Measurement of the Proton Structure Function F_2 and \sigma_{tot}^{\gamma^* 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.