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 $0.6< Q^2 < 17 {\rm GeV}^2$ 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 $Q^2 \leq 0.65{\rm GeV}^2$ 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 $Q^2\sim 1{\rm GeV}^2$, 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.