We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS and LHC energies. Employing the UrQMD hybrid transport model we study the effects of the final hadron/resonance expansion phase on the hadron multiplicities established at hadronization. The bulk meson yields freeze out directly at hadronization whereas the baryon-antibaryon sector is subject to significant alterations, due to annihilation and regeneration processes. We quantify the latter changes by survival factors for each species which are applied to modify the statistical model predictions for the data. The modified SM analysis recovers the hadronization points, which coincide with the recent lattice QCD predictions of the parton-hadron transition line at finite baryochemical potential.
We introduce a transport approach which combines partonic and hadronic degrees of freedom on an equal footing and discuss the resulting reaction dynamics. The initial parton dynamics is modeled in the framework of the parton cascade model, hadronization is performed via a cluster hadronization model and configuration space coalescence, and the hadronic phase is described by a microscopic hadronic transport approach. The resulting reaction dynamics indicates a strong influence of hadronic rescattering on the space-time pattern of hadronic freeze-out and on the shape of transverse mass spectra. …
We argue that a detailed analysis of the spin alignement of vector mesons can serve as a probe of two little-understood aspects of spin dynamics in the vortical fluid: The degree of relaxation between vorticity and parton spin polarization, and the degree of coherence of the hadron wavefunction at freeze-out. We illustrate these with a coalescence model.
We analyze the reaction dynamics of central Pb+Pb collisions at 160 GeV/nucleon. First we estimate the energy density pile-up at mid-rapidity and calculate its excitation function: The energy density is decomposed into hadronic and partonic contributions. A detailed analysis of the collision dynamics in the framework of a microscopic transport model shows the importance of partonic degrees of freedom and rescattering of leading (di)quarks in the early phase of the reaction for E > 30 GeV/nucleon. The energy density reaches up to 4 GeV/fm^3, 95% of which are contained in partonic degrees of fre…
A finite unbound system which is equilibrium in one reference frame is in general nonequilibrium in another frame. This is a consequence of the relative character of the time synchronization in the relativistic physics. This puzzle was a prime motivation of the Cooper--Frye approach to the freeze-out in relativistic hydrodynamics. Solution of the puzzle reveals that the Cooper--Frye recipe is far not a unique phenomenological method that meets requirements of energy-momentum conservation. Alternative freeze-out recipes are considered and discussed.
New solutions of 3+1D covariant kinetic theory are presented for nuclear collisions in the energy domain Ecm ~ 200 AGeV. They are obtained using MPC, a new Monte-Carlo parton transport technique that employs very high parton subdivision that is necessary to preserve covariance. The transport results are compared with ideal hydrodynamics solutions. We show that the transport evolution differs significantly from hydrodynamics. In addition, we compare the transport freeze-out distributions to those obtained from ideal hydrodynamics with the Cooper-Frye isotherm freeze-out prescription. The transp…
An addendum to our previous papers in Phys. Lett. B539 (2002) 46 and Phys. Lett. B502 (2001) 51, contributed to the CERN meeting "First data from the LHC heavy ion run", March 4, 2011
Thermal model fit indicates early chemical freeze-out of multi-strange hadrons with small collective velocities at 200AGeV Au-Au collisions at RHIC. In this work, we present our recent results by SPheRIO hydrodynamical calculations inspired by this picture. In our model, multi-strange hadrons go through chemical freeze-out when the system reaches some temperature close to the phase transition, stopping to make inelastic collisions, and their abundances are therefore determined only by partonic EOS. At a lower temperature thermal freeze-out takes place where elastic collisions are brought to a …
Driven by the Beam Energy Scan (BES) program at the RHIC, researches and discussions on the QCD phase diagram have flourished recently. In order to provide a reference from microscopic transport models, we performed a systematic analysis, using a multiphase transport (AMPT) model for the particle yields and a statistical model (THERMUS) for the thermal fit, for Au+Au collisions at $\sqrt{s_{\text{NN}}}$=7.7-200 GeV. It is found that at a fixed collision centrality the chemical freeze-out parameter, temperature $T_{\text{ch}}$, increases with collision energy and somehow saturates at certain va…
Extracting freeze-out conditions in beam energy scan via functional QCD
Peking University（北京大学）；Justus-Liebig-Universität Gießen（吉森大学）；GSI Helmholtzzentrum für Schwerionenforschung（德国重离子研究中心）；Beijing Institute of Technology（北京理工大学）；Center for High Energy Physics, Peking University（北京大学高能物理中心）；Collaborative Innovation Center of Quantum Matter（量子物质协同创新中心）；Universität Heidelberg（海德堡大学）；ExtreMe Matter Institute EMMI, GSI（GSI极端物质研究所）
The transverse momentum spectra of charged pions, kaons, and protons produced at mid-rapidity in central nucleus-nucleus (AA) collisions at high energies are analyzed by considering particles to be created from two participant partons which are assumed to be contributors from the collision system. Each participant (contributor) parton is assumed to contribute to the transverse momentum by a Tsallis-like function. The contributions of the two participant partons are regarded as the two components of transverse momentum of the identified particle. The experimental data measured in high-energy AA…
Ideally, the Statistical Hadronization Model (SHM) freeze-out curve should reveal the QCD parton-hadron phase transformation line in the ($T$,$μ_B$) plane. We discuss the effects of various final state interaction phenomena, like baryon-antibaryon annihilation, core-corona effects or QCD critical point formation, which shift or deform the SHM freezeout curve. In particular, we present a method to remove the annihilation effects by quantifying them with the microscopic hadron transport model UrQMD. We further discuss the new aspects of hadronization that could be associated with the relatively …
The hadronization scheme for parton transport in relativistic heavy ion collisions is considered in detail. It is pointed out that the traditional scheme for particles being freezed out one by one leads to serious problem on unreasonable long lifetime for partons. A super-cooling of the parton system followed by a collective phase transition is implemented in a simple way. It turns out that the modified model with a global phase transition is able to reproduce the experimental longitudinal distributions of final state particles better than the original one does. The encouraging results indicat…
Employing a perturbative QCD based parton cascade we calculate the elliptic flow $v_2$ and its transverse momentum dependence $v_2(p_T)$ for the gluon matter created in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. To make comparisons with the experimental data at the BNL Relativistic Heavy Ion Collider (RHIC), parton-hadron duality is assumed. We find that whereas the integrated $v_2$ matches the experimental data, the gluon (or pion) $v_2(p_T)$ is about 20-50% smaller than the experimental data. Hadronization via gluon fragmentation and quark recombination seems to be the key to explaining th…
Generalized parton distributions: Theory meets experiment
Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；University of California, Berkeley（加州大学伯克利分校）；RIKEN BNL Research Center and Physics Department, Brookhaven National Laboratory（布鲁克海文国家实验室理研BNL研究中心及物理系）；School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen（香港中文大学（深圳）理工学院）
We consider an idealized situation where the Quark-Gluon Plasma (QGP) is described by a perfect, 3+1 dimensional fluid dynamic model starting from an initial state and expanding until a final state where freeze-out and/or hadronization takes place. We study the entropy production with attention to effects of (i) numerical viscosity, (ii) late stages of flow where the Bag Constant and the partonic pressure are becoming similar, (iii) and the consequences of final freeze-out and constituent quark matter formation.
Hadronic and partonic composition of QCD matter across the crossover
Taras Shevchenko National University of Kyiv（基辅塔拉斯舍甫琴科国立大学）；Bogolyubov Institute for Theoretical Physics（博戈柳博夫理论物理研究所）；Jan Kochanowski University（扬科扎诺夫斯基大学）；University of Houston（休斯顿大学）
We consider a physical scenario for ultra-relativistic heavy-ion collisions where, at the early stage, only transverse degrees of freedom of partons are thermalized, while the longitudinal motion is described by free streaming. When the energy density of the partonic system drops to a certain critical value, the partons hadronize and the newly formed hadronic system freezes out. This sudden change is described with the help of the Landau matching conditions followed by the simulations done with THERMINATOR. The proposed scenario reproduces well the transverse-momentum spectra, the elliptic flo…
The parton rescattering effect on the charged particle production in ultra-relativistic p+p collisions is studied by the parton and hadron cascade model, PACIAE, based on PYTHIA. We have calculated charged particle pseudorapidity density ($dN_{ch}/dη$) at mid-rapidity and the pseudorapidity distribution in inelastic and non-single diffractive p+p collisions at $\sqrt s$=200, 900, 5500, and 14000 GeV with the PYTHIA and PACIAE models. The calculated results of $\sqrt s$=900 GeV are well compared with the ALICE data. The calculated $dN_{ch}/dη$ as a function of center-of-mass energy well meets w…
Jet energy loss in anisotropic plasmas meets limiting attractors
SUBATECH UMR 6457 (IMT Atlantique, Université de Nantes, IN2P3/CNRS)（SUBATECH联合研究单位6457（大西洋理工大学，南特大学，法国国家科学研究中心/法国原子能和替代能源委员会））；Institute for Theoretical Physics, TU Wien（维也纳工业大学理论物理研究所）；Department of Physics, ETH Zürich（苏黎世联邦理工学院物理系）；MIT Center for Theoretical Physics – a Leinweber Institute, Massachusetts Institute of Technology（麻省理工学院理论物理中心——莱因韦伯研究所）
Elliptic flow of hadrons observed at relativistic heavy-ion collision experiments at Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC), provides us an important signature of possible de-confinement transition from hadronic phase to partonic phase. However, hadronization processes of de-confined partons back into final hadrons are found to play a vital role in the observed hadronic flow. In the present work, we use coalescence mechanism also known as Recombination (ReCo) to combine quarks into hadrons. To get there, we have used Boltzmann transport equation in relaxation ti…
We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS and LHC energies. We determine the effects of baryon and antibaryon annihilation and/or regeneration occurring during the final cascade expansion stage of the collisions by deriving "survival factors" for each hadronic species and all energies considered by employing the UrQMD hybrid model. These survival factors are shown to resemble the pattern of data deviation from the statistical equilibrium calculations with the statistical model. We apply them in the SM data analysis, obtaining new points in the $T-μ_B$ plane wh…
In an earlier paper we developed a Parton Bubble Model (PBM) for RHIC, high-energy heavy-ion collisions. PBM was based on a substructure of a ring of localized bubbles (gluonic hot spots) which initially contain 3-4 partons composed of almost entirely gluons. The bubbles emitted correlated particles at kinetic freezeout, leading to a lumpy fireball surface. For a selection of charged particles (0.8 GeV/c $<$ $p_t$ $<$ 4.0 GeV/c), the PBM reasonably quantitatively (within a few percent) explained high precision RHIC experimental correlation analyses in a manner which was consistent with the sma…
In an earlier paper we developed a QCD inspired theoretical parton bubble model (PBM) for RHIC/LHC. The PBM quantitatively agreed with the strong charged particle pair correlations observed by the STAR collaboration at RHIC in the highest energy Au + Au central collisions, and also agreed with the Hanbury Brown and Twiss (HBT) observed small final state source size approximately 2f radii in the transverse momentum range above 0.8 GeV/c. The model assumed a substructure of a ring of localized adjoining 2f radius bubbles(gluonic hot spots) perpendicular to the collider beam direction, centered o…
The widths of charge balance function in high energy hadron-hadron and relativistic heavy ion collisions are studied using the Monte Carlo generators PYTHIA and AMPT, respectively. The narrowing of balance function as the increase of multiplicity is found in both cases. The mean parton-freeze-out time of a heavy-ion-collision event is used as the characteristic hadronization time of the event. It turns out that for a fixed multiplicity interval the width of balance function is consistent with being independent of hadronization time.
We study the particle production in the early stage of the ultrarelativistic heavy-ion collisions. To this end the Boltzmann kinetic equations for gluons and pions with elastic rescattering are considered together with a simple model for the parton-hadron conversion process (hadronisation). It is shown that the overpopulation of the gluon phase space in the initial state leads to an intermediate stage of Bose enhancement in the low-momentum gluon sector which due to the gluon-pion conversion process is then reflected in the final distribution function of pions. This pattern is very similar to …
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 fluctuations in the matter produced in heavy ion collisions as it undergoes the phase transition, and their time evolution in later hadronic stage of the collisions. Using the coalescence model to describe the production of deuterons and tritons from nucleons at the kinetic freeze out, we find that the yield ratio $ N_\text{t}N_\text{p}/ N_\text{d}^2$, where $N_\text{p}$, $N_\text{d}$, and $N_\text{t}$ are, respectively, the proton, deute…
Coupled linear Boltzmann transport and hydrodynamic (CoLBT-hydro) model has been developed for simultaneous simulations of jet propagation and jet-induced medium excitation in heavy-ion collisions. Within this coupled approach, the final reconstructed jets in heavy-ion collisions include not only hadrons from the hadronization of medium modified jet shower partons from the linear Boltzmann transport (LBT) but also hadrons from the freeze-out of the jet-induced medium excitation in the hydrodynamic evolution of the bulk medium. Using the CoLBT-hydro model, we study medium modification of the fr…
Computationally Efficient Description of Medium Response to Jets in Heavy Ion Collisions
Universitat de Barcelona（巴塞罗那大学）；LIP - Laboratório de Instrumentação e Física Experimental de Partículas（粒子物理与实验仪器实验室）；Departamento de Física, Instituto Superior Técnico (IST), Universidade de Lisboa（里斯本大学高等理工学院物理系）；Departamento de Física, Universidad de Oviedo（奥维耶多大学物理系）；Instituto Universitario de Ciencias y Tecnologías Espaciales de Asturias (ICTEA)（阿斯图里亚斯太空科技大学研究所）；Center for Theoretical Physics — A Leinweber Institute, Massachusetts Institute of Technology（麻省理工学院莱因韦伯理论物理中心）；InQubator for Quantum Simulation, Department of Physics, University of Washington（华盛顿大学量子模拟创新中心）
We calculate the single transverse spin asymmetry (STSA) in polarized proton-proton ($p^{\uparrow}+p$) and polarized proton-nucleus ($p^{\uparrow}+A$) collisions ($A_N$) generated by a partonic lensing mechanism. The polarized proton is considered in the quark-diquark model while its interaction with the unpolarized target is calculated using the small-$x$/saturation approach, which includes multiple rescatterings and small-$x$ evolution. The phase required for the asymmetry is caused by a final-state gluon exchange between the quark and diquark, as is standard in the lensing mechanism of Brod…
Within a parton cascade approach we investigate the scaling of the differential elliptic flow $v_2(p_T)$ with eccentricity $ε_x$ and system size and its sensitivity to finite shear viscosity. We present calculations for shear viscosity to entropy density ratio $η/s$ in the range from $1/4π$ up to $1/π$, finding that the $v_2$ saturation value varies by about a factor 2. Scaling of $v_2(p_T)/ε_x$ is seen also for finite $η/s$ which indicates that it does not prove a perfect hydrodynamical behavior, but is compatible with a plasma at finite $η/s$. Introducing a suitable freeze-out condition, we …
Proton-proton (pp) collisions have been traditionally used as a baseline measurement in the search for a deconfined state of matter in heavy-ion collisions at ultrarelativistic energies. The unprecedented collision energies that are available at the Large Hadron Collider (LHC) at the European Laboratory for Nuclear Research (CERN) have illuminated new challenges in understanding the possible formation of droplets of this deconfined matter of partonic degrees of freedom in hadronic collisions, especially in high-multiplicity events. Enhancement of multi-strange particles compared to pions, degr…
At low to moderate collision energies where the parton formation time $τ_F$ is not small compared to the nuclear crossing time, the finite nuclear thickness significantly affects the energy density $ε(t)$ and net conserved-charge densities such as the net-baryon density $n_B(t)$ produced in heavy ion collisions. As a result, at low to moderate energies the trajectory in the QCD phase diagram is also affected by the finite nuclear thickness. Here, we first discuss our semi-analytical model and its results on $ε(t)$, $n_B(t)$, $n_Q(t)$, and $n_S(t)$ in central Au+Au collisions. We then compare t…
We analyze the multiplicity correlations between distant forward and backward rapidity regions in relativistic heavy-ion collisions in a superposition framework, where the particle production occurs through independent emission from correlated sources. This in principle allows for inferring information on the long-range forward-backward correlations of the sources in the earliest phase of the collision, based solely on the experimental information on the statistical features of the observed particle distributions. Our three-stage study incorporates the fluctuations of parton production in the …
We study the effects of friction on the chiral symmetry restoration which may take place temporarily in high energy heavy ion collisions. The equations of motion with friction are introduced to describe the time evolution of the chiral condensates within the framework of the linear $σ$ model. Four types of friction are used to study how the result is sensitive to the choice of the friction. For the thermalization stage, the time dependent temperature is parameterized so as to simulate the result of the parton-cascade model. It is parameterized according to the one dimensional scaling hydrodyna…
Femtoscopy as a New Probe of the Nuclear Equation of State
Central China Normal University（华中师范大学）；GSI Helmholtzzentrum für Schwerionenforschung GmbH（GSI亥姆霍兹重离子研究中心有限公司）；Helmholtz Research Academy Hesse for FAIR (HFHF)（黑森FAIR亥姆霍兹研究院）；Institut für Theoretische Physik, Johann Wolfgang Goethe-Universität（约翰·沃尔夫冈·歌德大学理论物理研究所）
The workshop on "Hadron-Hadron and Cosmic-Ray Interactions at multi-TeV Energies" held at the ECT* centre (Trento) in Nov.-Dec. 2010 gathered together both theorists and experimentalists to discuss issues of the physics of high-energy hadronic interactions of common interest for the particle, nuclear and cosmic-ray communities. QCD results from collider experiments -- mostly from the LHC but also from the Tevatron, RHIC and HERA -- were discussed and compared to various hadronic Monte Carlo generators, aiming at an improvement of our theoretical understanding of soft, semi-hard and hard parton…
