ZEUS Results on the Measurement and Phenomenology of F₂ at Low Cursive Greek Chi and Low Q²

Measurements of the proton structure function F2 for 0.6 < Q2 < 17 GeV2 and 1.2 × 10-5 < cursive Greek chi < 1.9 × 10 -3 from ZEUS 1995 shifted vertex data are presented. From ZEUS F2 data the slopes dF2/d ln Q2 at fixed cursive Greek chi and din F2/d ln(1/cursive Greek chi) for cursive Greek chi < 0.01 at fixed Q2 are derived. For the latter, E665 data are also used. The transition region in Q2 is explored using the simplest non-perturbative models and NLO QCD. The data at very low Q2 ≤ 0.65 GeV2 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 × 10-5 < cursive Greek chi < 0.7. Data from NMC and BCDMS constrain the fit at large cursive Greek chi. Assuming the NLO QCD description to be valid down to Q2 ∼ 1 GeV2, it is found that the qq̄ sea distribution is still rising at small cursive Greek chi and the lowest Q2 values whereas the gluon distribution is strongly suppressed.