Measurement of the proton structure function F₂ at very low Q² at HERA

A measurement of the proton structure function F₂(x, Q²) is presented in the kinematic range 0.045 GeV² < Q² < 0.65 GeV² and 6 · 10^-7 < x < 1 · 10^-3. The results were obtained using a data sample corresponding to an integrated luminosity of 3.9pb^-1 in e⁺ p reactions recorded with the ZEUS detector at HERA. Information from a silicon-strip tracking detector, installed in front of the small electromagnetic calorimeter used to measure the energy of the final-state positron at small scattering angles, together with an enhanced simulation of the hadronic final state, has permitted the extension of the kinematic range beyond that of previous measurements. The uncertainties in F₂ are typically less than 4%. At the low Q² values of the present measurement, the rise of F₂ at low x is slower than observed in HERA data at higher Q² and can be described by Regge theory with a constant logarithmic slope δlnF₂/δln(1/x). The dependence of F₂ on Q² is stronger than at higher Q² values, approaching, at the lowest Q² values of this measurement, a region where F₂ becomes nearly proportional to Q². (C) 2000 Elsevier Science B.V.