Measurement and Phenomenology of the Proton Structure Function $F_2$   from ZEUS at HERA
  Measurements of the proton structure function $F_2$ in the $Q^2$ range $0.6 - 17 \rm GeV^2$ from ZEUS 1995 shifted vertex data and $Q^2 \simeq 1.5 - 20000 \rm GeV^2$ from 1996 and 1997 ZEUS data are presented. From the former and other ZEUS $F_2$ data the slopes $d F_2 / d \ln Q^2$ at fixed $x$ and $d F_2 / d \ln(1/x)$ at fixed $Q^2$ are derived. $F_2$ data at $Q^2$ below $0.9 \rm GeV^2$ are described successfully by a combination of generalised vector meson dominance and Regge theory. Using a NLO QCD fit the gluon density in the proton is extracted in the range $3 \times 10^{-5} < x < 0.7$ from ZEUS 1994 and 1995 data. For $Q^2 \sim 1 \rm GeV^2$ it is found that the $q \bar{q}$ sea distribution is still rising at small $x$ whereas the gluon distribution id strongly suppressed. It is shown that these observations may be understood from the behaviour of the $F_2$ and $d F_2 / d \ln Q^2$ data themselves. 
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