Net Charge Fluctuation and String Fragmentation

We present the simulation results of the net charge fluctuation in Au Au collisions at √Snn=130 GeV from a dynamic model, JPCIAE, and its revisions. The simulations are done for the quark-gluon matter, the directly produced pions, the pion matter, and the hadron matter. The simulated net charge fluctuation of the quark-gluon matter is close to the thermal model prediction for the quark-gluon gas. However, the discrepancy exists comparing the simulated net charge fluctuation for directly produced pions and the pion matter with the thermal model prediction for pion gas and the resonance pion gas, respectively. The net charge fluctuation of hadron matter from default JPCIAE simulations is nearly 3.5 times larger than quark-gluon matter. A discussion is given for the net charge fluctuation as an evidence of QGP phase transition.     

Net Charge Fluctuation and String Fragmentation

We present the simulation results of the net charge fluctuation in Au Au collisions at /Snn=130 GeV froma dynamic model, JPCIAE, and its revisions. The simulations are done for the quark-gluon matter, the directly producedpions, the pion matter, and the hadron matter. The simulated net charge fluctuation of the quark-gluon matter is closeto the thermal model prediction for the quark-gluon gas. However, the discrepancy exists comparing the simulated netcharge fluctuation for directly produced pions and the pion matter with the thermal model prediction for pion gas andthe resonance pion gas, respectively. The net charge fluctuation of hadron matter from default JPCIAE simulations isnearly 3.5 times larger than quark-gluon matter. A discussion is given for the net charge fluctuation as an evidence ofQGP phase transition.     

Phase transition in hot pion matter

The equation of state for the pion gas is analyzed within the third virial approximation. The second virial coefficient is found from the ππ-scattering data, while the third one is considered as a free parameter. The proposed model leads to a first-order phase transition from the pion gas to a more dense phase at the temperature T_pt<136 MeV. Due to relatively low temperature this phase transition cannot be related to the deconfinement. This suggests that a new phase of hadron matter — ‘hot pion liquid’ — may exist.     

Reconstruction of hadronization stage in Pb+Pb collisions at 158 A GeV/c

Recent data on hadron multiplicities in central Pb+Pb collisions at 158 A GeV/c at mid-rapidity are analyzed within the concept of chemical freeze-out. A non-uniformity of the baryon chemical potential along the beam axis is taken into account. An approximate analytical solution of the hydrodynamic equations for a chemically frozen Boltzmann-like gas is found. The Cauchy conditions for hydrodynamic evolution of the hadron resonance gas are fixed at the thermal freeze-out hypersurface from analysis of one-particle momentum spectra and HBT correlations. The proper time of chemical freeze-out and physical conditions at the hadronization stage, such as energy density and averaged transverse velocity, are found.     

3π强子物理和实验

在强子物理研究中,3π产生的理论和实验有非常重要的意义,是目前世界上很多大型实验设备的重要研究对象。3π强子物理包含丰富的物理内容,可以作为探索低能区强相互作用的有力工具。同时,3π产生过程是寻找奇特轻介子态的主要途径之一。另外,通过研究3π产生反应道还可以寻找“失踪”共振态和重子激发态之间的级联衰变。介绍了目前国际各大高能物理实验室的3π产生过程的实验、理论研究以及分波分析技术现状,重点介绍了美国杰弗逊国家实验室（Jefferson Lab,简称JLab）的CLAS（CEBAF Large Acceptance Spectrometer）实验上的3π反应过程。最后,指出了3π强子物理研究的意义和未来的研究方向。Three pion productions off nucleon are of significance in the research of hadron physics. Meanwhile it is the subject of many big experimental apparatus in the world. Due to the variety of three pion productions, it can be applied as an effective tool to study QCD in low energy region. Three pion productions is one of the main reactions in searching light exotic meson state. Moreover, it is possible to find out the "missing" baryon resonance and the cascaded decay process between baryon excited states. We introduce present experiments and theories of three pion productions as well as the partial wave analysis technique. The emphasis is on the CLAS (CEBAF Large Acceptance Spectrometer) experiment at JLab (Jefferson Lab). In the end, we point out the significance of studying the three-pion hadron physics at JLab.     

Feynman’s decoherence

Gell-Mann’s quarks are coherent particles confined within a hadron at rest, but Feynman’s partons are incoherent particles which constitute a hadron moving with a velocity close to that of light. It is widely believed that the quark model and the parton model are two different manifestations of the same covariant entity. If this is the case, the question arises whether the Lorentz boost destroys coherence. It is pointed out that this is not the case, and it is possible to resolve this puzzle without inventing new physics. It is shown that this decoherence is due to the measurement processes which are less than complete.     

The excluded volume hadron gas model and pion production at the SPS

Experimental data for nucleus-nucleus collisions at the CERN SPS suggest that an ideal hadron gas model is unable to account simultaneously (same baryonic chemical potential and temperature at freeze-out) for the strange anti-baryon to baryon ratios and pion abundances. Using a thermodynamically consistent excluded volume model we examine possibilities to account for the observed excess of pions.     

Pion correlations and resonance effects in \bar{p}p annihilation to 4\pi^0 at rest

We study correlations in the exclusive reaction at rest with complete reconstruction of the kinematics for each event. The inclusive distribution is fairly flat at small invariant mass of the pion pair while a small enhancement in the double differential distribution is observed for small invariant masses of both pion pairs. Dynamical models with resonances in the final state are shown to be consistent with the data while the stochastic HBT mechanism is not supported by the present findings. Received: 26 February 2002 / Revised version: 22 July 2002 / Published online: 30 August 2002     

Kaon interferometry

2-kaon and 2-pion correlation functions for an expanding thermalized source are compared. In the Yano-Koonin-Podgoretskii parametrization of the correlation function, the HBT radius parameters are shown to obeyM _∟-scaling in the absence of collective transverse flow. This scaling is broken by transverse flow. An accurate comparison of pion and kaon correlations can thus resolve issue whether the observedM _∟-dependence of the transverse radius parameter is due to transverse collective flow or other transverse gradients. Effects from resonance decays are shortly discussed.     

Single-spin asymmetries for small-angle pion production in high-energy hadron collisions

Within the framework of a simple model, we study single-spin asymmetries for pion production in hadron–hadron collisions at high energies with one hadron polarized. The asymmetries are generated via a mechanism of final- (initial-) state interactions. For peripheral kinematics, we find nonzero asymmetries at the high-energy limit when the pion belongs to the fragmentation region of the polarized proton. Numerical results and comparison with existing experimental data are presented. We also discuss the relationship with Odderon exchange phenomenology. Received: 11 May 1999 / Revised version: 22 July 1999 / Published online: 3 November 1999     

Pion showers in highly granular calorimeters

New results on properties of hadron showers created by pion beam at 8?C80 GeV in high granular electromagnetic and hadron calorimeters are presented. Data were used for the first time to investigate the separation of the neutral and charged hadron showers. The result is important to verify the prediction of the PFA algorithm based up to now on the simulated data only. Next, the properties of hadron showers were compared to different physics lists of GEANT4 version 9.3.     

Hadron production experiments for neutrino physics

Precision neutrino experiments require input from hadron production experiments to be able to reach their design aims. Some of these hadron yield experiments are discussed. In particular the recent pion yield measurement from HARP for the K2K neutrino beam is shown.     

Effect of scattering media on decays of high-energy particles

The effect of multiple scattering on the decay of high-energy particles has been studied. The self-consistent method for the calculation of decay rates of particles undergoing multiple elastic collisions in an equilibrium medium has been developed. Influence of multiple scattering on the decay rate of a neutral pion in a hadron gas has been studied. Zh. éksp. Teor. Fiz. 116, 3–10 (July 1999)     

Intermediate energy bremsstrahlung and the structure of free and bound hadrons: Radiative pion proton scattering,pion decay and muon capture

Experimental data on pion-proton bremsstrahlung, radiative pion decay and radiative muon capture are reviewed to demonstrate that the study of intermediate energy bremsstrahlung processes can reveal additional information on hadron structure inaccessible in their non-radiative counterparts.Dedicated to the memory of M. Gmitro.     

Dynamical models of hadrons based on the string model and the behavior of strongly interacting matter at high density

We propose dynamical models of hadrons, the nucleation model and the free-decay model, in which results of the string model are used to represent interactions. The string model is examined by comparing its predictions with experimental data and parameters are fitted. The equilibrium properties of hadrons at high density are investigated in terms of the nucleation model; we find a singular behavior at energy density 3–5 GeV/fm³, where hadrons coalesce and create highly excited states. We argue that this singular behavior corresponds to the phase transition to quark-gluon plasma. The possibility of observing the production of high-density strongly interacting matter in collider experiments is discussed using the free-decay model, which produces pion distributions as decay products of resonances. We show that our free-decay model recovers features of hadron distributions obtained in hadron collison experiments. Finally, perspectives and extensions of the models are discussed.     

Parametrization of a nonlocal chiral quark model in the instantaneous three-flavor case. Basic formulas and tables

We describe the basic formulation of the parametrization scheme for the instantaneous nonlocal chiral quark model in the three-flavor case. We choose to discuss the Gaussian, Lorentzian-type, Woods-Saxon, and sharp cutoff (NJL) functional forms of the momentum dependence for the form factor of the separable interaction. The four parameters, light and strange quark masses and coupling strength (G _S) and range of the interaction (Λ), have been fixed by the same phenomenological inputs: pion and kaon masses and the pion decay constant and light quark mass in vacuum. The Woods-Saxon and Lorentzian-type form factors are suitable for an interpolation between sharp cutoff and soft momentum dependence. Results are tabulated for applications in models of hadron structure and quark matter at finite temperatures and chemical potentials, where separable models have been proven successfully. The text was submitted by the authors in English.     

Elastic neutrino proton scattering and the structure of the neutral current

Optimal constraints on the structure of a general V, A hadronic neutral current are derived from neutrino proton scattering and compared with corresponding results from inclusive neutrino scattering and single pion production by neutrinos. For an arbitrary axial component of the neutral current, restrictions for the vector coupling constants are obtained. It is shown that the most general neutral current which can be related to charged weak and electromagnetic currents accounts for all existing data on neutrino hadron scattering. The neutral current coupling constants are determined for the pure isovector model, the Salam-Weinberg model and the bottom-quark model. All three models lead to practically the same isovector couplings but they differ in the strength of the isoscalar current.     

Measurement of longitudinal structure of hadron showers up to twenty nuclear interaction lengths

Longitudinal profiles of pion and proton showers are measured, up to 20 nuclear interaction lengths at the energies of 20–180 GeV, Measurements have been performed in the iron-scintillation hadron Tile calorimeter of the ATLAS Project on the beams of the Super Proton Synchrotron (SPS) at the European Organization of Nuclear Research (CERN). Comparative analysis of longitudinal profiles of pion and proton showers was made. The experimental data were parameterized and compared with the results of Monte-Carlo simulation.     

Froissart bound from gluon saturation

We demonstrate that the dipole–hadron cross-section computed from the non-linear evolution equation for the Colour Glass Condensate saturates the Froissart bound in the case of a fixed coupling and for a small dipole (Q²Λ_QCD²). That is, the cross-section increases as the logarithm squared of the energy, with a proportionality coefficient involving the pion mass and the BFKL intercept (_sN_c/π)4ln2. The pion mass enters via the non-perturbative initial conditions at low energy. The BFKL equation emerges as a limit of the non-linear evolution equation valid in the tail of the hadron wavefunction. We provide a physical picture for the transverse expansion of the hadron with increasing energy, and emphasize the importance of the colour correlations among the saturated gluons in suppressing non-unitary contributions due to long-range Coulomb tails. We present the first calculation of the saturation scale including the impact parameter dependence. We show that the cross-section at high energy exhibits geometric scaling with a different scaling variable as compared to the intermediate energy regime.     

Quantitative assessment of target dependence of pion fluctuation in hadronic interactions – estimation through erraticity

Event-to-event fluctuation pattern of pions produced by proton and pion beams is studied in terms of the newly defined erraticity measures χ(p, q), $\chi_q^{\prime}$ and $\mu_q^{\prime}$ proposed by Cao and Hwa. The analysis reveals the erratic behaviour of the produced pions signifying the chaotic multiparticle production in high-energy hadron–nucleus interactions (π ^???–AgBr interactions at 350 GeV/c and p–AgBr interactions at 400 GeV/c). However, the chaoticity does not depend on whether the projectile is proton or pion. The results are compared with the results of the VENUS-generated data for the above interactions which suggests that VENUS event generator is unable to reproduce the event-to-event fluctuations of spatial patterns of final states. A comparative study of p–AgBr interactions and p–p collisions at 400 GeV/c from NA27, with the help of a quantitative parameter for the assessment of pion fluctuation, indicates conclusively that particle production process is more chaotic for hadron–nucleus interactions than for hadron–hadron interactions.