String Fragmentation Research Articles

Study of nuclear stopping in Au+Au collisions at alternating gradient synchrotron energies by the ultra-relativistic quantum molecular dynamic model

Nuclear stopping in Au+Au collisions at alternating gradient synchrotron energies is studied in the framework of the modified ultra-relativistic quantum molecular dynamics transport model, in which mean field potentials of both formed and “preformed” hadrons (from string fragmentation), medium-modified nucleon-nucleon elastic cross sections, and cluster recognition criteria are considered. It is found that nuclear stopping is influenced by both the mean field potentials of formed and “preformed” hadrons and the medium modification of nucleon-nucleon elastic cross section. The free proton number is higher than that from the experimental rapidity distribution in the central region of rapidity distribution, which can be understood by considering the new criteria of judging fragments.

Thermodynamic Treatment of High Energy Heavy Ion Collision

The hadron production in heavy ion collision is treated in the framework of thermodynamic vision. Thermodynamic system formed during central collision of Pb-Pb at high energies is considered, through which binary collision is assumed among the valance quarks. The partition function of the system is calculated; accordingly the free available energy, the entropy and the chemical potential are calculated. The concept of string fragmentation and defragmentation are used to form the newly produced particles. The average multiplicity of the newly produced particles are calculated and compared with the recent experimental results.

Lambda-to-Kaon Ratio Enhancement in Heavy Ion Collisions at Several TeV

We introduced recently a new theoretical scheme which accounts for hydrodynamically expanding bulk matter, jets, and the interaction between the two. Important for the particle production at intermediate values of transverse momentum (p(t)) are jet hadrons produced inside the fluid. They pick up quarks and antiquarks (or diquarks) from the thermal matter rather than creating them via the Schwinger mechanism-the usual mechanism of hadron production from string fragmentation. These hadrons carry plasma properties (flavor, flow) but also the large momentum of the transversely moving string segment connecting quark and antiquark (or diquark). They therefore show up at quite large values of p(t), not polluted by soft particle production. We will show that this mechanism leads to a pronounced peak in the Lambda-to-kaon ratio at intermediate p(t). The effect increases substantially with centrality, which reflects the increasing transverse size with centrality.

Formation time dependence of femtoscopic ππ correlations in p+p collisions at = 7 TeV

We investigate femtoscopic ππ correlations using the UrQMD approach combined with a correlation afterburner. The dependence of ππ correlations on the charged particle multiplicity and formation time in p+p collisions at = 7 TeV is explored and compared to present ALICE data. The data allow us to constrain the formation time in the string fragmentation to τf ⩽ 0.8 fm/c.

Effects of nuclear absorption on theΛ¯/p¯ratio in relativistic heavy-ion collisions

An enhanced antiLambda/antiproton ratio in heavy-ion relative to p+p collisions has been proposed as one of the signatures for the Quark-Gluon Plasma (QGP) formation. A significantly large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio of 3.5 has been observed in the mid-rapidity and low transverse momentum region in central Au+Au collisions at the nucleon-nucleon center-of-mass energy of 4.9 GeV at the Alternating Gradient Synchrotron (AGS). This is an order of magnitude larger than the values in peripheral Au+Au collisions and p+p collisions at the corresponding energy. By using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport model, we demonstrate that the observed large ratio can be explained by strong absorption of antiprotons (~99.9%) and antiLambdas (~99%) in dense nuclear matter created in central collisions. We find within the model that the initial antiLambda/antiproton ratio, mainly from string fragmentation, does not depend on the collision centrality, and is consistent with that observed in p+p collisions. This suggests that the observed large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio at the AGS does not necessarily imply the formation of the QGP. We further study the excitation function of the ratio in UrQMD, which may help in the search and study of the QGP.

Scaled momentum distributions for $ K_S^0 $KS and $ \Lambda /\bar{\Lambda } $ in DIS at HERA

Abstract Scaled momentum distributions for the strange hadrons $ K_{\text{S}}^0 $ and $ \Lambda /\bar{\Lambda } $ were measured in deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of 330 pb−1. The evolution of these distributions with the photon virtuality, Q 2, was studied in the kinematic region 10 < Q 2 < 40000 GeV2 and 0.001 < x < 0.75, where x is the Bjorken scaling variable. Clear scaling violations are observed. Predictions based on different approaches to fragmentation were compared to the measurements. Leading-logarithm parton-shower Monte Carlo calculations interfaced to the Lund string fragmentation model describe the data reasonably well in the whole range measured. Next-to-leading-order QCD calculations based on fragmentation functions, FFs, extracted from e + e − data alone, fail to describe the measurements. The calculations based on FFs extracted from a global analysis including e + e −, ep and pp data give an improved description. The measurements presented in this paper have the potential to further constrain the FFs of quarks, anti-quarks and gluons yielding $ K_{\text{S}}^0 $ and $ \Lambda /\bar{\Lambda } $ strange hadrons.

Longitudinal correlation of the triangular flow event plane in a hybrid approach with hadron and parton cascade initial conditions

The longitudinal long-range correlations of the triangular flow event plane angles are calculated in a Boltzmann + hydrodynamics hybrid approach. The potential to disentangle different energy deposition scenarios is explored by utilizing two different transport approaches for the early non-equilibrium evolution. In the hadronic transport approach the particle production in high energy heavy ion reactions is mainly governed by string excitation and fragmentation processes which are absent in the parton cascade approach. We find that in both approaches the initial state shows a strong longitudinal correlation of the event plane angles which is diluted but still persists in the final state momentum space distributions of the produced particles. A ridge-like structure can also be caused by near-collinear gluon radiation in a parton cascade approach and does not necessarily prove longitudinal flux tubes in the initial state.

Initial-state fluctuations at the RHIC and the LHC in event-by-event ideal hydrodynamics

Triangular flow is a new observable in heavy ion collisions that provides interesting information about the structure of the initial state that enters (viscous) hydrodynamic simulations. The longitudinal correlations of the initial state triangularity is investigated to evaluate the potential to distinguish different energy deposition scenarios. We find that radiation processes in a parton cascade introduce similar longitudinal structures as the string fragmentation processes in a hadron cascade. The weak centrality dependence of triangular flow is reproduced within an event-by-event hybrid approach and confirms its sensitivity to initial state fluctuations. The transverse momentum-dependent triangular flow of identified particles at LHC energies is predicted and shows a mass splitting similar to the one observed for elliptic flow.

Effect of final state interactions on particle production ind+Aucollisions at energies available at the BNL Relativistic Heavy Ion Collider

We show that particle species dependence of enhanced hadron production at intermediate transverse momentum (${p}_{T}$) for $d+\text{Au}$ collisions at RHIC can be understood in terms of the hadronization from string fragmentation and the subsequent hadronic rescatterings in the final state. A multiphase transport model (AMPT) with two different hadronization mechanisms, string fragmentation or parton coalescence, is used in our study. When the hadrons are formed from string fragmentation, the subsequent hadronic rescatterings will result in particle mass dependence of the nuclear modification factor ${R}_{\mathrm{CP}}$, which is consistent with the present experimental data. On the other hand, in the framework of parton coalescence, the mass dependence disappears and the strangeness plays an important role in hadron production.

Systematic uncertainties in air shower measurements from high-energy hadronic interaction models

Hadronic interaction models at cosmic ray (CR) energies are inherently uncertain due to the lack of a fundamental theoretical description of soft hadronic and nuclear interactions and the large extrapolation required from collider energies to the range of the most energetic cosmic rays observed (>10 20 eV). Model uncertainties are evaluated within the QGSJET-II model, by varying some of the crucial parameters in the limits allowed by collider data, and between QGSJET-II and other models commonly used in air shower simulations. The crucial parameters relate to hard processes, string fragmentation, diffraction and baryon production. Results on inelastic cross sections, on secondary particle production and on the properties of air showers measured by ground detectors from energies of 10 12 to 10 19 eV are presented.

A study of the double hadron neutrino production on nuclei

The nuclear medium influence on the dihadron neutrinoproduction is investigated for the first time, using the data obtained with SKAT bubble chamber. An indication is obtained that the nuclear attenuation of the dihadron is more expressed for kinematically closest hadron pairs. The experimental data on the dihadron attenuation and on the ratio of the dihadron to single-hadron yields are compared with predictions of the two-scale string fragmentation model.

Charged ρ-meson production in neutrino-induced reactions at <E ν >≈ 10 GeV

The neutrinoproduction of charged $\rho$ mesons on nuclei and nucleons is investigated for the first time at moderate energies ($<E_\nu > \approx$ 10 GeV), using the date obtained with SKAT bubble chamber. No strong nuclear effects are observed in $\rho^+$ and $\rho^-$ production. The fractions of charged and neutral pions originating from $\rho$ decays are obtained and compared with higher energy data. From analysis of the obtained and available data on $\rho^+$ and $K^{*+}$(892) neutrinoproduction, the strangeness suppression factor in the quark string fragmentation is extracted: $\lambda_s = 0.18\pm0.03$. Estimations are obtained for cross sections of quasiexclusive single $\rho^+$ and coherent $\rho^+$ neutrinoproduction on nuclei. The estimated coherent cross section $\sigma_{\rho^+}^{coh}$ = (0.29$\pm0.16)\cdot 10^{-38}$ cm$^2$ is compatible with theoretical predictions.

An investigation of hadronization mechanism at a Z 0 factory

We briefly review the hadronization pictures adopted in the LUND String Fragmentation Model (LSFM), Webber Cluster Fragmentation Model (WCFM) and Quark Combination Model (QCM), respectively. Predictions of hadron multiplicity, baryon to meson ratios and baryon-antibaryon flavor correlations, especially those related to heavy hadrons at a Z 0 factory obtained by LSFM and QCM, are reported.

Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160A GeV

Nuclear stopping in heavy-ion collisions over a beam energy range from SIS and AGS up to SPS is studied in the framework of the modified Ultrarelativistic Quantum Molecular Dynamics transport model, in which mean field potentials of both formed and ``preformed'' hadrons (from string fragmentation) and medium-modified nucleon-nucleon elastic cross sections are considered. It is found that nuclear stopping is influenced by both the stiffness of the equation of state and medium modifications of nucleon-nucleon cross sections at SIS energies. At high SPS energies, a two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood by considering the preformed hadron potentials.

Wigner function of produced particles in string fragmentation

We show that QCD4 with transverse confinement can be approximately compactified into QCD2 with a transverse quark mass $m_{{}_T}$ that is obtained by solving a set of coupled transverse eigenvalue equations. In the limits of a strong coupling and a large number of flavors, QCD2 further admits Schwinger QED2-type bosonized solutions. We therefore examine phenomenologically the space-time dynamics of produced particles in string fragmentation by studying the Wigner function of produced bosons in Schwinger QED2, which mimics many features of string fragmentation in quantum chromodynamics. We find that particles with momenta in different regions of the rapidity plateau are produced at the initial moment of string fragmentation as a quark pulls away from an antiquark at high energies, in contrast to classical pictures of string fragmentation with longitudinal space-momentum-time ordering.

Gribov-Regge theory, partons, remnants, strings – and the EPOS model for hadronic interactions

We discuss the key ingredients of realistic hadronic interaction models for air shower developments, like parton evolutions, high parton density effects, remnants, multiple scattering, and string fragmentation. We will focus in particular on the EPOS approach, which has been extensively tested against RHIC, SPS, and TEVATRON data. We will also discuss recent developments concerning collective effects like the hydrodynamic evolution of a quark gluon plasma in hadronic interactions.

Chapter 5 Hadronic Fragmentation

The following sections are included: String Fragmentation in the Lund Model The Lund Area Law Solutions of the Lund Area Law Preliminary Multiplicity Parameter Tuning

A model for net-baryon rapidity distribution

In nuclear collisions, a sizable fraction of the available energy is carried away by baryons. As baryon number is conserved, the net-baryon $B-\bar{B}$ retains information on the energy-momentum carried by the incoming nuclei. A simple and consistent model for net-baryon production in high energy proton–proton and nucleus–nucleus collisions is presented. The basic ingredients of the model are valence string formation based on standard PDFs with QCD evolution and string fragmentation via the Schwinger mechanism. The results of the model are presented and compared with data at different centre-of-mass energies and centralities, as well as with existing models. These results show that a good description of the main features of the net-baryon data is possible in the framework of a simplistic model, with the advantage of making the fundamental production mechanisms manifest.

Study of charm fragmentation into D * ± mesons in deep-inelastic scattering at HERA

The process of charm quark fragmentation is studied using $D^{*\pm}$ meson production in deep-inelastic scattering as measured by the H1 detector at HERA. Two different regions of phase space are investigated defined by the presence or absence of a jet containing the $D^{*\pm}$ meson in the event. The parameters of fragmentation functions are extracted for QCD models based on leading order matrix elements and DGLAP or CCFM evolution of partons together with string fragmentation and particle decays. Additionally, they are determined for a next-to-leading order QCD calculation in the fixed flavour number scheme using the independent fragmentation of charm quarks to $D^{*\pm}$ mesons.

Particles production in expanding color flux tube

This article generalizes Schwinger’s mechanism for particle production in the time dependent field volume in the quasi-static approximation. Solution of DGLAP equations in the double leading log approximation for a small x gluon distribution function was used to derive the new formula for initial chromofield energy density. This initial chromofield energy is distributed among color neutral clusters or strings. These strings are stretched by the receding nucleus. From the proposed mechanism of string fragmentation or color field decay, the new formula for the rapidity spectrum of produced partons was derived.