ZEUS Results on the Measurement and Phenomenology of $F_2$ at Low x and Low 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 $d{F}_2/d\ln Q^2$ 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 $Q^2$ $\le 0.65{GeV}^2$ 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.

arXiv comment36 pages including 12 figures

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