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J. Kwiecinski

Publications and source records attributed to J. Kwiecinski.

At least 37 records · Page 2Linked to original sources
arXiv2000

Parametrisation of F_2^gamma at low Q^2 and of sigma(gamma,gamma) and sigma(gamma^*,gamma) at high energies

A parametrisation of the real photon structure function F_2^gamma in the low Q^2, low x region is formulated. It includes both the VMD and the QCD components, the latter suitably extrapolated to the low Q^2 region and based on arbitrary parton distributions in the photon. The parametrisation used together with the GRV and GRS' parton densities describes reasonably well the existing high energy data on F_2^gamma, sigma(gamma,gamma) and the low Q^2 data on sigma(gamma^*, gamma). Predictions for sigma(gamma,gamma) and for sigma(gamma^* ,gamma) for energies which may become accessible in future linear colliders are also given.

hep-ph↗
arXiv2000

The unintegrated gluon distribution from the CCFM equation

The gluon distribution f(x, k_t^2,mu^2), unintegrated over the transverse momentum k_t of the gluon, satisfies the angular-ordered CCFM equation which interlocks the dependence on the scale k_t with the scale μof the probe. We show how, to leading logarithmic accuracy, the equation can be simplified to a single scale problem. In particular we demonstrate how to determine the two-scale unintegrated distribution f(x,k_t^2,mu^2) from knowledge of the integrated gluon obtained from a unified scheme embodying both BFKL and DGLAP evolution.

hep-ph↗
arXiv2000

Ultrahigh energy neutrino physics

We discuss a problem concerning the ultrahigh energy neutrino propagation through the Earth. We present calculation of the neutrino-nucleon cross section at high energies, based on the unifed evolution equation at small x. We also show the solution of the transport equation for different neutrino fluxes originating from active galactic nuclei, gamma ray bursts and top-down model.

hep-ph↗
arXiv2000

The QCD pomeron at TESLA - motivation and exclusive J/psi production

We briefly present the motivation for studying the processes mediated by the QCD pomeron at high energy e+e- colliders. We describe the behaviour of the cross-section for the reaction gamma gamma to J/psi J/psi obtained from the BFKL equation with dominant non-leading corrections. We give the predictions for the rates of anti-tagged e+e- to e+e- J/psi J/psi events in TESLA and conclude that such reactions may be excellent probes of the hard pomeron.

hep-ph↗
arXiv1999

QCD predictions for polarised deep inelastic scattering accompanied by a forward jet in the low x region of possible HERA measurements

We estimate the cross-section and asymmetry relevant for the production of forward jets in polarised deep inelastic scattering in the region of small values of $x$ which can be probed at possible polarised HERA measurements. The kinematical cuts implemented in our analysis are the same as those which were used in the unpolarised deep inelastic scattering at HERA. The calculations are based on the double ln^2(1/x) resummation which controls the polarised deep inelastic scattering for small values of the Bjorken parameter x. We show that this resummation substantially enhances the corresponding cross-section and asymmetry. The predicted value of the asymmetry is found to vary between -0.01 and -0.04 within the small x region which can possibly be probed at polarised HERA measurements.

hep-ph↗
arXiv1999

Spin dependent structure function g_1 at low x and low Q^2

Theoretical description of the spin dependent structure function g_1(x,Q^2) in the region of low values of x and Q^2 is presented. It contains the Vector Meson Dominance contribution and the QCD improved parton model suitably extended to the low Q^2 domain. Theoretical predictions are compared with the recent experimental data in the low x, low Q^2 region.

hep-ph↗
arXiv1999

Polarised deep inelastic scattering accompanied by a forward jet as a probe of the $\ln^2(1/x)$ resummation

We argue that the production of forward jets in polarised deep inelastic scattering can be a useful tool for probing the double $ln^2(1/x)$ resummation effects which control the polarised deep inelastic scattering for small values of the Bjorken parameter $x$. We solve the corresponding integral equations generating the double $ln^2(1/x)$ resummation and calculate the differential structure function describing the forward jet production in the small $x$ regime which can possibly be probed by the polarised HERA measurements. We show that these structure functions should exhibit the characteristic increase with decreasing $x/x_J$, where $x_J$ denotes the longitudinal momentum fraction of the parent proton carried by a jet, and we quantify this increase.

hep-ph↗
arXiv1999

Probing the QCD pomeron in doubly tagged e^+ e^- collisions

We calculate the total cross-section for $γ^* γ^*$ collisions and for the process $e^+ e^- \to e^+ e^- + hadrons$ with two tagged leptons assuming dominance of the QCD pomeron exchange. We solve the BFKL equation including the dominant subleading effects generated by the consistency constraint which restricts the available phase-space of the emitted gluons to the region inwhich the virtuality of the exchanged gluons is dominated by their transverse momentum squared. Estimate of the possible soft pomeron contribution to the $γ^* γ^*$ cross-section is also presented. At very high CM energies $W$ the calculated total $γ^* γ^*$ cross-section exhibits effective power-law $(W^2)^λ$ behaviour with $λ\sim 0.3$. We confront our results with the recent measurements at LEP and give predictions for the energies which can be accessible at TESLA and at other future linear $e^+ e^-$ colliders.

hep-ph↗
arXiv1999

Penetration of the Earth by ultrahigh energy neutrinos and the parton distributions inside the nucleon

In this talk we would like to present recent calculations of the cross sections for the ultrahigh energy neutrinos interacting with nucleons. We briefly present the framework of unifed BFKL/DGLAP equations which resums all the leading log(1/x) effects as well as the subleading terms. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. We then use these parton distributions to calculate the cross section for the neutrino-nucleon interactions at very high energies, up to 10^12 GeV. We do also investigate the attenuation of neutrinos when traversing through the Earth. We use the transport equation which we solve for different fluxes of high energy neutrinos (Active Galactic Nucleai, Gamma Ray Bursts, and top-down models). We study the effects of the regeneration of the neutrino flux via neutral current interaction.

hep-ph↗
arXiv1999

Small x QCD effects in DIS with a forward jet or a forward pi0

We compare predictions based on small x (QCD) dynamics with recent data for deep inelastic events containing forward jets or forward pi0 mesons. We quantify the effect of imposing the (higher order) consistency condition on the BFKL equation and study uncertainties inherent in the QCD predictions. We also estimate the cross-section for the forward production of two jets.

hep-ph↗
arXiv1999

The QCD pomeron in e+e- collisions

The contribution of the QCD pomeron to the processes: e+e- to e+e- J/psi J/psi and e+ e- to e+ e- hadrons (with tagged electrons) is discussed. We focus on reactions which occur via photon-photon collisions, with virtual photons coming from the Weizsaecker-Wiliams spectrum of the electrons. We stress the importance of the non-leading corrections to the BFKL equation and take 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. The γ* γ* cross-sections are found to increase with increasing γ* γ* CM energy W as (W^2)^{λ_P} while the cross-section for γγto J/psi J/psi is found to increase as (W^2)^{2λ_P}. The parameter λ_P is slowly varying with energy W and takes the values lambda_P \sim 0.23 - 0.35 depending on the process. We also analyze the contribution of the soft pomeron for the total γ* γ* cross-section. We compare results of our calculations to the recent data from LEP.

hep-ph↗
arXiv1999

Predictions for dijet production in DIS using small x dynamics

We study the properties of dijet production in deep inelastic scattering using a unified BFKL/DGLAP framework, which includes important subleading ln (1/x) contributions. We calculate the azimuthal decorrelation between the jets. We compute the cross section for dijet production as a function of Q^2 and the jet transverse momentum, as well as calculate the total dijet rate. We compare the predictions with HERA data.

hep-ph↗
arXiv1999

F_2^gamma at low Q^2 and sigma_{γγ} at high energies

The parametrisation of the photon structure function in the low Q^2 region is formulated. It includes the VMD contribution and the QCD improved parton model component suitably extrapolated to the low Q^2 region. The parametrisation describes reasonably well existing experimental data on sigma_{γγ} for real photons and the low Q^2 data on sigma_{γ^* γ}. Predictions for sigma_{γγ} and for sigma_{γ^* γ} for energies which may be accesssible in future linear colliders are also given.

hep-ph↗
arXiv1999

QCD expectations for the spin structure function $g_1$ in the low $x$ region

The structure function $g_1(x,Q^2)$ is analysed within the formalism based on unintegrated spin dependent parton distributions incorporating the LO Altarelli-Parisi evolution and the double $ln^2(1/x)$ resummation at low $x$. We quantitatively examine possible role of the latter for the small $x$ behaviour of the structure functions $g_1$ of the nucleon within the region which may be relevant for the possible polarized HERA measurements. We show that while the non-singlet structure function is dominated at low $x$ by ladder diagrams the contribution of the non-ladder bremsstrahlung terms is important for the singlet structure function. Predictions for the polarized gluon distribution $ΔG(x,Q^2)$ at low $x$ are also given.

hep-ph↗
arXiv1998

Penetration of the Earth by ultrahigh energy neutrinos predicted by low x QCD

We calculate the cross sections for neutrino interactions with (isoscalar) nuclear targets in the energy domain all the way up to 10^12 GeV. Small x QCD effects are included by using a unified BFKL/DGLAP formalism which embodies non-leading log(1/x) contributions. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. The attenuation of neutrinos transversing the Earth at different nadir angles is calculated for a variety of energy spectra for neutrinos originating from different sources (from Active Galactic Nuclei, Gamma ray bursts, top-down models), as well as for atmospheric neutrinos. For this purpose we solve the transport equation which includes regeneration due to neutral current neutrino interactions, besides attenuation.

astro-ph↗
arXiv1998

Diffractive dijet photoproduction as a probe of the off-diagonal gluon distribution

We propose exclusive diffractive dijet photoproduction as an ideal measure of the off-diagonal gluon distribution at high scales. We solve the off-diagonal evolution equations for the gluon and quark singlet over the full kinematic domain. We discuss the nature of the solutions of these equations, which embody both DGLAP and ERBL evolution. We give predictions for the transverse momentum distribution of the jets. In particular we quantify the enhancement arising from the evolution of the off-diagonal parton distributions.

hep-ph↗
arXiv1998

Spin Dependent Structure Function $g_1$ at Low $x$

Theoretical expectations concerning small $x$ behaviour of the spin dependent structure function $g_1$ are summarised. This includes discussion of the Regge pole model predictions and of the double $ln^2(1/x)$ effects implied by perturbative QCD. The quantitative implementation of the latter is described within the unified scheme incorporating both Altarelli-Parisi evolution and the double $ln^2(1/x)$ resummation. The double $ln^2(1/x)$ effects are found to be important in the region of $x$ which can possibly be probed at HERA. Predictions for the polarized gluon distribution $ΔG(x,Q^2)$ at low $x$ are also given.

hep-ph↗