Multiplicities in electroproduction and e+e− annihilation

It is shown how more sophisticated versions of the parton model can replace the naive expectation of final state hadrons being produced in ep or e⁺e^? collisions in two jets of finite multiplicity, well-separated in rapidity, with a picture that permits the rapidity gap to be filled in and allows the multiplicity to rise with energy, even up to the limit imposed by energy conservation. Large multiplicity is shown to imply a rather slow approach to scaling and a significant tail in the transverse momentum distribution.     

Inclusive photoproduction of neutral pions in the photon hemisphere at HERA

The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below GeV. The measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of in the centre-of-mass frame. The transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon. Received: 14 June 2000 / Published online: 27 November 2000     

The rapidity distribution of the cascade inside deuteron in πd high energy collisions

The problem of the existence of internuclear cascade and its characteristics are analysed in the π^? d experiment at 205 GeV/c. Two versions of the additive quark model are used to obtain the mean multiplicity and the rapidity distribution of charged particles produced through cascading. It is shown that the cascading effect does exist. The particles produced through this effect have the rapidity distribution very similar as in elementary low energy hadron-nucleon interactions. Both the rapidity distribution and the mean multiplicity do not depend on the version of the additive quark model which was used to obtain them.     

On multiplicity correlations in the STAR data

The STAR data on the multiplicity correlations between narrow pseudorapidity bins in the pp and AuAu collisions are discussed. The PYTHIA 8.145 generator is used for the pp data, and a na?ve superposition model is presented for the AuAu data. It is shown that the PYTHIA generator with default parameter values describes the pp data reasonably well, whereas the superposition model fails to reproduce the centrality dependence seen in the data. Some possible reasons for this failure and a comparison with other models are presented.     

Transverse-momentum fluctuations in pi(+)p and K(+)p Collisions at 250 GeV/c

We report results on event-by-event fluctuations of transverse momentum, Phi(p(t)), in pi(+)p and K(+)p collisions at 250 GeV/c. For the first time, their dependence on rapidity region, transverse momentum acceptance, multiplicity, mean transverse momentum per event, and on the correlation between transverse momentum and multiplicity are systematically presented. The results are compared with those from the PYTHIA Monte Carlo generator. The fluctuations under the same acceptance cuts as used in current heavy-ion experiments are also presented.     

强子-强子碰撞的快度关联花样

本文利用强子-强子碰撞的PYTHIA模型, 模拟了质心系能量为s=22, 200GeV的质子-质子碰撞. 研究了快度空间中末态粒子在固定bin和任意bin中, 以及相邻bin中的关联花样. 结果发现, 相邻bin的快度关联在中心快度区最强, 向边缘区逐渐减弱; 固定bin和任意bin的关联在固定bin取不同快度位置时, 具有不同的花样.     

Superposition models and the multiplicity fluctuations in heavy-ion collisions

A class of simple superposition models based on the Glauber picture of multiple collisions is compared with the data on the centrality dependence of the multiplicity distributions in a central rapidity bin. We show how the results depend on the specific assumptions concerning the distributions in the number of participants and their relations to the distributions of the number of produced hadrons in various phase space bins. None of the versions of the model describes satisfactorily the centrality dependence of the scaled dispersion.     

高能P■碰撞末态粒子的结团产生和前后关联

本工作在三火球模型的基础上引进结团（ｃｌｕｓｔｅｒ）机制对多种窗口的前后关联进行ＭｏｎｔｅＣａｒｌｏ模拟．与＝２００，５４６，９００ＧｅＶ的实验数据的符合表明了模型的合理性．同时从理论上说明了ｃｌｕｓｔｅｒ机制对多重数关联的影响表现为短程加强，长程减弱。     

The Clustered Production of Final Particles and ForwardBackward Correlation in High Energy pp Collision

In this paper, forward-backward multiplicity correlation in various rapidity windows in high energy p collision is obtained by using Monte Carlo simulation based on Three Pire Ball model with cluster mechanism. The results fit the exprimental data of s=200,546,900GeV, indicating that the model is reasonable. It is shownthat the effect of cluster mechanism makes forward-backward multiplicity correlation increase in short range and decrease in long range.     

Measures of Charge Fluctuation in Nucleonic and Nuclear Collisions

The dependencies of all suggested measures for charge fluctuation on the size of rapidity window are compared using the nucleonic and nuclear collision models—PYTHIA and RQMD. It is found that such dependency is model dependent. However, the charge correlations have the best measure for the effects of global charge conservation and short-range rapidity correlation and is therefore recommended.     

Hadronization from color interactions

A quark coalescence model, based on semi-relativistic molecular dynamics with color interactions among quarks, is presented and applied to pp collisions. A phenomenological potential with two tunable parameters is introduced to describe the color interactions between quarks and antiquarks. The interactions drive the process of hadronization that finally results in different color neutral clusters, which can be identified as hadrons based on some criteria. A Monte Carlo generator PYTHIA is used to generate quarks in the initial state of hadronization, and different values of tunable parameters are used to study the final state distributions and correlations. Baryon-to-meson ratio, transverse momentum spectra, pseudorapidity distributions and forward-backward multiplicity correlations of hadrons produced in the hadronization process, obtained from this model with different parameters, are compared with those from PYTHIA.     

NONDIFFRACTIVE HADRON-HADRON COLLISIONS AT EXTREME EMERGIES

Based on a statistical model which successfully describes the data in energy interval √s=14 GeV to √s=540 GeV, predictions for multiplicity and rapidity distributions at √s=900 GeV, 2TeV and 40 TeV are made.     

Multiplicity Distributions for Different Rapidity Windows in High Energy Collisions

On the assumption that ,under the constraints of energy conservation and transverse-momentum cutoff,the rapidities of produced particles by the fireball are distributed with equal probability,multiplicity distributions for different rapidity windows are calculated by using the multisource model.The calculated results fit the experimental data well.     

Intermittency model for rapidity particle density fluctuations

The one-dimensional model of intermittency is applied to particle density fluctuations in rapidity. Intermittency may explain the “anomalous” events with large density of particles in small rapidity windows and the, approximately energy-independent, linear relation between the average maximum rapidity density and the multiplicity. The connection of “host-spots” in rapidity with large transverse energy jets and the formation of the quark-gluon plasma is briefly discussed.     

Comparison of the Hard and Soft Processes in High Energy Hadron-Hadron Colisions

Using the“con ealgorithm”in QCD to identify jet,the hard and soft processes in high energy hadroncic olisions are analysed indetail.A Monte Carlo simulation event sample is produced by PYTHIA.The resultant pseudorapidity distribution of jets has a plateau in the central region.The particle and transverse energy density in jets decreases exponentially with the increase of the distance from particles to jet axis.Through comparisons,we find the event average transverse momentum or maximum transverse momentum is positively correlated with multiplicity for the whole event sample,and negatively correlated with multiplicity for jet event subsample.Two ratio variables well describe the hardness of jet events,and they are positively correlated with the event average transverse momentum and negatively correlated with multiplicity,which confirms the event average transverse momentum is a better characteristic quantity to describe the hardness of jet events than multiplicity.     

Multiparticle production in hadron-nucleus collisions around 100 GeV

Experimental results of various groups are analysed in order to establish empirical laws on how the multiplicity and rapidity distributions of produced particles depend on the nature of the projectile hadron, its energy E and on the number v of struck nucleons. Global observables of these distributions (mean multiplicity, mean rapidity and variances) are parametrized. The parameters which govern the effects of multiple scattering are found independent of the energy E and of the nature of the hadron. Then, using identical hadron-nucleon input data, we evaluate the predictions of eight theoretical models and compare them with the empirical laws. Some models can be ruled out, while others, though based on very different physical assumptions, are compatible with experiment.     

Semi-inclusive rapidity correlations in cluster emission models

From an approach to multiparticle production in which clusters of hadrons are emitted, the formalism is developed for an analysis of rapidity correlations in the central region at fixed multiplicity. Predictions are given for multiplicity, energy, and rapidity dependence of correlation functions.     

Study on‘soft＇ and ‘hard＇ interactions in ^-pp （pp） collisions using HIJING and PYTHIA

This paper presents a study on ＇hard＇ and ＇soft＇ interactions in ^-pp （pp） collisions using a phenomenological model of HIJING, the jet-cone reconstruction method is employed to select the ＇hard＇ and ＇soft＇ event sub-samples from minimum bias events. It is found that the HIJING model can reproduce the energy scaling behaviour of mean transverse momentum （〈PT〉） distributions of charged hadrons versus multiplicity （Nch） in ＇soft＇ events. From the PYTHIA simulation comparing with the HIJING model, the enhancement of the kaon and proton yields from ＇hard＇ interactions comparing with ＇soft＇ interactions is observed to be due to the mini-jets effect. These mechanisms responsible for the increase of charged hadron＇s （PT） are different in ＇soft＇ and ＇hard＇ interactions.     

Multiplicity distributions in a multiperipheral fireball model

Multiplicity distributions are studied in the thermodynamic model. The approximate Koba, Nielsen, Olesen scaling behaviour of the data and Wroblewski's linear law can be understood in the thermodynamic model with multiperipheral production of fireballs. This production mechanism is also essential to understand two-particle rapidity correlations especially in the ISR energy range. There are contributions from diffractive and non-diffractive production of fireballs in the model, which agrees in this sense with the two-component models used recently to discuss multiplicity distributions. In contrast to other models, however, the non-diffractive component taken separately exhibits approximate KNO-scaling and accounts rather well for the behaviour of the data up to 300 GeV. For the fireball production mechanism choosen the cross section of the diffractive component is expected to be smaller than in other models and can lead to a two-maxima multiplicity distribution only at energies higher than predicted in other models.     

Energy Dependence of String Fragmentation Function and φ Meson Production

The φ meson productions in Au+Au and/or Pb+Pb collisions at AGS, SPS, RHIC, and LHC energies have been studied systematically with a hadron and string cascade model LUCIAE. After considering the energy dependence of the model parameter α in string fragmentation function and adjusting it to the experimental data of charged multiplicity to a certain extent, the model predictions for φ meson yield, rapidity, and transverse mass distributions are compatible with the experimental data at AGS, SPS and RHIC energies. A calculation for Pb+Pb collisions at LHC energy is given as well. The obtained fractional variable in string fragmentation function shows a saturation in energy dependence. It is discussed that the saturation of fractional variable in string fragmentation function might be a qualitative representation of the energy dependence of nuclear transparency.