Measurement of the Proton Structure Function F_2 at Very Low Q^2 at HERA

TL;DR

This paper reports on measuring the proton structure function F_2 at very low Q^2 and x values using HERA data, revealing a slower rise at low x and a near proportionality to Q^2 at the lowest Q^2, extending previous measurements.

Contribution

The study extends the measurement of F_2 to lower Q^2 and x ranges with improved precision and detector techniques, providing new insights into proton structure at these scales.

Findings

F_2 rises more slowly at low x and low Q^2 than at higher Q^2.

At the lowest Q^2, F_2 is nearly proportional to Q^2.

Uncertainties in F_2 are typically less than 4%.

Abstract

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 3.9pb^-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 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. 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.