### Regular Series

#### Vol. 32 (2001), No. 12, pp. 3927 - 4215

Bose–Einstein Correlations in $Z$ Fragmentation and Other Reactions

Acta Phys. Pol. B 32, 3927 (2001)

page 3927 •

abstract

Recent experimental studies of Bose–Einstein Correlations in $Z$ fragmentation are reviewed in view of the need to understand their apparent suppression for pions originating from different $W$’s. Particular features discussed are source elongation, position-momentum correlation, non-Gaussian shape of the correlator, transverse-mass dependence, density dependence and dilution, space–time shape of the emission function, neutral-pion and genuine higher-order correlations.

Correlations in $WW$ Events

Acta Phys. Pol. B 32, 3973 (2001)

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abstract

A critical summary is given of the present status of the study of Bose–Einstein correlations in $W$-pair production at LEP 2. In particular, the evidence is reviewed for or against the existence of Bose–Einstein correlations between pions originating both from a different of the two $W$’s. If present, such an inter-$W$ interference would not only form a potential bias in the determination of the $W$ mass, but also would provide a laboratory to measure the space-time development of the overlap. If absent, this would drastically change the conventional (Hanbury Brown and Twiss) picture of pion interferometry in high energy physics.

Some Questions Concerning Bose–Einstein Correlations in Multiple Particle Production Processes

Acta Phys. Pol. B 32, 3983 (2001)

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abstract

Most models of Bose-Einstein correlations in multiple particle production processes can be ascribed to one of the following three broad classes: models based on the original idea of the Goldhabers, Lee and Pais, hydrodynamic models and string models. We present for discussion some basic questions concerning each of these classes of models.

The Lund Fragmentation of a Multigluon String State

Acta Phys. Pol. B 32, 3993 (2001)

page 3993 •

abstract

I will present a new fragmentation model for a multigluon string state that will exactly fulfil the Area Law that is at the basis of the original (1+1)-dimensional Lund Model. This means that I will have to briefly discuss string motion, in particular the description of the general string. As the string surface is a minimal surface it is mathematically completely determined by its boundary curve and I will show how to use the symmetries of string dynamics to devise a process along this boundary curve. I will also show that the new model is closely related to the $T$ functional and the $\lambda$ measure that we have repeatedly used in investigations in the Lund Group.

QCD Phenomenology and Light-Front Wavefunctions

Acta Phys. Pol. B 32, 4013 (2001)

page 4013 •

abstract

A natural calculus for describing the bound-state structure of relativistic composite systems in quantum field theory is the light-front Fock expansion which encodes the properties of a hadrons in terms of a set of frame-independent $n$-particle wavefunctions. Light-front quantization in the doubly-transverse light-cone gauge has a number of remarkable advantages, including explicit unitarity, a physical Fock expansion, the absence of ghost degrees of freedom, and the decoupling properties needed to prove factorization theorems in high momentum transfer inclusive and exclusive reactions. A number of applications are discussed in these lectures, including semileptonic $B$ decays, two-photon exclusive reactions, diffractive dissociation into jets, and deeply virtual Compton scattering. The relation of the intrinsic sea to the light-front wavefunctions is discussed. Light-front quantization can also be used in the Hamiltonian form to construct an event generator for high energy physics reactions at the amplitude level. The light-cone partition function, summed over exponentially-weighted light-cone energies, has simple boost properties which may be useful for studies in heavy ion collisions. I also review recent work which shows that the structure functions measured in deep inelastic lepton scattering are affected by final-state rescattering, thus modifying their connection to light-front probability distributions. In particular, the shadowing of nuclear structure functions is due to destructive interference effects from leading-twist diffraction of the virtual photon, physics not included in the nuclear light-cone wavefunctions.

Differences in High $p_{\rm T}$ Meson Production Between CERN SPS and RHIC Heavy Ion Collisions

Acta Phys. Pol. B 32, 4069 (2001)

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abstract

In this talk we present a perturbative QCD improved parton model calculation for light meson production in high energy heavy ion collisions. In order to describe the experimental data properly, one needs to augment the standard pQCD model by the transverse momentum distribution of partons (“intrinsic ${k}_{\rm T}$”). Proton–nucleus data indicate the presence of nuclear shadowing and multi-scattering effects. Further corrections are needed in nucleus–nucleus collisions to explain the observed reduction of the cross section. We introduce the idea of proton dissociation and compare our calculations with the SPS and RHIC experimental data.

$J/{\mit \Psi }$ Suppression in an Expanding Hadron Gas

Acta Phys. Pol. B 32, 4079 (2001)

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abstract

A model for $J/{\mit \Psi }$ suppression at a high energy heavy ion collision is presented. The main (and the only) reason for the suppression is $J/{\mit \Psi }$ inelastic scattering within hadron matter. The hadron matter is in the form of a multi-component non-interacting gas. Also the evolution of the gas, both longitudinal and transverse, is taken into account. It is shown that under such circumstances and with $J/{\mit \Psi }$ disintegration in nuclear matter added, $J/{\mit \Psi }$ suppression evaluated agrees well with NA38 and NA50 data.

Dynamically Generating the Quark-Level SU(2) Linear Sigma Model

Acta Phys. Pol. B 32, 4093 (2001)

page 4093 •

abstract

First we study Nambu-type gap equations, $\delta f_\pi \!=\!f_\pi$ and $\delta m_{q}\!=\!m_{q}$. Then we exploit the dimensional regularization lemma, subtracting quadra- tic from log-divergent integrals. The nonperturbative quark loop L$\sigma$M solution recovers the original Gell–Mann–Levy (tree level) equations along with $m_\sigma = 2m_{q}$ and meson-quark coupling $g = 2\pi / \sqrt {N_{\rm c}}$. Next we use the Ben Lee null tadpole condition to reconfirm that $N_{\rm c} = 3$ even through loop order. Lastly we show that this loop order L$\sigma$M (a) reproduces the (remarkably successful) Vector Meson Dominance (VMD) scheme in tree order, and (b) could be suggested as the infrared limit of low energy QCD.

Acta Phys. Pol. B 32, 4105 (2001)

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abstract

In this talk we describe the application of the AdS/CFT correspondence for a confining background to the study of high energy scattering amplitudes in gauge theory. We relate the energy behaviour of scattering amplitudes to properties of minimal surfaces of the helicoidal type. We describe the results of hep-th/0003059 and hep-th/0010069 for amplitudes with vacuum quantum number exchange and, very briefly, hep-th/0110024 on the extension of this formalism to Reggeon exchange.

full authors' list

L. Bogacz, Z. Burda, J. Jurkiewicz, A. Krzywicki, C. Petersen, B. Petersson

Dirac Operator and Ising Model on a Compact 2D Random Lattice

Acta Phys. Pol. B 32, 4121 (2001)

page 4121 •

abstract

Lattice formulation of a fermionic field theory defined on a randomly triangulated compact manifold is discussed, with emphasis on the topological problem of defining spin structures on the manifold. An explicit construction is presented for the two-dimensional case and its relation with the Ising model is discussed. Furthermore, an exact realization of the Kramers–Wannier duality for the two-dimensional Ising model on the manifold is considered. The global properties of the field are discussed. The importance of the GSO projection is stressed. This projection has to be performed for the duality to hold.

Absolute Neutrino Masses

Acta Phys. Pol. B 32, 4169 (2001)

page 4169 •

abstract

Since the recent convincing evidence for massive neutrinos in oscillation experiments, the next task is to determine the absolute masses of neutrinos. A unique pattern of neutrino masses will be hopefully fixed in the future superbeam experiments and neutrino factories. However, the determination of the exact scale is more complicated and depends on the mass of the lightest neutrino $\left ( m_{\nu } \right ) _{\min }$. If $\left ( m_{\nu } \right ) _{\min }\gtrsim 0.35$ eV, the future tritium ${\beta }$ decay experiments (e.g. KATRIN) will have a chance to establish absolute neutrino masses. For smaller masses, 0.004 eV$\leq \left ( m_{\nu } \right ) _{\min }\leq 0.35$ eV, if neutrinos are Majorana particles, an additional information can be derived from the neutrinoless double ${\beta }$ decay $\left ( {\beta } {\beta } \right )_{0 \nu }$ of nuclei and again the absolute neutrino masses can be fixed. If, however, $\left ( m_{\nu } \right ) _{\min }\leq 0.004\;$ eV, none of the present and foreseeable future experiments is known to be able to fix the mass scale.

The Enigmatic Piston

Acta Phys. Pol. B 32, 4183 (2001)

page 4183 •

abstract

The use of ensemble theory to describe systems in thermal equilibrium is justified by the fact that it explains a large variety of experiments. The theoretical understanding, as embodied in the work of Boltzmann and Gibbs, is based on the assumption that all microscopic states with the same energy occur with equal a priori probability. Efforts to explain this assumption or to avoid it by using the microscopic equations of motion, are doomed to fail, because of the extreme complexity of these equations. In the present paper, however, we consider a system for which this complexity is reduced to a minimum. It consists of a finite one-dimensional tube, filled with an ideal gas, in which a piston forms an adiabatic separation between the two parts. Analytical and numerical investigation of this system reveals a very slow approach to a final state in which the piston still performs some non-chaotic motion, which is probably related to the formation of shock fronts. The general question of how much complexity is needed for a system to approach thermal equilibrium is, however, still an open problem.

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