Measurement of the Proton Structure Function F_2 and \sigma_{tot}^{\gamma^* p} at Low Q^2 and Very Low x at HERA

Deep inelastic electron-photon scattering is studied using e+e- data collected by the OPAL detector at centre-of-mass energies sqrt{s_ee} ~ M_{Z^0}. The photon structure function F_2^gamma(x,Q^2) is explored in a Q^2 range of 1.1 to 6.6 GeV/c^2 at lower x values than ever before. To probe this kinematic region events are selected with a beam electron scattered into one of the OPAL luminosity calorimeters at scattering angles between 27 and 55 mrad. A measurement is presented ...

Results on the derivative of log(F_2) with respect to log(x) at fixed Q^2 are presented. The measured derivatives are within errors independent of x for Q^2 >= 0.85 GeV^2 and increase linearly with log(Q^2) for 0.0001 <= x <= 0.01 and Q^2 >= 3 GeV^2. The results are based on preliminary and published H1 data which at Q^2 below 2 GeV^2 are combined with NMC and ZEUS data.

The photon-proton total cross section has been measured in the process e+ p -> e+ gamma p -> e+ X with the ZEUS detector at HERA. Events were collected with photon virtuality Q^2 < 0.02 GeV^2 and average gamma-p center-of-mass energy W_{gamma p} = 209 GeV in a dedicated run, designed to control systematic effects, with an integrated luminosity of 49 nb^{-1}. The measured total cross section is sigma_{tot}^{gamma p} = 174 +- 1 (stat.) +- 13 (syst.) microbarns. The energy depen...

This contribution presents recent ZEUS results on proton structure functions at HERA. The inclusive phi(1020)-meson cross section was measured, and it was used to determine the s-quark content of the proton. The structure function F_2 was extracted using initial-state radiative events. Neutral and charged current cross sections were used to extract the structure function xF_3 and measure the mass of the W boson, respectively. A NLO QCD fit to ZEUS data and fixed target cross ...

Measurements of the total cross section for real and virtual photons on protons at center-of-mass energies in the range from 20 GeV to 270 GeV with photon virtualities up to 5000 GeV^2 are presented. For real photons this cross section can be described by Regge-motivated models while for virtual photons perturbative QCD can be applied. The measurements of the two HERA collider experiments ZEUS and H1 open the possibility to investigate the interplay between the two theoretica...

I investigate what a direct measurement of the longitudinal structure function F_L(x,Q^2) could teach us about the structure of the proton and the best way in which to use perturbative QCD for structure functions. I assume HERA running at a lowered beam energy for approximately 4-5 months and examine how well the measurement could distinguish between different theoretical approaches. I conclude that such a measurement would provide useful information on how to calculate struc...

QED Compton scattering at HERA is discussed in terms of the information it may reveal on the proton structure at low momentum transfers Q2. Detailed Monte Carlo studies are performed which show that the analysis of inelastic QED Compton events allows the extension of present HERA structure function measurements to a kinematic regime, which up to now was only accessed in fixed target data. For these studies an improved version of the COMPTON event generator is used, where spec...

Self-similar properties of proton structure in the kinematic region of low Bjorken x are introduced and studied numerically. A description of the proton structure function F2(x,Q2) reflecting self-similarity is proposed with a few parameters which are fitted to recent HERA data. The specific parameterisation provides an excellent description of the data which cover a region of four momentum transfer squared, 0.045 < Q2 < 120 GeV2, and of Bjorken x, 6.2E-7 < x < 0.01.

Measurements of the proton structure function F_2 for 0.11 < Q^2 < 20000 GeV^2 and 1.2 X 10^-5 < x < 0.65 from ZEUS 1994-1997 measurements are presented. From ZEUS 1994 and 1995 F_2 data the slopes d(F_2)/d(ln Q^2) at fixed x and d(ln F_2)/d(ln(1/x)) for x < 0.01 at fixed Q^2 are derived. For the latter E665 data are also used. The transition region in Q^2 is explored using the simplest non-perturbative models and NLO QCD. The data at very low Q^2 <= 0.65 GeV^2 are described ...

A fit is made to the data for the proton structure function up to Q^2=10 GeV^2, including the real gamma p total cross-section. It is economical and simple, and its form is motivated by physical principles. It is extrapolated down to very small values of x. Data for the ratio (nu W_2^n/nu W_2^p) are also fitted. A FORTRAN program for the fit to (nu W_2^p) is available by email on request Figure 5 from the original version has been deleted.