Jan Kwiecinski

Small x phenomenology: Summary and status

Abstract. The aim of this paper is to summarize the general status of our understanding of small-x physics. It is based on presentations and discussions at an informal meeting on this topic held in Lund, Sweden, in March 2001. This document also marks the founding of an informal collaboration between experimentalists and theoreticians with a special interest in small-x physics. This paper is dedicated to the memory of Bo Andersson, who died unexpectedly from a heart attack on March 4th, 2002.The aim of this paper is to summarize the general status of our understanding of small-x physics. It is based on presentations and discussions at an informal meeting on this topic held in Lund, Sweden, in March 2001. This document also marks the founding of an informal collaboration between experimentalists and theoreticians with a special interest in small-x physics. This paper is dedicated to the memory of Bo Andersson, who died unexpectedly from a heart attack on March 4th, 2002.

Screening effects in the ultra-high energy neutrino interactions

Abstract.We study possible saturation effects in the total crosssections describing the interaction of ultra-high energy neutrinos with nucleons. This analysis is performed within two approaches, i.e., within the Golec-Biernat-Wüsthoff saturation model and within the scheme unifying the DGLAP and BFKL dynamics incorporating non-linear screening effects which follow from the Balitzki-Kovchegov equation. The structure functions in both approaches are constrained by HERA data. It is found that screening effects affect the extrapolation of the neutrino-nucleon total cross-sections to ultra-high neutrino energies

Shadowing in gluon distributions in the small-x region☆

E_{\nu}

Shadowing in inelastic lepton-deuteron scattering

and reduce their magnitude by a factor equal to about 2 at

Diffractive J/Ψ production in high energy γγ collisions as a probe of the QCD pomeron

E_{\nu} \sim 10^{12}

Unintegrated parton distributions of pions and nucleons from the CCFM equations in the single-loop approximation

GeV. This reduction becomes amplified by nuclear shadowing in the case of the neutrino-nucleus cross-sections and an approximate estimate of this effect is performed.

Saturation model for two-photon interactions at high energies

The nonlinear evolution equation for the gluon distribution in a proton containing effects of shadowing is solved. We compare the solution with the unshadowed distributions in the case when the latter corresponds to a very steep input behaving as 1x1.5 in the small-x limit. The effect of shadowing is found to be substantial in the region 10−4 < xBj < 10−2. The bare distributions are found to be reduced by a factor of about 2.5 when x = 10−4 and Q2 = 4 GeV2.

Low Q**2, low x region in electroproduction: An Overview

Abstract Shadowing in inelastic lepton-deuteron scattering is analysed using the double interaction formalism where we relate shadowing to inclusive diffractive processes. Both the vector meson and parton contributions are considered for low and high Q2 values including QCD corrections with parton recombination for high Q2. These Q2 values were chosen to correspond to existing experimental data and to the possible HERA measurements. Detailed discussion of various shadowing mechanisms is given. As expected the shadowing effects are found to be very small, less than 2% or so, in agreement with the recent precise measurements performed by the New Muon Collaboration.

Spin structure function g 1 (x, Q) and the DHGHY integral I(Q) at low Q: predictions from the GVMD model

Abstract The reaction γγ → J / ΨJ / Ψ is discussed assuming dominance of the QCD BFKL pomeron exchange. We give prediction for the cross-section of this process for LEP2 and TESLA energies. We solve the BFKL equation in the non-forward configuration taking into account dominant non-leading effects which come from the requirement that the virtuality of the exchanged gluons along the gluon ladder is controlled by their transverse momentum squared. We compare our results with those corresponding to the simple two gluon exchange mechanism and with the BFKL pomeron exchange in the leading logarithmic approximation. The BFKL effects are found to generate a steeper t -dependence than the two gluon exchange. The cross-section is found to increase with increasing CM energy W as ( W 2 ) 2 λ . The parameter λ is slowly varying with W and takes the values λ ∼0.23–0.28. The magnitude of the total cross-section for the process γγ → J / ΨJ / Ψ is found to increase from 4 to 26 pb within the energy range accessible at LEP2. The magnitude of the total cross-section for the process e + e − → e + e − J / ΨJ / Ψ with antitagged e + and e − is estimated to be around 0.1 pb at LEP2.

Shadowing effects in nuclear parton distributions for small values ofx

The unintegrated quark and gluon distributions in the pion and nucleon are analyzed using the CCFM equations in the single-loop approximation. We utilize the transverse-coordinate (or impact-parameter) representation which diagonalizes the equations and study in detail the dependence on the transverse coordinate induced by the CCFM evolution. We find considerable broadening of the transverse-momentum distributions with an increasing magnitude of the hard scale Q. For instance, at the Bjorken