelastic scattering

TL;DR

This work conducts a comprehensive next-to-leading-order QCD analysis of ZEUS deep inelastic scattering data, augmented by fixed-target measurements, to extract proton parton distribution functions and the strong coupling constant α_s(M_Z), while rigorously accounting for correlated experimental uncertainties. By performing both a standard ZEUS-S fit with α_s fixed and a simultaneous ZEUS-α_s fit, the study demonstrates precise determinations of the gluon and sea densities and a consistent α_s(M_Z) around 0.1166, with results in agreement with world averages. A dedicated ZEUS-O fit using ZEUS data alone shows that HERA data predominantly constrain gluon and sea at low x, and the analysis also probes the limits of the NLO DGLAP formalism by testing very low Q^2, where the framework begins to fail. Overall, the paper highlights the power of high-precision HERA data to shape proton PDFs and quantify α_s, while clearly identifying the applicability limits of perturbative QCD in DIS.

Abstract

Next-to-leading order QCD analyses of the ZEUS data on deep inelastic scattering together with fixed-target data have been perfomed, from which the gluon and the quark densities of the proton and the value of the strong coupling constant, alpha_s(M_Z), were extracted. The study includes a full treatment of the experimental systematic uncertainties including point-to-point correlations. The resulting uncertainties in the parton density functions are presented. A combined fit for alpha_s(M_Z) and the gluon and qurak densities yields a value of alpha_s(M_Z) in agreement with the world average. The parton density functions derived from ZEUS data alone indicate the importance of HERA data in determining sea quark and gluon distributions at low x. The limits of applicability of the theoretical formalism have been explored by comparing the fit predictions to ZEUS data at very low Q^2.

Figures (16)

• Figure 1: The ZEUS-S NLO QCD fit compared to ZEUS 96/97 and proton fixed-target $F_2$ data. The error bands of the fit represent the total experimental uncertainty from both correlated and uncorrelated sources.
• Figure 2: The ZEUS-S NLO QCD fit compared to ZEUS 96/97 and proton fixed-target $F_2$ data. The error bands are defined in the caption to Fig. \ref{['fig:f2lowall']}.
• Figure 3: The ZEUS-S NLO QCD fit compared to the ZEUS high-$Q^2$$e^+p$ and $e^-p$ neutral current reduced cross sections. The error bands are defined in the caption to Fig. \ref{['fig:f2lowall']}. Note that the $e^+p$ data were taken at $\surd s = 300 \hbox{\rm GeV}$, whereas the $e^-p$ data were taken at $\surd s = 318 \hbox{\rm GeV}$.
• Figure 4: The ZEUS-S NLO QCD fit compared to the ZEUS high-$Q^2$$e^+p$ and $e^-p$ charged current reduced cross sections. The error bands are defined in the caption to Fig. \ref{['fig:f2lowall']}. Note that the $e^+p$ data were taken at $\surd s = 300 \hbox{\rm GeV}$, whereas the $e^-p$ data were taken at $\surd s = 318 \hbox{\rm GeV}$.
• Figure 5: (a) The gluon, sea, $u$ and $d$ valence distributions extracted from the standard ZEUS-S NLO QCD fit at $Q^2 = 10\hbox{\rm GeV}^2$. The error bands in this figure show the uncertainty from statistical and other uncorrelated sources separately from the total uncertainty including correlated systematic uncertainties. (b) The gluon, sea, $u$ and $d$ valence distributions extracted from the ZEUS-S NLO QCD fit at $Q^2 = 10\hbox{\rm GeV}^2$, compared to those extracted from the fits MRST2001 epj:c23:73 and CTEQ6 hep-ph-0201195.