Energy and Q^2 dependence of elastic vectormeson production and the proton structure function F_2

In the framework of the Model of the Stochastic Vacuum elastic hadron-hadron scattering, photo- and electroproduction of vectormesons and also can be well described at center of mass energy approximately 20 GeV. The scattering amplitude is derived by smearing the color dipole-dipole scattering, which is calculated nonperturbatively in the Model of the Stochastic Vacuum, with appropriate wavefunctions. For the considered processes the dipoles have extensions in the range of hadron sizes. We now extend this idea to small dipoles and high energies. The energy dependence is modeled in a phenomenological way: we assume that there a two pomerons, the soft- and the hard-pomeron, each being a simple pole in the complex angular plane. We couple dipoles of hadronic size to the soft-pomeron and small dipoles to the hard-pomeron. For small dipoles we take the perturbative gluon exchange into account. By that way we obtain an energy dependent dipole-dipole scattering amplitude which can be used for all the processes with the same parameters. We show that this approach can describe in addition to all the low energy results (20 GeV) also the HERA data for the considered processes in a large energy and range. Especially the right transition from the soft to the hard behavior is observed. Received: 27 July 1998 / Published online: 5 October 1998     

Small-x behaviour of the polarized photon structure function F_3^\gamma(x,Q^2)

We study the small-x behaviour of the polarized photon structure function F₃^gF_3^{\gamma}, measuring the gluon transversity distribution, in the leading logarithmic approximation of perturbative QCD. There are two contributions, both arising from two-gluon exchange. The leading contribution to small-x is related to the BFKL pomeron and behaves like x^-1-w₂x^{-1-\omega_2}, w₂ = O(a_S)\omega_2 ={\cal O}(\alpha_S). The other contribution includes in particular the ones summed by the DGLAP equation and behaves like x^1-w₀(+)x^{1-\omega_0^{(+)}}, w₀⁽⁺⁾ = O(?{a_S})\omega_0^{(+)} = {\cal O}(\sqrt{\alpha_S}).     

Measurement of neutrino and antineutrino structure functions in hydrogen and iron

The CDHS neutrino detector has been used to measure events originating in a tank of liquid hydrogen and in the iron of the detector. Total cross-sections, differential cross-sections, and structure functions are given for hydrogen and compared with those in iron. The measurements are in agreement with the expectations of the quark parton model. No significant differences indicative of nuclear binding effects in corresponding structure functions of protons and iron are observed. This may be of special interest in the case of the sea structure functions, since large differences are expected in some models.     

Soft and hard Pomeron in the structure function of the proton at low x and low Q^2

We study inclusive electroproduction on the proton at low x and low using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, , proposed by López and Ynduráin. This extension allows to consider both the case and the region of higher on the basis of the same parametrization. A good simultaneous fit to the data on and on the cross section of real photoproduction is obtained for . With four parameters we achieve a for 222 data points. In addition, we use our model of the inclusive interaction to compute the longitudinal structure function . Received: 6 December 1998 / Revised version: 19 April 1999 / Published online: 18 June 1999     

g_1 at low x and low Q^2 with Polarized ep colliders

Measurements of at low x and low are expected to provide a sensitive probe of the transition from Regge to perturbative QCD dynamics, offering a new testing ground for models of small x physics. We discuss the potential of polarized ep colliders (Polarized HERA and eRHIC) to investigate this physics – varying between 0.01 and 1 GeV– and to constrain the high-energy part of the Drell-Hearn-Gerasimov sum-rule for polarized photoproduction. Received: 28 August 2000 / Revised version: 9 November 2000 / Published online: 21 December 2000     

Semi-inclusive neutrino scattering and pion structure functions

We show that a study of the processes

can provide information on the pion structure functions. Using a realistic parton distribution for the pion we calculate cross sections and relevant distributions for these processes. The results show that such measurements are experimentally accessible.     

QCD analysis of non-singlet neutrino structure functions

We present a systematic analysis of all available neutrinoxF ₃ data, using an analytic parametrisation which incorporates second order QCD predictions forQ ² evolution. We find $$\Lambda _{\overline {MS} } = 0.30 \pm 0.13{\rm{ }}GeV$$ where the error is statistical, and both Fermi andW-boson propagator corrections have been made to the data. Neglect of the former has negligible effects, but neglect of the latter would increase Λ by ～0.1 GeV. We find that higher-twist effects are small and negative, and estimate their effect on the determination of Λ. We are unable to fit the data with higher-twist terms alone.     

Double Q^2-rescaling model and the nuclear effect of the parton distribution functions

In order to overcome the shortcoming of nonconservation of nuclear momentum existing in the original -rescaling model (ORM) and avoid introducing nuclear shadowing factor, we proposed a double -rescaling model (DRM) for the parton distributions of the bound nucleon. Using the experimental data of lepton-nucleus deep inelastic scattering (DIS) and the condition of the nuclear momentum conservation, the -rescaling parameters of various partons for Sn, Fe, Ca and C nuclei are determined. The rescaling parameters of valence quark distributions are larger than unity and gradually increase with atomic number A, on the contrary, the rescaling parameters of sea quark distributions and gluon distributions are smaller than unity, and slowly decrease with A. By using this model, the experimental data of the DIS process, the nuclear Drell-Yan process and ph otoproduction process are consistently and quite satisfactorily explained. Received: 18 August 1997 / Published online: 20 February 1998     

Polarized structure functions and asymmetries in deep inelastic neutrino scattering

A new approach to extracting the nucleon polarized structure functions from measurable asymmetries in the deep inelastic lepton-nucleon scattering with a charged weak current applicable to any target is proposed.     

Measurement of the low-x behaviour of the photon structure function F_2^\gamma

The photon structure function has been measured using data taken by the OPAL detector at centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in ranges of 1.5–30.0 GeV (LEP1), and 7.0–30.0 GeV (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in at low-x as expected from QCD. Received: 9 June 2000 / Published online: 13 November 2000     

Measurement of neutral and charged current cross sections in electron-proton collisions at high Q^2

The inclusive single and double differential cross sections for neutral and charged current processes are measured with the H1 detector at HERA, in the range of four-momentum transfer squared between 150 and 30 000 GeV, and Bjorken x between 0.002 and 0.65. The data were taken in 1998 and 1999 with a centre-of-mass energy of 320 GeV and correspond to an integrated luminosity of 16.4 pb. The data are compared with recent measurements of the inclusive neutral and charged current cross sections. For clear evidence is observed for an asymmetry between and neutral current scattering and the generalised structure function is extracted for the first time at HERA. A fit to the charged current data is used to extract a value for the W boson propagator mass. The data are found to be in good agreement with Standard Model predictions. Received: 21 December 2000 / Published online: 15 March 2001