Quark-gluon Plasma Formation Research Articles

Evidences of quark-gluon plasma formation in central nuclear collisions

Due to the absence of clear and unambiguous theoretical signals of the deconfinement transition from hadron matter to quark-gluon plasma (QGP) the experimental searches of QGP formation are based the analysis of various irregularities in the collision energy dependence of thermodynamic and hydrodynamic quantities. Here we present several remarkable irregularities at chemical freeze-out (CFO) of hadrons which are found using an advanced version of the hadron resonance gas model (HRGM). Among them are the sharp peaks of the trace anomaly and baryonic density which are seen at the center of mass energies √sNN = 4.9 GeV and √sNN = 9.2 GeV, and the two sets of highly correlated quasi-plateaus in the collision energy dependence of the entropy per baryon, total pion number per baryon, and thermal pion number per baryon which we found at the center of mass energies 3.8-4.9 GeV and 7.6-10 GeV. In addition we found a significant change of slope of the hadron yield ratios and , when the center of mass collision energy increases from 4.3 GeV to 4.9 GeV and from 7.6 GeV to 9.2 GeV [1]. The increase of slopes of these ratios at the collision energy interval 4.3-4.9 GeV is accompanied by a dramatic growth of resonance decays at CFO. We argue that such a strong correlation between the previously found irregularities and an enhancement of strangeness production can serve as the quark-gluon plasma formation signature. Hence, we conclude that a dramatic change of the system properties seen in the narrow collision energy range √sNN = 4.3-4.9 GeV may open entirely new possibilities for experimental studies of QGP properties at NICA JINR and FAIR GSI accelerators.

Thermodynamical string fragmentation

The observation of heavy-ion-like behaviour in pp collisions at the LHC suggests that more physics mechanisms are at play than traditionally assumed. The introduction e.g. of quark-gluon plasma or colour rope formation can describe several of the observations, but as of yet there is no established paradigm. In this article we study a few possible modifications to the Pythia event generator, which describes a wealth of data but fails for a number of recent observations. Firstly, we present a new model for generating the transverse momentum of hadrons during the string fragmentation process, inspired by thermodynamics, where heavier hadrons naturally are suppressed in rate but obtain a higher average transverse momentum. Secondly, close-packing of strings is taken into account by making the temperature or string tension environment-dependent. Thirdly, a simple model for hadron rescattering is added. The effect of these modifications is studied, individually and taken together, and compared with data mainly from the LHC. While some improvements can be noted, it turns out to be nontrivial to obtain effects as big as required, and further work is called for.

High baryon densities achievable in the fragmentation regions at RHIC and LHC

We use the McLerran-Venugopalan model of the glasma energy-momentum tensor to compute the rapidity loss and excitation energy of the colliding nuclei in the fragmentation regions followed by a space-time picture to obtain their energy and baryon densities. At the top RHIC energy we find baryon densities up to 3 baryons/fm3, which is 20 times that of atomic nuclei. Assuming the formation of quark-gluon plasma, we find initial temperatures of 200 to 300 MeV and baryon chemical potentials of order 1 GeV. Assuming a roughly adiabatic expansion it would imply trajectories in the T − μ plane which would straddle a possible critical point.

Overview of recent results from the STAR experiment

The Solenoidal Tracker at RHIC (STAR) experiment utilizes its excellent mid-rapidity tracking and particle identification capabilities to study the emergent properties of Quantum Chromodynamics (QCD). The STAR heavy-ion program at vanishingly small baryon density is aimed to address questions about the quantitative properties of the strongly-interacting Quark Gluon Plasma (QGP) matter created in high energy collisions (η/s, qˆ, chirality, transport parameters, heavy quark diffusion coefficients…). At finite baryon density, the questions concern the phases of nuclear matter (the QCD phase diagram) and the nature of the phase transition, namely: what is the onset collision energy for the formation of QGP? What is the nature of phase transition in heavy-ion collisions? Are there two phase transition regions? If yes, where is the critical point situated? At Quark Matter 2015, the STAR collaboration has presented a wealth of new experimental results which address these questions. In these proceedings I highlight a few of those results.

PHENIX results on collectivity tests in high-multiplicity p + p and p + Au collisions at [formula omitted

The observation of possible collective effects in high-multiplicity p+p and p+Pb collisions at the LHC and in d+Au and 3He+Au collisions at RHIC challenge our understanding of the ingredients necessary for quark-gluon plasma formation. For further investigation of these effects, the PHENIX collaboration has taken high statistics data in p+p and p+Au and p+Al collisions in 2015. For these data sets, high-multiplicity triggers were implemented using the forward silicon detector (FVTX) and the beam-beam counter (BBC) covering pseudo-rapidity 1.0<|η|<3.0 and 3.1<|η|<3.9, respectively. The multi-hundred million high-multiplicity event samples recorded enable highly differential analysis to look for collective effects. We report results on large pseudo-rapidity separation correlations to elucidate if the so-called ridge phenomena exists in certain p+p event classes at RHIC. The flow coefficients from azimuthal anisotropies in p+Au are extracted and compared with theoretical expectations in various models, including viscous hydrodynamics where the elliptic flow strength is expected to be substantially smaller than in d+Au and 3He+Au at the same energy due to the smaller initial spatial eccentricity.

Recent developments in the theory of electromagnetic probes in relativistic heavy-ion collisions

The theoretical developments in the study of electromagnetic radiation in relativistic heavy-ion collisions are reviewed. The recent progress in the rates for photon and lepton pair production is discussed. Together with the improvements in the hydrodynamic descriptions of the bulk medium, the combined effort is discussed to resolve the “direct photon flow puzzle” in the RHIC and the LHC experiments. Further prediction of the direct photon production in high multiplicity proton-nucleus collisions at the LHC energy can serve as a signature of the quark gluon plasma formation in these small systems. Phenomenological study of dilepton production at finite net baryon density is highlighted at the collision energies available for the RHIC beam energy scan program.

Quarkonium cross sections and polarizations in pp collisions with CMS

According to NRQCD, quarkonium production starts with the production of a pre-resonant quark-antiquark (QQ‾) pair that then binds into the observed quarkonium state through a possible interaction with surroundings. Understanding this interaction would play a crucial role in the interpretation of the quarkonium suppression patterns as signatures of quark-gluon plasma formation. Unlike the measurements of quarkonium differential cross sections, which reflect partonic processes and are sensitive to initial-state effects, the polarization measurements are unambiguous probes of the final-state quarkonium formation process, determined by the quantum properties of the QQ‾: a change in polarization would be a direct signal, virtually unaffected by initial-state phenomena, of a change in binding mechanism. This report presents the ψ(nS) and ϒ(nS) cross sections and polarizations measured by CMS, using dimuon samples collected in pp collisions at s=7 TeV. The cross sections of five S-wave states extend up to or exceed pT=100 GeV, probing kinematical windows where the theory calculations are supposed to be the most reliable. The polarizations are measured up to high pT at midrapidity, and the Upsilon polarizations are also shown as a function of the number of charged particles produced in the pp collision.

PHENIX Results in d + Au Collisions

The PHENIX experiment at the Relativistic Heavy Ion Collider has performed a comprehensive set of measurements in d + Au collisions. Observables in d + Au collisions were originally conceived as a control experiment where no quark-gluon plasma is formed and one could isolate so-called cold nuclear matter effects, including nuclear modified parton distributions and parton multiple scattering. However, recent data from the PHENIX experiment in d + Au, in conjunction with new p + Pb results at the Large Hadron Collider, give strong evidence for a very different picture. We present new results that hint at the formation of a small quark-gluon plasma, that though short lived, leaves a fingerprint of evidence on final state observables. These new results will be discussed in the context of competing theoretical interpretations.

Flavor dependence of jet quenching in pp collisions and its effect on R AA for heavy mesons

We study the flavor dependence of the medium modification factor $R_{pp}$ for $pp$ collisions for scenario with formation of a small-size quark-gluon plasma (QGP) for RHIC ($\sqrt{s}=0.2$ TeV) and LHC ($\sqrt{s}=2.76$ TeV) energies. We find that at $p_T\sim 10$ GeV the pion spectrum is suppressed by $\sim 20-30\,(25-35)$\% for RHIC (LHC), for $D$~($B$) mesons the suppression effect is smaller by a factor of $\sim 0.7-0.8$ ($0.5$). The flavor hierarchy $R_{pp}^\pi<R_{pp}^D<R_{pp}^B$ is held at $p_T\lesssim 20$ GeV for RHIC and at $p_T\lesssim 70$ GeV for LHC. This gives a sizeable reduction of the heavy-to-light ratios of the nuclear modification factors $R_{AA}$ as compared to that in the standard scenario without the QGP production in $pp$ collisions.

Physics perspectives of heavy-ion collisions at very high energy

Heavy-ion collisions at very high colliding energies are expected to produce a quark-gluon plasma (QGP) at the highest temperature obtainable in a laboratory setting. Experimental studies of these reactions can provide an unprecedented range of information on properties of the QGP at high temperatures. We report theoretical investigations of the physics perspectives of heavy-ion collisions at a future high-energy collider. These include initial parton production, collective expansion of the dense medium, jet quenching, heavy-quark transport, dissociation and regeneration of quarkonia, photon and dilepton production. We illustrate the potential of future experimental studies of the initial particle production and formation of QGP at the highest temperature to provide constraints on properties of strongly interaction matter.

A possible evidence of observation of two mixed phases in nuclear collisions

Using an advanced version of the hadron resonance gas model we have found several remarkable irregularities at chemical freeze-out. The most prominent of them are two sets of highly correlated quasi-plateaus in the collision energy dependence of the entropy per baryon, total pion number per baryon, and thermal pion number per baryon which we found at center of mass energies 3.6-4.9 GeV and 7.6-10 GeV. The low energy set of quasi-plateaus was predicted a long time ago. On the basis of the generalized shock-adiabat model we demonstrate that the low energy correlated quasi-plateaus give evidence for the anomalous thermodynamic properties of the mixed phase at its boundary to the quark-gluon plasma. The question is whether the high energy correlated quasi-plateaus are also related to some kind of mixed phase. In order to answer this question we employ the results of a systematic meta-analysis of the quality of data description of 10 existing event generators of nucleus-nucleus collisions in the range of center of mass collision energies from 3.1 GeV to 17.3 GeV. These generators are divided into two groups: the first group includes the generators which account for the quark-gluon plasma formation during nuclear collisions, while the second group includes the generators which do not assume the quark-gluon plasma formation in such collisions. Comparing the quality of data description of more than a hundred of different data sets of strange hadrons by these two groups of generators, we find two regions of the equal quality of data description which are located at the center of mass collision energies 4.3-4.9 GeV and 10.-13.5 GeV. These two regions of equal quality of data description we interpret as regions of the hadron-quark-gluon mixed phase formation.

Multiplicity and theremalization time in heavy-ions collisions

We present a concise review of quark-gluon plasma formation in heavy-ions collisions within the holographic approach. In particular, we discuss how to get the total multiplicity in heavy ions collision to fit the recent experimental data. We also discuss theoretical estimations of time formation of QGP in heavy ions collision and show that different observables can give the different times of QGP formation.

Classical gluon production amplitude in heavy-ion collisions

The initial condition for the formation of the Quark-Gluon Plasma, the distribution of quarks and gluons produced in the initial stages of nuclear collisions, is the fundamental building block of heavy-ion theory. In the framework of saturation physics and Wilson lines formalism we calculate the scattering amplitude, beyond the leading order, of the classical gluon produced in heavy-ion collisions.known as the initial condition of the Quark-Gluon Plasma formation, is the fundamental building block of heavy-ion theory.

Measurements of Elliptic and Triangular Flow in High-Multiplicity 3He+Au Collisions at √(s(NN))=200 GeV.

We present the first measurement of elliptic (v(2)) and triangular (v(3)) flow in high-multiplicity (3)He+Au collisions at √(s(NN))=200 GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in (3)He+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the (3)He+Au system. The collective behavior is quantified in terms of elliptic v(2) and triangular v(3) anisotropy coefficients measured with respect to their corresponding event planes. The v(2) values are comparable to those previously measured in d+Au collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three (3)He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.

Melting hadrons, boiling quarks

In the context of the Hagedorn temperature half-centenary I describe our understanding of the hot phases of hadronic matter both below and above the Hagedorn temperature. The first part of the review addresses many frequently posed questions about properties of hadronic matter in different phases, phase transition and the exploration of quark-gluon plasma (QGP). The historical context of the discovery of QGP is shown and the role of strangeness and strange antibaryon signature of QGP illustrated. In the second part I discuss the corresponding theoretical ideas and show how experimental results can be used to describe the properties of QGP at hadronization. The material of this review is complemented by two early and unpublished reports containing the prediction of the different forms of hadron matter, and of the formation of QGP in relativistic heavy ion collisions, including the discussion of strangeness, and in particular strange antibaryon signature of QGP.

Observables in ultrarelativistic heavy-ion collisions from two different transport approaches for the same initial conditions

For nucleus-nucleus collisions at Relativistic-Heavy-Ion Collider (RHIC) energies we calculate observables in two conceptually transport theories, i.e. the n-body molecular dynamical model RSP and in the Parton-Hadron-String-Dynamics (PHSD), starting out from the same distribution of the initial energy density at the quark gluon plasma (QGP) formation time. The RSP is based on the Nambu Jona-Lasinio (NJL) Lagrangian whereas in PHSD the partons are described by the dynamical quasi-particle model (DQPM). Despite of the very different description of the parton properties, their interactions and of the hadronization in both approaches the final transverse momentum distributions of pions and kaons are rather similar. Thus it will be difficult to conclude from the final experimenal data on the details of the initial plasma properties.

Photon radiation from hot and dense matter of quark gluon plasma

We work on photon radiation from hot and dense matter of quark gluon plasma (QGP). The total photon spectra of leading order (LO) process is produced in relativistic massive collisions of two nuclei using various value of quark phenomenological flow parameter dependent on quark chemical potential in quark mass. The total photon measurement is strongly increasing function of quark flow parameter ranging 2γg ≤ γq ≤ 8γg and also find appreciable enhancement at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN as a result of high temperature and chemical potential using quark phenomenological flow parameter. The results obtained are presented on the study of photon production of leading order process. Thus, the study of photon radiation from hot and dense matter provides a valuable information in the formation and evolution of QGP and in the subsequent investigation of its properties. The results are compared with other theoretical work.

Formula omitted] production in high energy nuclear collisions

We investigate the production of Ωccc baryon in high energy nuclear collisions via quark coalescence mechanism. The wave function of Ωccc is solved from the Schrödinger equation for the bound state of three charm quarks by using the hyperspherical method. The production cross section of Ωccc per binary collision in a central Pb+Pb collision at sNN=2.76 TeV reaches 9 nb, which is at least two orders of magnitude larger than that in a p+p collision at the same energy. Therefore, it is most probable to discover Ωccc in heavy ion collisions at LHC, and the observation will be a clear signature of the quark–gluon plasma formation.

Measurement of ϒ production in p+p collisions at = 500 GeV in the STAR experiment

Quarkonium suppression was proposed as a signature of the quark-gluon plasma (QGP) formation. Different members of the quarkonium family are expected to melt at different temperatures providing an estimate of the temperature reached in the QGP. The ϒ states offer a clean probe of the QGP, thanks to small secondary production and cold nuclear matter effects. Previous measurements at RHIC focused on colliding system and centrality dependence of ϒ nuclear modification factor RAA. The ϒ RAA as a function of pT will provide additional information about ϒ interaction with the QGP, which requires a pT spectrum in p+p collisions as a baseline. Measurement of the ϒ pT spectrum in p+p collisions at = 500 GeVmay provide such a baseline with high precision. It can be rescaled based on pQCD calculations and used as a baseline for Au+Au collisions at = 200 GeV.In this paper, the status of ϒ measurements in p+p collisions at = 500 GeVin the STAR experiment is presented. In addition, with the help of the newly installed Muon Telescope Detector (MTD) it will be possible to measure ϒ 1S, 2S and 3S states separately.

Shock waves in Lifshitz-like spacetimes

We construct shock waves for Lifshitz-like geometries in four- and fivedimensional effective theories as well as in D3-D7 and D4-D6 brane systems. The solutions to the domain wall profile equations are found. Further, the study makes a connection with the implications for the quark-gluon plasma formation in heavy-ion collisions. According to the holographic approach, the multiplicity of particles produced in heavy-ion collisions can be estimated by the area of the trapped surface formed in shock wave collisions. We calculate the areas of trapped surfaces in the geometry of two colliding Lifshitz domain walls. Our estimates show that for five-dimensional cases with certain values of the critical exponent the dependence of multiplicity on the energy of colliding ions is rather close to the experimental data $$ \mathrm{\mathcal{M}} $$ ∼ s 0.15 observed at RHIC and LHC.