Partonic effects in heavy ion collisions at RHIC

Effects of partonic interactions in heavy ion collisions at RHIC are studied in a multiphase transport model (AMPT) that includes both initial partonic and final hadronic interactions. It is found that a large parton scattering cross section is needed to understand the measured elliptic flow of pions and two-pion correlation function.     

Pseudorapidity dependence of short-range correlations from a multi-phase transport model

Using a multi-phase transport model(AMPT) that includes both initial partonic and hadronic interactions, we study neighboring bin multiplicity correlations as a function of pseudorapidity in Au+Au collisions at (sNN)~(1/2) = 7.7- 62.4 GeV.It is observed that for (sNN)~(1/2) 19.6 GeV Au+Au collisions, the short-range correlations of final particles have a trough at central pseudorapidity, while for (sNN)~(1/2) 19.6 GeV AuAu collisions,the short-range correlations of final particles have a peak at central pseudorapidity. Our findings indicate that the pseudorapidity dependence of short-range correlations should contain some new physical information, and are not a simple result of the pseudorapidity distribution of final particles. The AMPT results with and without hadronic scattering are compared. It is found that hadron scattering can only increase the short-range correlations to some level, but is not responsible for the different correlation shapes for different energies. Further study shows that the different pseudorapidity dependence of short-range correlations are mainly due to partonic evolution and the following hadronization scheme.     

Possible signatures of the hadronisation scale in parton jets

Models for hadron production in hard collisions differ widely in the energy scale characteristic of the transition from the primary partonic to the secondary hadronic phase of jet evolution. We investigate possible experimental signatures for the existence of both phases. In particular, we consider multiplicity and energy moments, long range charge correlations and angular correlations as a function of total energy or near the exclusive two body limit ine ⁺ e ^? annihilation and deep inelastic scattering processes. The possibility of a dual correspondence between hadronic and partonic states is discussed.     

Inclusive heavy flavor hadroproduction in NLO QCD: The exact analytic result

We present the first exact analytic result for all partonic channels contributing to the total cross section for the production of a pair of heavy flavors in hadronic collisions in NLO QCD. Our calculation is a step in the derivation of the top quark pair production cross section at NNLO in QCD, which is a cornerstone of the precision LHC program. Our results uncover the analytical structures behind observables with heavy flavors at higher orders. They also reveal surprising and non-trivial implications for kinematics close to partonic threshold.     

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.     

ee0A GeV S+Au反应中光子产生的低横动量增强

用建立在LUND弦模型特别是PYTHIA事件产生器基础上的描写极端相对论性核一核碰撞的强子和弦级联模型—JPCIAE研究了入射能量为200A GeV的S+ Au中心碰撞中光子产生.模型同时考虑了部分子QCD散射过程、强子末态相互作用以及强子衰变等光子产生反应道并作了协调处理.JPCIAE模型计算结果很好再现了WA93实验数据所呈现的低横动量增强效应.     

Evidence for nonhadronic degrees of freedom in the transverse mass spectra of kaons from relativistic nucleus-nucleus collisions?

We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions within two independent transport approaches that are based on quark, diquark, string, and hadronic degrees of freedom. Both transport models show their reliability for elementary pp as well as light-ion (C+C, Si+Si) reactions. However, for central Au+Au (Pb+Pb) collisions above approximately 5A GeV the measured K+/- transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus, the pressure generated by hadronic interactions in the transport models above approximately 5A GeV is lower than observed in the experimental data. This finding shows that the additional pressure--as expected from lattice QCD calculations at finite quark chemical potential and temperature--is generated by strong partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.     

Examining the crossover from the hadronic to partonic phase in QCD

A mechanism, consistent with color confinement, for the transition between perturbative and physical vacua during the gradual crossover from the hadronic to partonic phase is proposed. The essence of this mechanism is the appearance and growing up of a kind of grape-shape perturbative vacuum inside the physical one. A percolation model based on simple dynamics for parton delocalization is constructed to exhibit this mechanism. The crossover from hadronic matter to sQGP (strongly coupled quark-gluon plasma) as well as the transition from sQGP to weakly coupled quark-gluon plasma with increasing temperature is successfully described by using this model.     

Virtual Compton scattering at low energies with a positron beam

The European Physical Journal A - Using a transport model that includes a first-order chiral phase transition between the partonic and the hadronic matter, we study the development of density...     

Comparative study of the forward and backward methods for calculating jet properties in pp collisions at s~(1/2)=7 TeV

We propose a forward method based on PYTHIA6.4 to study the jet properties in ultra-relativistic pp collisions.In the forward method,the partonic initial states are first generated with PYTHIA6.4 and then hadronized in the Lund string fragmentation model,and finally the hadronic jets are constructed from the created hadrons.Jet properties calculated with the forward method for pp collisions at √s=7 TeV are comparable to those calculated with the usual anti-kt algorithm(backward method)in PYTHIA6.4.The comparison between the backward and forward methods may contribute to the understanding of the partonic origin of jets in the backward method.     

Multiplicity structure of the hadronic final state in diffractive deep-inelastic scattering at HERA

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type , where Y is a hadronic system with mass GeV and where the squared momentum transfer at the pY vertex, t, is limited to , is studied as a function of the invariant mass of the system X. Results are presented on multiplicity distributions and multiplicity moments, rapidity spectra and forward-backward correlations in the centre-of-mass system of X. The data are compared to results in annihilation, fixed-target lepton-nucleon collisions, hadro-produced diffractive final states and to non-diffractive hadron-hadron collisions. The comparison suggests a production mechanism of virtual photon dissociation which involves a mixture of partonic states and a significant gluon content. The data are well described by a model, based on a QCD-Regge analysis of the diffractive structure function, which assumes a large hard gluonic component of the colourless exchange at low . A model with soft colour interactions is also successful. Received: 27 April 1998 / Published online: 3 September 1998     

On AdS/QCD correspondence and the partonic picture of deep inelastic scattering

We critically examine the question of scaling of the Deep Inelastic Scattering process in the medium Bjorken x region on a scalar boson in the framework of the AdS/QCD correspondence. To get the right polarization structure of the forward electroproduction amplitude, we show that one needs to add (at least) the scalar to scalar and scalar to vector hadronic amplitudes. This illustrates how the partonic picture may emerge from a simple scenario based on the AdS/QCD correspondence, provided one allows the conformal dimension of the hadronic field to equal 1 and use the concept of “hadron–parton duality”.     

A general derivation of differential cross section in quark–quark and quark–gluon scatterings at fixed impact parameter

We propose a general derivation of differential cross section in quark–quark and quark–gluon scatterings at fixed impact parameters. The derivation is well defined and free of ambiguity in the conventional one. The approach can be applied to a variety of partonic and hadronic scatterings in low- or high-energy particle collisions.     

On local parton-hadron duality

The multiplicity measure on colour-connected partonic states introduced in [1] is further generalized and we show how to partition the properties of the final state hadronic multiplicity distributions into one part depending upon the partonic cascade and one depending upon the fragmentation inside the Lund model framework. We also show the stability of the measure with respect to variations in the energy and the possible cascade cutoffs and investigate the approximations made in the analytical formulas by means of Monte Carlo simulated events.     

In-medium effects on electromagnetic probes

We discuss some of the aspects of the physics of relativistic nuclear collisions, in particular those having to do with the observation of electromagnetic radiation. We concentrate on what such measurements tell us about the local, in-medium properties of the environment from which they emerge. The contributions from different sources are considered: that from the partonic sector of QCD, and that from the confined hadronic phase. Specifically, we discuss the observation of real photons and of lepton pairs at the SPS and at RHIC, and make predictions for the LHC. The role of jets is discussed.Arrival of the final proofs: 18 July 2005PACS: 25.75.-q, 12.38.Mh     

Violation of KNO scaling and the NBD phenomenon in the framework of the statistical bootstrap model

The connection is considered of multiplicity distributions in three stages: partonic, hadronization, and hadronic. An interpretation of the LoPHD parameter is found. It is shown that under specific hypotheses on the form of the mass spectrum, the statistical bootstrap model leads to the negative binomial distribution (NBD) at the hadronic stage of development of the multiple production process with specific analytic dependences of the parameters of the NBD.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 28–32, May, 1991.One of the authors (V. I. Kuvshinov) is thankful to R. Hagedorn and G. Burgers for numerous discussions.