ZEUS Results on the Measurement and Phenomenology of $F_2$ at Low x and Low Q^2

TL;DR

This paper presents ZEUS measurements of the proton structure function $F_2$ at low $x$ and $Q^2$, exploring non-perturbative models and extracting the gluon density, revealing a rising sea quark distribution and suppressed gluons at low scales.

Contribution

It provides new measurements of $F_2$ at low $x$ and $Q^2$, compares data with models, and extracts the gluon density in a previously unexplored kinematic range.

Findings

$F_2$ data are described by vector meson dominance and Regge theory at very low $Q^2$.

The gluon density is suppressed at low $Q^2$ but the sea quark distribution rises at small $x$.

The data constrain the proton's parton distribution functions at low $x$ and $Q^2$.

Abstract

Measurements of the proton structure function $F_2$ for $0.6 < Q^2 < 17 {GeV}^2$ and $1.2 \times 10^{-5} < x <1.9 \times 10^{-3}$ from ZEUS 1995 shifted vertex data are presented. From ZEUS $F_2$ data the slopes $dF_2/d\ln Q^2$ 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 $Q^2$ $\leq 0.65 {GeV}^2$ are described successfully by a combination of generalised vector meson dominance and Regge theory. From a NLO QCD fit to ZEUS data the gluon density in the proton is extracted in the range $3\times 10^{-5} < x < 0.7$. Data from NMC and BCDMS constrain the fit at large $x$. Assuming the NLO QCD description to be valid down to $Q^2\sim 1 {GeV}^2$, it is found that the $q\bar{q}$ sea distribution is still rising at small $x$ and the lowest $Q^2$ values whereas the gluon distribution is strongly suppressed.