Structure Functions, the Gluon Density, and PQCD Tests

TL;DR

This paper presents measurements of the proton structure function F_2 over a wide range of Q^2 and x, analyzes its slopes, compares data with models, and extracts the gluon density, testing perturbative QCD predictions.

Contribution

It provides new measurements of F_2 from ZEUS, explores the transition between non-perturbative and perturbative regimes, and extracts the gluon density in the proton using NLO QCD fits.

Findings

F_2 data are well described by Regge theory at very low Q^2.

Gluon density in the proton is extracted and found to be suppressed at low x and Q^2.

The sea quark distribution continues to rise at small x and low Q^2.

Abstract

Measurements of the proton structure function F_2 for 0.11 < Q^2 < 20000 GeV^2 and 1.2 X 10^-5 < x < 0.65 from ZEUS 1994-1997 measurements are presented. From ZEUS 1994 and 1995 F_2 data the slopes d(F_2)/d(ln Q^2) at fixed x and d(ln F_2)/d(ln(1/x)) for x < 0.01 at fixed Q^2 are derived. For the latter E665 data are also used. The transition region in Q^2 is explored using the simplest non-perturbative models and NLO QCD. The data at very low Q^2 <= 0.65 GeV^2 are described successfully by Regge theory. From a NLO QCD fit to ZEUS data the gluon density in the proton is extracted in the range 3 X 10^-5 < x < 0.7. Data from NMC and BCDMS constrain the fit at large x. Assuming the NLO QCD description to be valid down to Q^2 of 1 GeV^2, it is found that the qqbar sea distribution is still rising at small x and the lowest Q^2 values whereas the gluon distribution is strongly suppressed. Preliminary ZEUS 1996-1997 $F_2$ measurements are also presented.