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J. Kwiecinski

Publications and source records attributed to J. Kwiecinski.

At least 19 recordsLinked to original sources
arXiv2004

Search for Chargino and Neutralino Production at sqrt(s) = 192-209 GeV at LEP

Approximately 438 pb-1 of e+e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 Gev, are analyzed to search for evidence of chargino pair production, e+e- -> tilde chi^+_1 tilde chi^-_1, or neutralino associated production, e+e- -> tilde chi^0_2 tilde chi^0_1. Limits are set at the 95% confidence level on the product of the cross-section for the process e+e- -> tilde chi^+_1 tilde chi^-_1 and its branching ratios to topologies containing jets and missing energy, of jest with a lepton and missing energy, and on the product of the cross-section for e+e- -> tilde chi^0_2 tilde chi^0_1 and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the tilde chi^+-_1, tilde chi^0_1 and tilde chi^0_2, and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on sigma(e+e- -> tilde chi^+_1 tilde chi^-_1) with the assumption that each chargino decays via a W boson, and on sigma(e+e- -> tilde chi^0_2 tilde chi^0_1) with the tilde chi^0_2 assumed to decay via a Z^0.

hep-ex↗
arXiv2003

Unintegrated CCFM parton distributions and transverse momentum of gauge bosons

Transverse momentum distribution of gauge bosons $W^{\pm}$ and $Z^0$ is calculated using unintegrated parton distributions obtained by solving leading order CCFM equation(s) in the impact parameter space. For illustration we compare the results of the fixed-order collinear, soft-gluon resummation and CCFM approaches. A parameter of a nonperturbative form factor is adjusted to the D0 data. In contrast to the collinear approach, the leading order calculation based on the unintegrated CCFM parton distributions generates transverse momentum distributions of gauge bosons, which are almost consistent with experimental data provided the parameter of the nonperturbative form factor is appropriately adjusted. The $W^+$-$W^-$ asymmetry is calculated as a function of the boson rapidity and transverse momentum. Predictions are given for RHIC and LHC.

hep-ph↗
arXiv2002

Unintegrated gluon distributions in a photon from the CCFM equation in the single loop approximation

The system of CCFM equations for unintegrated parton distributions in a photon is considered in the single loop approximation. We include quarks and non-singular parts of the splitting functions in the corresponding evolution equations. We solve the system of CCFM equations utilising the transverse coordinate representation which diagonalises these equations in the single loop approximation. The results for the unintegrated gluon distributions in a photon are presented and confronted with the approximate form expressing those distributions in terms of the integrated gluon and quark distributions and a suitably defined Sudakov-like form factor.

hep-ph↗
arXiv2002

Two-photon cross-sections from the saturation model

A saturation model for the total gamma-gamma and gamma*-gamma* cross-sections and for the real photon structure function F_2^gamma(x,Q^2) is described. The model is based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property with the saturation radius taken from the GBW analysis of the gamma*-p interaction at HERA. The model is combined with the QPM and non-pomeron reggeon contributions an it gives a very good description of the data on the gamma-gamma total cross-section, on the photon structure function F_2^gamma(x,Q^2) at low x and on the gamma*-gamma* cross-section. Production of heavy quarks in gamma-gamma collisions is also studied.

hep-ph↗
arXiv2002

Saturation model for 2-gamma physics

We introduce a saturation model for photon-photon interactions, based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property. This pomeron-like contribution is supplemented with QPM and non-pomeron reggeon contributions. The model gives a very good description of the data on the gamma-gamma total cross-section, on the photon structure function F_2^gamma(x,Q^2) at low x and on the gamma*-gamma* cross-section.

hep-ph↗
arXiv2002

Geometric scaling and QCD evolution

We study the impact of the QCD DGLAP evolution on the geometric scaling of the gluon distributions which is expected to hold at small x within the saturation models. To this aim we solve the DGLAP evolution equations with the initial conditions provided along the critical line Q^2=Q_s^2(x) where Q_s^2(x) = Q_0^2 x^{-λ}and satisfying geometric scaling. Both fixed and running coupling cases are studied. We show that in the fixed coupling case the geometric scaling at low x is stable against the DGLAP evolution for sufficiently large values of the parameter λand in the double logarithmic approximation of the DGLAP evolution this happens for λ> 4N_cα_s/π. In the running coupling case geometric scaling is found to be approximately preserved at very small x. The residual geometric scaling violation in this case can be approximately factored out and the corresponding form-factor controlling this violation is found.

hep-ph↗
arXiv2002

Small x Phenomenology: Summary and Status

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.

hep-ph↗
arXiv2002

Unintegrated gluon distributions from the transverse coordinate representation of the CCFM equation in the single loop approximation

We utilise the fact that the CCFM equation in the single loop approximation can be diagonalised by the Fourier-Bessel transform. The analytic solution of the CCFM equation for the moments $f_ω(b,Q)$ of the scale dependent gluon distribution is obtained, where $b$ is the transverse coordinate conjugate to the transverse momentum of the gluon. The unintegrated gluon distributions obtained from this solution are analysed. It is shown how the approximate treatment of the exact solution makes it possible to express the unintegrated gluon distributions in terms of the integrated ones. The corresponding approximate expressions for the unintegrated gluon distribution are compared with exact solution of the CCFM equation in the single loop approximation.

hep-ph↗
arXiv2001

Saturation model for two-photon interactions at high energies

We formulate and analyse a saturation model for the total gamma gamma and gamma* gamma* cross-sections and for the real photon structure function F_2^gamma(x,Q^2). The model is based on a picture in which the gamma* gamma* total cross-section for arbitrary photon virtualities is driven by the interaction of colour dipoles, into which the virtual photons fluctuate. The cross-section describing this interaction is assumed to satisfy the saturation property with the saturation radius taken from the Golec-Biernat and Wusthoff analysis of the gamma* p interaction at HERA. The model is supplemented by the QPM and non-pomeron reggeon contributions. The model gives a very good description of the data on the gamma gamma total cross-section, on the photon structure function F_2^gamma(x,Q^2) at low x and on the gamma* gamma* cross-section extracted from LEP double tagged events. Production of heavy flavours in gamma gamma collisions is also studied. Predictions of the model for the very high energy range which will be probed at future linear colliders are given.

hep-ph↗
arXiv2001

Azimuthal decorrelation of forward and backward jets at the Tevatron

We analyse the azimuthal decorrelation of Mueller-Navelet dijets produced in the collisions at Tevatron energies using a BFKL framework which incorporates dominant subleading effects. We show that these effects significantly reduce the decorrelation yet they are still insufficient to give satisfactory description of experimental data. However a good description of the data is obtained after incorporating within formalism the effective rapidity defined by Del Duca and Schmidt.

hep-ph↗
arXiv2001

Gold-plated processes at photon colliders

We review the most important topics and objectives of the physics program of the gamma-gamma, gamma-electron collider (photon collider) option for an e+e- linear collider.

hep-ph↗
arXiv2001

Gluon shadowing in the low x region probed by the LHC

Starting from an unintegrated gluon distribution which satisfies a `unified' equation which embodies both BFKL and DGLAP behaviour, we compute the shadowing corrections to the integrated gluon in the small x domain that will be accessible at the LHC. The corrections are calculated via the Korchegov equation, which incorporates the leading ln(1/x) triple-Pomeron vertex, and are approximately resummed using a simple Pade technique. We find that the shadowing corrections to xg(x,Q^2) are rather small in the HERA domain, but lead to a factor of 2 suppression in the region x \sim 10^-6, Q^2 \sim 4 GeV^2 accessible to experiments at the LHC.

hep-ph↗
arXiv2001

Introduction to low x physics and diffraction

The basic concepts relevant for the theoretical description of deep inelastic scattering within the QCD improved parton model are introduced. Recent developments in low $x$ DIS and in deep inelastic diffraction are briefly summarised. This includes discussion of the BFKL dynamics including the subleading effects and of the saturation model. The dedicated measurements, which probe the QCD pomeron are also discussed.

hep-ph↗
arXiv2000

QCD predictions for spin dependent photonic structure function $g_1^γ(x,Q^2)$ in the low x region of future linear colliders

Spin dependent structure function $g_1^γ(x,Q^2)$ of the polarised photon is analysed within the formalism based upon the unintegrated spin dependent parton distributions incorporating the LO Altarelli-Parisi evolution and the double $ln^2(1/x)$ resummation at low values of Bjorken parameter x. We analyse the effects of the double $ln^2(1/x)$ resummation on the behaviour of $g_1^γ(x,Q^2)$ in the low x region which may be accessible in future linear $e^+e^-$ and $eγ$ colliders. Sensitivity of the predictions on the possible nonperturbative gluon content of the polarised photons is analysed. Predictions for spin dependent gluon distribution $Δg^γ (x,Q^2)$ are also given.

hep-ph↗
arXiv2000

Geometric scaling for the total gamma^* p cross section in the low x region

We observe that the saturation model of deep inelastic scattering, which successfully describes inclusive and diffractive data at small x, predicts a geometric scaling of the total gamma^* p cross section in the region of small Bjorken variable x. The geometric scaling in this case means that the cross section is a function of only one dimensionless variable tau = Q^2 R_0^2(x), where the function R_0(x) (called saturation radius) decreases with decreasing x. We show that the experimental data from HERA in the region x<0.01 confirm the expectations of this scaling over a very broad region of Q^2. We suggest that the geometric scaling is more general than the saturation model.

hep-ph↗
arXiv2000

Theoretical description of the total gamma*gamma* cross-section and its confrontation with the LEP data on doubly tagged e+e- events

We perform a detailed analysis of the total inelastic cross-section for gamma*gamma* collisions. Different contributions coming from the quark box diagram, reggeons, the soft and hard pomeron are included. The QCD pomeron contribution contains a dominant part of subleading effects which reduces its intercept and delays the onset of the asymptotic pomeron to high energies. Estimates of the cross-section for doubly-tagged (e+e- to e+e- hadrons) events are presented and compared with the existing LEP data. Good agreement between the theoretical results and the experimental data is found. We also comment on the extraction of the BFKL pomeron intercept from the available LEP measurements.

hep-ph↗
arXiv2000

QCD at Photon Colliders

The novel possibilities of probing the photon structure and high energy limit of QCD at photon colliders are summarised. We discuss the photon structure function $F_2^γ(x,Q^2)$, the gluon distribution in the photon and the spin dependent structure function $g_1^γ(x,Q^2)$ of the photon and emphasise advantages of the photon colliders for measuring these quantities. The possibility of probing the QCD pomeron and odderon in $γγ$ and $e γ$ collisions is also described.

hep-ph↗