Measurement of the proton structure function F(2) at very low Q(2) at HERA

A measurement of the proton structure function F(2)(x, Q(2)) is presented in the kinematic range 0.045 GeV(2) < Q(2) < 0.65 GeV(2) and 6 . 10(-7) < x < 1 . 10(-3). The results were obtained using a data sample corresponding to an integrated luminosity of 3.9 pb(-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 F(2) are typically less than 4%. At the low Q(2) values of the present measurement, the rise of F(2) at low x is slower than observed in HERA data at higher Q(2) and can be described by Regge theory with a constant logarithmic slope partial derivative lnF(2)/partial derivative ln(1/x). The dependence of F(2) on Q(2) is stronger than at higher Q(2) values, approaching, at the lowest Q(2) values of this measurement, a region where F(2) becomes nearly proportional to Q(2). (C) 2000 Elsevier Science B.V. All rights reserved.