Quark-gluon Plasma Droplets Research Articles

Probing the shape of the quark-gluon plasma droplet via event-by-event quark-gluon plasma tomography

This study investigates quark-gluon plasma in heavy-ion collisions through two avenues: high-p⊥ frameworks and hydrodynamic modeling. Using the TRENTo model, we find that the p=0 value aligns well with high-p⊥ data, in agreement with Bayesian analysis of the low-p⊥ regime. While adjusting p values may improve a fit to a particular high-p⊥ observable, it does not permit an earlier onset of transverse expansion. Published by the American Physical Society 2024

Use of the Meta-Analysis and Kolmogorov Criteria in the Finding of Singularities of a Nuclear Matter Created in Ultra-Relativistic Nuclear Collisions

Published theoretical data from several models – PHSD/HSD both with and without chiral symmetry restoration (CSR), applied to experimental data on nuclear collisions from BEVALAC/SIS to LHC energies were analyzed using meta-analysis and Kolmogorov criteria. This made it possible to localize possible features of nuclear matter created in central nucleus-nucleus collisions. Ignition of a drop of quark-gluon plasma (QGP) begins already at an energy of about √s<SUB>NN</SUB> = 2 GeV. The assessment showed that this QGP droplet occupies a small fraction, 15% (average radius of about 5.3 fm, if the fireball radius is 10 fm), of the total volume of the fireball created at √s<SUB>NN</SUB> = 2.7 GeV. A drop of exotic matter undergoes a split phase transition – separated boundaries of sharp (1<sup>st</sup> order) crossover and CSR in chiral limit, between QGP and Quarkyonic matter at an energy about √s<SUB>NN</SUB> = 3.5 GeV. The critical endpoint of 2<sup>nd</sup> order probably cannot be reached in nuclear collisions. The triple phase area occupies interval from √s<SUB>NN</SUB> =12 GeV to 15 GeV, the critical endpoint of 1<sup>st</sup> order – at around √s<SUB>NN</SUB> = 20 GeV. The boundary of smooth (2<sup>nd</sup> order) crossover transition with CSR in chiral limit between Quarkyonic matter and QGP was localized between √s<SUB>NN</SUB> = 9.3 GeV and 12 GeV, and between Hadronic and QGP in the interval from √s<SUB>NN</SUB> = 15 GeV to 20 GeV, the boundary of sharp (1<sup>st</sup> order) crossover transition with CSR in chiral limit between Hadronic matter and QGP was localized after √s<SUB>NN</SUB> = 20 GeV. The phase trajectory of the hadronic corona, enveloping the exotic droplet, always remains in the hadronic phase. The possible phase diagram of nuclear matter created in mid-central heavy ion collisions is also presented in the same energy range as for central collisions. Taking into account the quantum nature of the fireball created in nuclear collisions, emphasis is on the existence of events in central nuclear collisions at energy range from √s<SUB>NN</SUB> = 2 GeV to 2.76 TeV, at which no exotic matter is created and nuclear matter in the fireball remains in the hadronic phase throughout its (fireball) evolution.

Strong Constraints on Jet Quenching in Centrality-Dependent p+Pb Collisions at 5.02TeV from ATLAS.

Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including pp and p+Pb collisions. In contrast, while jet quenching is observed in Pb+Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb^{-1} of p+Pb and 3.6 pb^{-1} of pp collisions at 5.02TeV. The yields of charged hadrons with p_{T}^{ch}>0.5 GeV near and opposite in azimuth to jets with p_{T}^{jet}>30 or 60GeV, and the ratios of these yields between p+Pb and pp collisions, I_{pPb}, are reported. The collision centrality of p+Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The I_{pPb} values are consistent with unity within a few percent for hadrons with p_{T}^{ch}>4 GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p+Pb collisions.

Searching for QGP droplets with high- pT hadrons and heavy flavor

The search for the smallest quark-gluon plasma (QGP) droplets in nature has motivated recent small collisions system programs at RHIC and LHC. Unambiguous identification of jet quenching due to final-state interactions is key to confirming QGP formation in these reactions. We compute the nuclear modification factors $R_{AA}$ and $R_{p(d)A}$ of charged hadrons and heavy flavor mesons in large (Au-Au, Xe-Xe, Pb-Pb) and small ($d$-Au, $p$-Pb, O-O) colliding systems, respectively. Our results include the Cronin effect and initial-state parton energy loss in cold nuclear matter. In the final state, hard partons undergo collisional energy loss and branching that was recently derived using Soft-Collinear-Effective-Theory with Glauber Gluon (SCET$_{\rm G}$). In large colliding systems, medium-modified QCD evolution of the fragmentation functions dominates the nuclear correction. As the system size decreases, we find that cold nuclear matter effects, collisional energy loss, and QGP-induced radiations can become equally important. A systematic scan over the medium size and mass/flavor dependence of $R_{AA}$ provides the opportunity to separate these individual contributions and identify QGP signatures in small systems. Predictions for $R_{AA}^{h}$, $R_{AA}^{D}$, $R_{AA}^{B}$ in O-O collisions at $\sqrt{s}=7$ TeV are presented with and without the formation of a QGP and contrasted with the corresponding $R_{p(d)A}$ calculations. Upcoming single-hadron measurements at the LHC will not only test the O-O predictions for both light and heavy flavor production, but will shed light on the possibly very different dynamics of $p$-A and A-A reactions at similar soft particle production multiplicities.

Holographic entropy production in a Bjorken expanding hot and dense strongly coupled quantum fluid

The authors study the dynamics of droplets of quark-gluon plasma created in heavy iron collisions. The relativistic quantum fluid undergoes Bjorken flow expansion and is modeled as conformal $N=4$ supersymmetric Yang-Mills plasma. In particular, they investigate the holographic entropy production far from equilibrium where the horizon area is no longer a proper measure for the entropy but the area of a dynamic apparent horizon is assumed to be the right dual observable.

Three loop correction in the formation of QGP droplet

Quark–gluon plasma (QGP) droplet formation is re-considered with the addition of three loop correction to the earlier loop factors in the mean field potential. The correction of the three loop factor increases stability in the droplet formation of QGP at different parametrization factors of the QGP fluid and it is in a better agreement in comparison with the lattice results of pressure, energy density and other thermodynamic relations. This implies that the contribution of the three loop enhances in showing the characteristic features of the QGP fluid. It also shows that increasing the loop increased the strength of parametrization value defined in earlier papers as a number parameter of fluid dynamics. It indicates that the model with the loop correction boosts in explaining about the formation of QGP droplet in the expansion of early universe.

Freezing out fluctuations in Hydro+ near the QCD critical point

We introduce a freeze-out procedure to convert the critical fluctuations in a droplet of quark-gluon plasma (QGP) that has, as it expanded and cooled, passed close to a posited critical point on the phase diagram into cumulants of hadron multiplicities that can subsequently be measured. The procedure connects the out-of-equilibrium critical fluctuations described in concert with the hydrodynamic evolution of the droplet of QGP by extended hydrodynamics, known as Hydro+, with the subsequent kinetic description in terms of observable hadrons. We introduce a critical scalar isoscalar field sigma whose fluctuations cause correlations between observed hadrons due to the couplings of the sigma field to the hadrons via their masses. We match the QGP fluctuations obtained by solving the Hydro+ equations describing the evolution of critical fluctuations before freeze-out to the correlations of the sigma field. In turn, these are imprinted onto correlations and fluctuations in the multiplicity of hadrons, most importantly protons, after freeze-out via the generalization of the familiar half-century-old Cooper-Frye freeze-out prescription which we introduce. The proposed framework allows us to study the effects of critical slowing down and the consequent deviation of the observable predictions from equilibrium expectations quantitatively. We also quantify the suppression of cumulants due to conservation of baryon number. We demonstrate the procedure in practice by freezing out a Hydro+ simulation in an azimuthally symmetric and boost invariant background that includes radial flow discussed in arXiv:1908.08539.

Heavy and light flavor jet quenching in different collision systems at energies available at the CERN Large Hadron Collider

Recent experiments have observed large anisotropic collective flows in high multiplicity proton-lead collisions at the CERN Large Hadron Collider (LHC), which indicates the possible formation of mini quark-gluon plasma (QGP) in small collision systems. However, no jet quenching has been confirmed in such small systems so far. To understand this intriguing result, the system size scan experiments have been proposed to bridge the gap between large and small systems. In this work, we perform a systematic study on both heavy and light flavor jet quenching in different collision systems at the LHC energies. Using our state-of-the-art jet quenching model, which combines the next-to-leading-order perturbative quantum chromodynamics framework, a linear Boltzmann transport model and the ($3+1$)-dimensional viscous hydrodynamics simulation, we provide a good description of nuclear modification factor ${R}_{\mathrm{AA}}$ for charged hadrons and $D$ mesons in central and midcentral $\mathrm{Pb}+\mathrm{Pb}$ and $\mathrm{Xe}+\mathrm{Xe}$ collisions measured by the CMS collaboration. We further predict the transverse momentum and centrality dependences of ${R}_{AA}$ for charged hadrons, $D$ and $B$ mesons in $\mathrm{Pb}+\mathrm{Pb}$, $\mathrm{Xe}+\mathrm{Xe}$, $\mathrm{Ar}+\mathrm{Ar}$, and $\mathrm{O}+\mathrm{O}$ collisions at the LHC energies. Our numerical results show a clear system size dependence for both light and heavy flavor hadron ${R}_{AA}$ across different collision systems. Sizable jet quenching effect is obtained for both heavy and light flavor hadrons in central $\mathrm{O}+\mathrm{O}$ collisions at the LHC energies. Our study provides a significant bridge for jet quenching from large to small systems, and should be helpful for finding the smallest QGP droplet and the disappearance of QGP in relativistic nuclear collisions.

Quark–gluon plasma and nucleons à la Laughlin

Inspired by Laughlin’s theory of the fractional quantum Hall effect, we explore the topological nature of the quark–gluon plasma (QGP) and the nucleons in the context of the Clifford algebra. In our model, each quark is transformed into a composite particle via the simultaneous attachment of a spin monopole and an isospin monopole. This is induced by a novel kind of meson endowed with both spin and isospin degrees of freedom. The interactions in the strongly coupled quark–gluon system are governed by the topological winding number of the monopoles, which is an odd integer to ensure that the overall wave function is antisymmetric. The states of the QGP and the nucleons are thus uniquely determined by the combination of the monopole winding number [Formula: see text] and the total quark number [Formula: see text]. The radius squared of the QGP droplet is expected to be proportional to [Formula: see text]. We anticipate the observation of such proportionality in the heavy ion collision experiments.

Kinematic dependence of azimuthal anisotropies in p+Au, d+Au, and He3+Au at sNN=200GeV

There is strong evidence for the formation of small droplets of quark-gluon plasma in $p/d/^{3}$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in $p$+$p$/Pb collisions at the Large Hadron Collider. In particular, the analysis of data at RHIC for different geometries obtained by varying the projectile size and shape has proven insightful. In the present analysis, we find excellent agreement with the previously published PHENIX at RHIC results on elliptical and triangular flow with an independent analysis via the two-particle correlation method, which has quite different systematic uncertainties and an independent code base. In addition, the results are extended to other detector combinations with different kinematic (pseudorapidity) coverage. These results provide additional constraints on contributions from nonflow and longitudinal decorrelations.

Quark-Gluon Plasma Fireball Formation in the Environment of Strong Magnetic Field

Using a theoretical model based on quasi-particle, we calculated the free energy evolution of quark-gluon plasma (QGP) in the presence of strong magnetic field generated in the high energy heavy ion collisions. The finite quark mass is used under the assumption of vanishing chemical potential. We found that the size of QGP fireball enhanced and more stable in the presence of strong magnetic field. This indicates that magnetic field plays an important role to describe the dynamics of QGP fireball under suitable conditions. Current results are useful in order to create a finite size of large QGP droplet. We noticed that the current results are also improved as compared to our earlier work where the contribution of magnetic field was neglected at RHIC and LHC. Thus, our results with quasi-particle model are of relevance in connection with the relativistic heavy ion collisions as well as for cosmological quark-hadron phase transition. In near future, experimentalists may provide the data for the existence of QGP fireball at RHIC, LHC and FAIR experiments.

Thermodynamic and transport properties of matter formed in pp, p-Pb, Xe–Xe and Pb–Pb collisions at the Large Hadron Collider using color string percolation model

To have a better understanding of the matter formed in ultra-relativistic collisions, using the color string percolation model, we have estimated initial energy density, mean free path, squared speed of sound, shear viscosity to entropy density ratio (η/s), bulk viscosity to entropy density ratio and bulk modulus for pp, p-Pb, Xe–Xe and Pb–Pb collisions at the Large Hadron Collider (LHC). These observables are studied as a function of final state charged particle multiplicity density normalized by nuclear overlap area and initial state percolation temperature. The results from this work are compared with the results from well known QCD models and with the transport properties of matter found in day-to-day life. These observables allow us to conclude an array of important findings about the matter formed in high-energy collisions at the LHC. For the high-multiplicity events, the estimated η/s are comparable with that is observed for heavy-ion collisions, which is an indication of a strongly coupled quark gluon plasma. This makes LHC high-multiplicity pp collision events as probable candidates for the search of quark-gluon plasma droplets.

Vortex rings from high energy central p+A collisions

Relativistic $p+A$ collisions may produce droplets of quark-gluon plasma (QGP) that quickly develop a toroidal vortex structure similar to that of an expanding smoke ring. We present viscous relativistic hydrodynamic calculations of ultracentral $p+A$ collisions and develop an experimental observable to probe the structure, correlating the polarization and momentum of hyperons emitted from the collision. This effect is robust against changes in the definition of vorticity used to calculate the polarization. Experiments at the Brookhaven National Laboratory Relativistic Heavy Ion Collider and the CERN Large Hadron Collider may test the existence and strength of the vortex toroids, bringing new evidence to bear on the question of collectivity in the smallest QGP droplets.

Jet wake from linearized hydrodynamics

We explore how to improve the hybrid model description of the particles originating from the wake that a jet produced in a heavy ion collision leaves in the droplet of quark-gluon plasma (QGP) through which it propagates, using linearized hydrodynamics on a background Bjorken flow. Jet energy and momentum loss described by the hybrid model become currents sourcing linearized hydrodynamics. By solving the linearized hydrodynamic equations numerically, we investigate the development of the wake in the dynamically evolving droplet of QGP, study the effect of viscosity, scrutinize energy-momentum conservation, and check the validity of the linear approximation. We find that linearized hydrodynamics works better in the viscous case because diffusive modes damp the energy-momentum perturbation produced by the jet. We calculate the distribution of particles produced from the jet wake by using the Cooper-Frye prescription and find that both the transverse momentum spectrum and the distribution of particles in azimuthal angle are similar in shape in linearized hydrodynamics and in the hybrid model. Their normalizations are different because the momentum-rapidity distribution in the linearized hydrodynamics analysis is more spread out, due to sound modes. Since the Bjorken flow has no transverse expansion, we explore the effect of transverse flow by using local boosts to add it into the Cooper-Frye formula. After including the effects of transverse flow in this way, the transverse momentum spectrum becomes harder: more particles with transverse momenta bigger than 2 GeV are produced than in the hybrid model. Although we defer implementing this analysis in a jet Monte Carlo, as would be needed to make quantitative comparisons to data, we gain a qualitative sense of how the jet wake may modify jet observables by computing proxies for two example observables: the lost energy recovered in a cone of varying open angle, and the fragmentation function. We find that linearized hydrodynamics with transverse flow effects added improves the description of the jet wake in the hybrid model in just the way that comparison to data indicates is needed. Our study illuminates a path to improving the description of the wake in the hybrid model, highlighting the need to take into account the effects of both transverse flow and the broadening of the energy-momentum perturbation in spacetime rapidity on particle production.

γ -hadron spectra in p+Pb collisions at sNN=5.02 TeV

Under the assumption that a quark-gluon plasma droplet is produced and its evolution can be described by hydrodynamics in p + A collisions, $\gamma$-triggered hadron spectra are studied within a next-to-leading-order perturbative QCD parton model with the medium-modified parton fragmentation functions. The initial conditions and space-time evolution of the small QGP droplet are provided by the superSONIC hydrodynamic model simulations and parton energy loss in such a medium is described by the high-twist (HT) approach. The scaled jet transport coefficient $\hat{q}/T^3$ in this HT approach is extracted from single hadron suppression in central A + A collisions at the same colliding energy. Numerical results for this scenario show that $\gamma$-hadron spectra at $p_{\rm T}^\gamma=12-40$ GeV/$c$ are suppressed by 5\% $\sim$ 10\% in the most central 0 - 10\% p + Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. The suppression becomes weaker at higher transverse momentum of the $\gamma$ trigger. As a comparison, $\gamma$-hadron suppression in Pb + Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ and 5.02 TeV is also predicted.

Effect of curvature on the nucleation of quark-gluon plasma droplets from a hadronic gas

We study the nucleation of quark-gluon plasma (QGP) droplets in a hadronic gas phase (HG). This process is driven by statistical fluctuations which are determined by the difference in free energy in the QGP and hadronic gas phases. We investigate this difference in free energy in two cases; when considering the curvature term additionally to the volume term in the density of states of particles constituting the QGP and without considering it, and this for the color singlet SU(3) representation of the QGP. The behavior of the obtained change in free energy is examined with varying radius of the QGP droplet, at vanishing chemical potential, in the two cases. The obtained results with and without the curvature contribution in the density of states are compared, and the effect of the curvature term on the nucleation of QGP droplets from a hadronic gas is analyzed.

Evidence for top quark production in nucleus-nucleus collisions

Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting quantum chromodynamics (QCD) matter, are routinely produced in heavy nuclei high-energy collisions. Although the experimental signatures marked a paradigm shift away from expectations of a weakly coupled QGP, a challenge remains as to how the locally deconfined state with a lifetime of a few fm can be resolved. The only colored particle that decays mostly within the QGP is the top quark. Here we demonstrate, for the first time, that top quark decay products are identified, irrespective of whether interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). Using 1.7±0.1 nb−1 of lead-lead (A = 208) collision data recorded by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV, we report evidence of top quark pair (tt¯) production. Dilepton final states are selected, and the cross section (σtt¯) is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The σtt¯ measurement is additionally performed considering the jets originating from the hadronization of bottom quarks, which improve the sensitivity to the tt¯ signal process. After background subtraction and analysis corrections, the measured σtt¯ is 2.56 ± 0.82(tot) and 2.02 ± 0.69(tot)μb in the two cases, respectively, consistent with predictions from perturbative QCD.

Understanding the out-of-equilibrium dynamics near a critical point in the QCD phase diagram

Upcoming experimental programs will look for signatures of a possible critical point in the QCD phase diagram in fluctuation observables. To understand and predict these signatures, one must account for the fact that the dynamics of any critical fluctuations must be out-of-equilibrium: because of critical slowing down, the fluctuations cannot stay in equilibrium as the droplet of QGP produced in a collision expands and cools. Furthermore, their out-of-equilibrium dynamics must also influence the hydrodynamic evolution of the cooling droplet. The recently developed Hydro+ formalism allows for a consistent description of both the hydrodynamics and the out-of-equilibrium fluctuations, including the feedback between them. We shall explicitly demonstrate how this works, setting up a Hydro+ simulation in a simplified setting: a rapidity-independent fireball undergoing radial flow with an equation of state in which we imagine a critical point close to the $\mu_B=0$ axis of the phase diagram. Within this setup, we show that we can quantitatively capture non-equilibrium phenomena, including critical fluctuations over a range of scales and memory effects. Furthermore, we illustrate the interplay between the dynamics of the fluctuations and the hydrodynamic flow of the fireball: as the fluid cools and flows, the dynamical fluctuations lag relative to how they would evolve if they stayed in equilibrium; there is then a backreaction on the flow itself due to the out-of-equilibrium fluctuations; and, in addition, the radial flow transports fluctuations outwards by advection. Within our model, we find that the backreaction from the out-of-equilibrium fluctuations does not yield dramatically large effects in the hydrodynamic variables. Further work will be needed in order to check this quantitative conclusion in other settings but, if it persists, this will considerably simplify future modelling.

Droplets of quark gluon plasma: PHENIX results on small systems at RHIC

In these proceedings we discuss PHENIX results on small systems at RHIC, specifically those relating to collectivity and flow. We discuss the small systems geometry scan, which comprises collisions of p+Au, d+Au, and 3He+Au at and is designed to exploit differences in intrinsic geometry. We also discuss the small systems beam energy scan, which comprises d+Au collisions at , 62.4 GeV, 39 GeV, and 19.6 GeV and is designed to explore differences in system size and lifetime while keeping the geometry fixed. We find the data in nearly all cases are well-described by hydrodynamical calculations involving a QGP phase, suggestion formation of QGP droplets in a wide variety of collisions.

Bulk viscosity and cavitation in heavy ion collisions

Relativistic heavy ion collisions generate nuclear-sized droplets of quark-gluon plasma (QGP) that exhibit nearly inviscid hydrodynamic expansion. Smaller collision systems such as $p+\mathrm{Au}, d+\mathrm{Au}$, and $^{3}\mathrm{He}+\mathrm{Au}$ at the BNL Relativistic Heavy Ion Collider, as well as $p+\mathrm{Pb}$ and high-multiplicity $p+p$ at the CERN Large Hadron Collider may create even smaller droplets of QGP. If so, the standard time evolution paradigm of heavy ion collisions may be extended to these smaller systems. These small systems present a unique opportunity to examine pre-hydrodynamic physics and extract properties of the QGP, such as the bulk viscosity, where the short lifetimes of the small droplets make them more sensitive to these contributions. Here, we focus on the influence of bulk viscosity, its temperature dependence, and the implications of negative pressure and potential cavitation effects on the dynamics in small and large systems using the publicly available hydrodynamic codes sonic and music. We also discuss pre-hydrodynamic physics in different frameworks including anti--de Sitter/conformal field theory strong coupling, ip-glasma weak coupling, and free streaming.