Measurement of the proton structure function F2 and σ at low and very low x at HERA

A small electromagnetic sampling calorimeter, installed in the ZEUS experiment in 1995, significantly enhanced the acceptance for very low x and low Q 2 inelastic neutral current scattering, e+p → e+X, at HERA. A measurement of the proton structure function F2 and the total virtual photon-proton ( γ ∗ p ) cross-section is presented for 0.11 ≤ Q 2 ≤ 0.65 GeV2 and 2 × 10−6 ≤ x ≤ 6 × 10−5, corresponding to a range in the γ ∗ p c.m. energy of 100 ≤ W ≤ 230 GeV. Comparisons with various models are also presented.