Three- and Four-Jet Production at Low x at HERA

Three- and four-jet production is measured in deep-inelastic ep scattering at low x and Q$^2$ with the H1 detector using an integrated luminosity of 44.2pb$^{−1}$. Several phase space regions are selected for the three-jet analysis in order to study the underlying parton dynamics from global topologies to the more restrictive regions of forward jets close to the proton direction. The measurements of cross sections for events with at least three jets are compared to fixed order QCD predictions of $O(\alpha^{2}_{s})$ and $O(\alpha^{3}_{s})$ and with Monte Carlo simulation programs where higher order effects are approximated by parton showers. A good overall description is provided by the $O(\alpha^{3}_{s})$ calculation. Too few events are predicted at the lowest x ∼ 10$^{−4}$, especially for topologies with two forward jets. This hints to large contributions at low x from initial state radiation of gluons close to the proton direction and unordered in transverse momentum. The Monte Carlo program in which gluon radiation is generated by the colour dipole model gives a good description of both the three- and the four-jet data in absolute normalisation and shape

Combined Measurement and QCD Analysis of the Inclusive e+- p Scattering Cross Sections at HERA

A combination is presented of the inclusive deep inelastic cross sections measured by the H1 and ZEUS Collaborations in neutral and charged current unpolarised ep scattering at HERA during the period 1994-2000. The data span six orders of magnitude in negative four-momentum-transfer squared, Q^2, and in Bjorken x. The combination method used takes the correlations of systematic uncertainties into account, resulting in an improved accuracy. The combined data are the sole input in a NLO QCD analysis which determines a new set of parton distributions HERAPDF1.0 with small experimental uncertainties. This set includes an estimate of the model and parametrisation uncertainties of the fit result