Measurement of the Proton Structure Function F_2 and $σ_{tot}^{γ^* p}$ at Low $Q^2$ 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, $e^{+}p\to e^{+}X$, at HERA. A measurement of the proton structure function $F_2$ and the total virtual photon-proton ($\gamma^*p$) cross-section is presented for $0.11 \le Q^{2} \le 0.65$ GeV$^2$ and $2 \times 10^{-6} \le x \le 6 \times 10^{-5}$, corresponding to a range in the $\gamma^{*}p$ c.m. energy of $100 \le W \le 230$ GeV. Comparisons with various models are also presented.