A Measurement of the proton structure function f2 (x, q**2) at low x and low q**2 at HERA

Abstract The results of a measurement of the proton structure function F 2 ( x , Q 2 ) and the virtual photon-proton cross section are reported for momentum transfers squared Q 2 between 0.35 GeV 2 and 3.5 GeV 2 and for Bjorken- x values down to 6 × 10 −6 using data collected by the HERA experiment H1 in 1995. The data represent an increase in kinematic reach to lower x and Q 2 values of about a factor of 5 compared to previous H1 measurements. Including measurements from fixed target experiments the rise of F 2 with decreasing x is found to be less steep for the lowest Q 2 values measured. Phenomenological models at low Q 2 are compared with the data.