Can we distinguish energy loss from hadron absorption?

Knowing whether a hadron is formed inside or outside the nuclear medium is very important for correctly interpreting jet quenching in heavy-ion collisions. The cleanest experimental environment to study the space-time evolution of hadronization is semi-inclusive DIS on nuclear targets. 2 frameworks are presently competing to explain the observed attenuation of hadron production: quark energy loss, with hadron formation outside the nucleus [2, 3], and nuclear absorption with hadronization starting inside the nucleus [4–6]. I demonstrate that the observed approximate A ^2/3 scaling of experimental data cannot conclusively establish the correctness of either energy loss or absorption.     

Space-time evolution of hadronization

Beside its intrinsic interest for the insights it can give into color confinement, knowledge of the space-time evolution of hadronization is very important for correctly interpreting jet-quenching data in heavy-ion collisions and extracting the properties of the produced medium. On the experimental side, the cleanest environment to study the space-time evolution of hadronization is semi-inclusive deeply inelastic scattering on nuclear targets. On the theoretical side, two frameworks are presently competing to explain the observed attenuation of hadron production: quark energy loss (with hadron formation outside the nucleus) and nuclear absorption (with hadronization starting inside the nucleus). I will discuss recent observables and ideas which will help to distinguish these two mechanisms and to measure the time scales of the hadronization process. PACS 25.30.-c; 25.75.-q; 24.85.+p; 13.87.Fh     

Ratio of a strange quark mass ms to up or down quark mass mu,d predicted by a quark propagator in the frameworkof the chiral perturbation theory

Based on the fully dressed quark propagator and chiral perturbation theory, we study the ratio of the strange quark mass m_s to up or down quark mass m_u,d. The ratio is related to the determination of quark masses which are fundamental input parameters of QCD Lagrangian in the Standard Model of particle physics and can not be directly measured since the quark is confined within a hadron. An accurate determination of these QCD free parameters is extremely important for both phenomenological and theoretical applications. We begin with a brief introduction to the non-perturbation QCD theory, and then study the mass ratio in the framework of the chiral perturbation theory (χ PT) with a parameterized fully dressed quark propagator which describes confining fully dressed quark propagation and is analytic everywhere in the finite complex p²-plane and has no Lehmann representation so there are no quark production thresholds in any theoretical calculations of observable data. Our prediction for the ratio m_s/m_u,d is consistent with other model predictions such as Lattice QCD, instanton model, QCD sum rules and the empirical values used widely in the literature. As a by-product of this study, our theoretical results, together with other predictions of physical quantities that used this quark propagator in our previous publications, clearly show that the parameterized form of the fully dressed quark propagator is an applicable and reliable approximation to the solution of the Dyson-Schwinger Equation of quark propagator in the QCD.     

THE QCD BEHAVIOUR FOR THE VALENCE QUARK DISTRIBUTION FUNCTION OF PION IN THE LEADING LOGARITHM APPROXIMATION AND RELEVANT DIFFERENTIAL CROSS-SECTION

Using the analytic expression for valence quark distribution function of hadron in the leading logarithm approximation given by paper^[11], we discuss the QCD behaviour for the valence quark distribution function of pion. The existing data for valence quark distribution function of pion are explained excellently. We also predict the differential cross section M³ dσ/dM for massive lepton pairr production with Drell-Yan mechanism and compare it with data.     

Parton transverse momentum distribution at the moment of hadronization at RHIC

We extract the transverse momentum distribution of effective partons using the spectra of Ω, Ξ, Λ and ϕ hadrons measured by the STAR Collaboration from Au + Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV at RHIC. The extracted momentum distribution of strange quarks is flatter than that of up/down quarks consistent with hydrodynamics expansion in partonic phase prior to hadronization. Consistency in quark ratios derived from various hadron spectra gives clear evidence for hadron production as suggested by quark coalescence or recombination model.     

Characteristics of heavy flavour production inep collisions

The production of heavy flavours inep collisions at the HERA energy is studied using a model based on boson-gluon fusion into a heavy quark-antiquark pair followed by gluon emission and string hadronization. Total cross-sections and dependence on basic kinematical variables are given as well as distributions of the most important variables at both the quark and hadron level, e.g. rapidity and transverse momentum. Charm, bottom and top production is compared and signatures for their separation are discussed in terms of decay muons and global event shape properties.     

Thermal hadron production in pp and p\bar p collisions

It is shown that the hadron production in high energy pp and $p\bar p$ collisions, calculated by assuming that particles originate in hadron gas fireballs at thermal and partial chemical equilibrium, agrees very well with the data. The temperature of the hadron gas fireballs, determined by fitting hadron abundances, does not seem to depend on the centre of mass energy, having a nearly constant value of about 170 MeV. This value is in agreement with that obtained in e⁺e^?collisions and supports a universal hadronization mechanism in all kinds of reactions consisting in a parton-hadron transition at critical values of temperature and pressure.     

CYLINDRICAL BAG MODEL OF PARTICLE PRODUCTION IN HIGH ENERGY e+e－ ANNIHILATION

The recent experimental data from TASSO and HRS collaborations for multiparticle production in high e⁺e^－ annihilation is analysed in a cylindrical bag model.It is shown that the observed approximate KNO scaling can be explained in terms of bag parameter B,which determines the hadronization probability densities proportional to the square of the materialization energy.     

Spin Transfer in High-Energy Hadronization Processes and Polarization of Hyperon in the Lepton-Induced Reactions

We show that measuring the polarization of hyperon in the lepton induced reactions is an ideal way to study the spin transfer in high energy hadronization processes. After a brief summary of the polarization of the initial quark or antiquark in these reactions,we give the calculation method of the polarization of hyperon. As an example,we calculate polarizations of different hyperons in e⁺e^－ annihilation at LEP energys and compare these results with the available data. The results show that measurements of such polarizations should provide important information about the question of which picture of spin structure of baryon is more suitable in describing the spin effects in the high energy fragmentation processes.     

Fragmentation Character of Resonance Events in BEPC e+e－ Annihilation

In this paper an important character was found that the final hadron multiplicities of direct J/ψ decays satisfy the same universal mass dependence as that in e⁺e^－→q₀q₀ continuum events. This indicates that the fragmentation of quark and gluon to hadron follows the same mechanism. We give a reasonable interpretation to final baryon multiplicity that is enhanced by 2.5 times in direct Y decays compared to the continuum through our calculating results. But we cannot find this similar phenomenon in direct J/ψ decays.     

Color neutralization in deep inelastic scattering on nuclei

Particle production in deep inelastic scattering on nuclei is reduced due to absorption of the produced particles in the nucleus. The photon ejects a quark from a bound nuclon which propagates through the nucleus forming a prehadron before turning into a hadron. Prehadron formation occurs on a shorter length scale than hadron formation and is therefore very important. We explicitly derive aA ^2/3 dependence of nuclear attenuation.     

A constituent quark model for particle production in collisions between hadrons

It is shown that a constituent picture of hadrons, with no other fundamental assumption than that only one constituent in each hadron interacts with one constituent in the other hadron, explains several features of particle production in high-energy collisions. We obtain KNO scaling of the particle multiplicity, a neutral for multiplicity for fixed charged multiplicity which is independent of the energy, and a rapidity distribution of the central fireball which depends on n/〈n〉 only. Numerical applications show that the quark momentum distribution which reproduces the experimental data in strong interactions is very similar to the valence quark momentum distribution derived from deep inelastic scattering experiments.     

Production and hadronization of heavy quarks

Heavy long-lived quarks, i.e. charm and bottom, are frequently studied both as tests of QCD and as probes for other physics aspects within and beyond the standard model. The long lifetime implies that charm and bottom hadrons are formed and observed. This hadronization process cannot be studied in isolation, but depends on the production environment. Within the framework of the string model, a major effect is the drag from the other end of the string that the c/b quark belongs to. In extreme cases, a small-mass string can collapse to a single hadron, thereby giving a non-universal flavor composition to the produced hadrons. We here develop and present a detailed model for the charm/bottom hadronization process, involving the various aspects of string fragmentation and collapse, and put it in the context of several heavy-flavor production sources. Applications are presented from fixed-target to LHC energies. Received: 15 May 2000 / Published online: 9 August 2000     

Kaon and pion ratio probes of jet quenching in nuclear collisions

Non-Abelian energy loss in quark gluon plasmas is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. Here we investigate pion and kaon production in pp and AA collisions in a perturbative QCD frame, suppression pattern and hadron ratios. The K^?/K⁺ and K⁺/π⁺ ratios are found to be most sensitive to the opacity (density) of the plasma. Experimental data indicate that the fragmentation dominated pQCD region will be reached only at higher p _T; in an intermediate p _Tregion other particle production mechanisms dominate the K/π ratios.     

Decays of Dsj*(2317) and Dsj(2460) Mesons in the Quark Model

We study the decay widths of the narrow resonances D_sj^*(2317) and D_sj(2460) in the chiral quark model, together with the well-known D^* and D_s^* mesons. All the parameters in our calculation are taken from Godfrey and Isgur's quark model except the π⁰－η mixing angle which is fixed by the D_s^* decay widths. The calculated electromagnetic decay widths agree with those from other groups and the experimental data available quite well. However, the pionic decay widths of D_sj(2317) and D_sj(2460) are too small to fit the experimental data. We suspect that the simple chiral quark pion axial-vector interaction Hamiltonian is not suitable for hadron strong decays of D_sj(2317) and D_sj(2460).     

Strangeness Suppression in e+e－ Light Flavour Jets

From the simple physical picture of quark combination model, we obtain the generate probabilities of various particles and relative ration in e⁺e^－→q₀q₀→h′s process, and find that the relationship between the ration of strange hadron to unstrange hadron γ and the strangeness suppression factor λ. Our results can be used to explain particle ration enhancement observed in experiments without assumption of strangeness suppression factor enhancement.     

Jet lag effect and leading hadron production

We propose a solution for the long standing puzzle of a too steeply falling fragmentation function for a quark fragmenting into a pion, calculated by Berger [E.L. Berger, Phys. Lett. B 89 (1980) 241] in the Born approximation. Contrary to the simple anticipation that gluon resummation worsens the problem, we find good agreement with data. Higher quark Fock states slow down the quark, an effect which we call jet lag. It can be also expressed in terms of vacuum energy loss. As a result, the space–time development of the jet shrinks and the z-dependence becomes flatter than in the Born approximation. The space–time pattern is also of great importance for in-medium hadronization.     

Hadrons Production from Light Quark Jets in e＋e－ Annihilation

From calculation and analysis of the mean light flavor quark production probability in light quark jet events, e^＋e^－→q₀q₀→h′s, which involves electromagnetic interaction and hadronic interaction, we determine the dependence of the mean production rates of particles in light (uds) quark jets on the primary quark flavor and number of quark pairs N. Moreover, the relative production rates of leading hadrons are obtained, which shows a strong correlation with the quark pair number N. These results provide a basis for furture studies of particle production in light favor jet at low center-of-mass energy range.