Search

Thermalization of heavy quarks in the quark-gluon plasma (QGP) is one of the most promising phenomena for understanding the strong interaction. The energy loss and momentum broadening at low momentum can be well described by a stochastic process with drag and diffusion terms. Recent advances in quantum computing, in particular quantum amplitude estimation (QAE), promise to provide a quadratic speed-up in simulating stochastic processes. We introduce and formalize an accelerated quantum circuit Monte-Carlo (aQCMC) framework to simulate heavy quark thermalization. With simplified drag and diffusion coefficients connected by Einstein's relation, we simulate the thermalization of a heavy quark in isotropic and anisotropic mediums using an ideal quantum simulator and compare that to thermal expectations. With Grover-like QAE, we calculate physical observables with quadratically fewer resources, which is a boost over the classical MC simulation that usually requires a large sampling number at the same estimation accuracy., Comment: 10 pages, 9 figures

We use QCD effective kinetic theory to calculate far-from-equilibrium dynamics on an event-by-event basis within the K\o{}MP\o{}ST framework. We present non-equilibrium charge response functions and the dynamical evolution of the conserved charge current pertinent to the early-time dynamics of heavy-ion collisions at the highest energies. The K\o{}MP\o{}ST framework with conserved baryon, strangeness, and electric charges can then be readily implemented into a multistage model allowing for the initialization of a non-equilibrium charge current in hydrodynamic simulations.

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio $\eta/s$ in the early stages of the collision., Comment: 7 pages, 3 figures

We present a simple approach to extract hydrodynamic sound modes and non-hydrodynamic modes in kinetic theories from response functions of the energy-momentum tensor. By comparing the response functions in four types of kinetic theories, namely the Relaxation-Time Approximation, scalar $\phi^4$ theory, SU(3) Yang-Mills theory and QCD kinetic theory, we find a remarkable degree of universality for the sound mode, even beyond the hydrodynamic regime., Comment: 6 pages, 4 figures; Proceeding on 11th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probe 2023), 26-31 March 2023, Aschaffenburg, Germany

Jets provide one of the primary probes of the quark-gluon plasma produced in ultrarelativistic heavy ion collisions and the cold nuclear matter explored in deep inelastic scattering experiments. However, despite important developments in the last years, a description of the real-time evolution of QCD jets inside a medium is still far from being complete. In our previous work, we have explored quantum technologies as a promising alternative theoretical laboratory to simulate jet evolution in QCD matter, to overcome inherent technical difficulties in present calculations. Here, we extend our previous investigation from the single particle $|q\rangle$ to the $|q\rangle+|qg\rangle$ Fock space, taking into account gluon production. Based on the light-front Hamiltonian formalism, we construct a digital quantum circuit that tracks the evolution of a multi-particle jet probe in the presence of a medium described as a stochastic color field. Studying the momentum broadening of the jet state, we observe sizable sub-eikonal effects by comparing to eikonal estimates. We also study the medium-induced modifications to the gluon emission probability, which exhibit small corrections compared to the vacuum splitting function. In addition, we study the time evolution of the von-Neumann entropy associated with the quark component; we find that the exponential of the entropy grows linearly in time for the bare quark but super-linearly when taking into account gluon emission., Comment: 16 pages, 12 figures

We study the response of the energy-momentum tensor in several kinetic theories, from the simple relaxation time approximation (RTA) to Quantum Chromodynamics (QCD). Irrespective of the differences in microscopic properties, we find a remarkable degree of universality in the response functions from conformal theories. We find that the response to scalar perturbations in kinetic theory can be effectively described by a pair of one hydrodynamic sound mode and one non-hydrodynamic mode. We find that even beyond the range of validity of hydrodynamics, the energy-momentum response in position space can be effectively described by one single mode with non-trivial dispersion relation and residue., Comment: 10 pages, 7 figures

Kinetic and chemical equilibrations play important roles in the formation of the quark-gluon plasma (QGP) in relativistic heavy-ion collisions (HICs). These processes further influence the production of hard and electromagnetic probes in HICs, in particular, the thermalization of heavy quarks, which are produced at an extremely early time before the formation of the QGP. We calculate the drag and diffusion coefficients of heavy quarks in the pre-hydrodynamic quantum chromodynamic (QCD) plasma with the state-of-the-art QCD effective kinetic theory (EKT) solver. We present the time, momentum, and angular dependencies of these coefficients for gluon and quark contributions separately, showing the effects of isotropization and chemical equilibration from the QCD plasma. We also provide a simple formula to estimate the heavy quark drag and diffusion coefficients, as well as its energy loss, within the pre-hydrodynamic plasma at different coupling strengths based on the attractor theory. We then discuss the validity of these estimations with leading-order calculations and leading-logarithmic rescaling factors., Comment: 9 pages, 8 figures

QCD jets provide one of the best avenues to extract information about the quark-gluon plasma produced in the aftermath of ultra relativistic heavy ions collisions. The structure of jets is determined by multiparticle quantum interference hard to tackle using perturbative methods. When jets evolve in a QCD medium this interference pattern is modified, adding another layer of complexity. By taking advantage of the recent developments in quantum technologies, such effects might be better understood via direct quantum simulation of jet evolution. In this work, we introduce a precursor to such simulations. Based on the light-front Hamiltonian formalism, we construct a digital quantum circuit that tracks the evolution of a single hard probe in the presence of a stochastic color background. In terms of the jet quenching parameter $\hat q$, the results obtained using classical simulators of ideal quantum computers agree with known analytical results. With this study, we hope to provide a baseline for future in-medium jet physics studies using quantum computers.

The evaluation of quarkonium regeneration in ultrarelativistic heavy-ion collisions (URHICs) requires the knowledge of the heavy-quark phase space distributions in the expanding quark-gluon plasma (QGP) fireball. We employ a semi-classical charmonium transport approach where regeneration processes explicitly account for the time-dependent spectra of charm quarks via Langevin simulations of their diffusion. The inelastic charmonium rates and charm-quark transport coefficients are computed from the same charm-medium interaction. The latter is modeled by perturbative rates, augmented with a $K$-factor to represent nonperturbative interaction strength and interference effect. Using central 5.02~TeV Pb-Pb collisions as a test case we find that a good description of the measured $J/\psi$ yield and its transverse-momentum dependence can be achieved if a large $K\gtrsim5$ is employed while smaller values lead to marked discrepancies. This is in line with open-charm phenomenology in URHICs, where nonperturbative interactions of similar strength are required. Our approach establishes a common transport framework for a microscopic description of open and hidden heavy-flavor (HF) observables that incorporates both nonperturbative and non-equilibrium effects, and thus enhances the mutual constraints from experiment on the extraction of transport properties of the QGP., Comment: 7 pages, 3 figures

Color screening and parton inelastic scattering modify the heavy-quark antiquark potential in the medium that consists of particles from quantum chromodynamics (QCD), leading to suppression of quarkonium production in relativistic heavy-ion collisions. Due to small charm/anti-charm ($c\bar{c}$) pair production number in proton-nucleus (pA) collisions, the correlation between different $c\bar{c}$ pairs is negligible, which makes the Schr\"odinger equation viable for tracking the evolution of only one $c\bar{c}$ pair. We employ the time-dependent Schr\"odinger equation with in-medium $c\bar{c}$ potential to study the evolution of charmonium wave functions in the hydrodynamic like QCD medium produced in pA collisions. We explore different parametrizations of real and imaginary parts of $c\bar{c}$ potential and calculate the nuclear modification factors ($R_{\rm pA}$) of $J/\psi$ and $\psi(2S)$ in $\sqrt{s_{NN}}=5.02$ TeV energy p-Pb collisions at Large Hadron Collider (LHC). Comparing a strong and a weak screening scenario with experimental data in this approach, we arrive at the conclusion that the color screening is weak at temperature close to deconfined phase transition. Moreover, the imaginary part of the potential is crucial to describe the experimental data which is consistent with widely studied semi-classical approaches where the dissociation rates are essential., Comment: 9 pages, 8 figures, accepted by Chinese Physics C

We use principal component analysis to study the hydrodynamic attractor in Yang-Mills kinetic theory undergoing the Bjorken expansion with Color Glass Condensate initial conditions. The late time hydrodynamic attractor is characterized by a single principal component determining the overall energy scale. How it is reached is governed by the disappearance of single subleading principal component characterizing deviations of the pressure anisotropy, the screening mass and the scattering rate. We find that for wide range of couplings the approach to the hydrodynamic attractor at late times is well described by an exponential. Its decay rate dependence on the coupling turns out to translate into a simple dependence on the shear viscosity to entropy density ratio., Comment: 12 pages, 7 figures

The spectrum of dileptons produced by the quark-gluon plasma in an ultrarelativistic nucleus-nucleus collision depends only, to a good approximation, on the transverse mass M_t of the dilepton. This scaling is exact as long as transverse flow is negligible, and the system is in local thermal equilibrium. We implement a state-of-the-art modelization of kinetic and chemical equilibration in the early stages of the evolution to study the modifications of the spectrum. Violations of M_t scaling resulting from these effects are evaluated as a function of the shear viscosity to entropy ratio (eta/s) that controls the equilibration time. We determine the dependence of the spectrum on system size, centrality, rapidity, and collision energy. We show that the quark-gluon plasma produces more dileptons than the Drell-Yan process up to invariant masses of order M = 4 GeV. Due to different kinematics, for a given M_t , the dependence of the dilepton yield on M is opposite for the two processes, so that experiment alone can in principle determine which process dominates., Comment: 9 pages, 3 figures

The production of dileptons with an invariant mass in the range 1 GeV < M < 5 GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory, and study how the dilepton mass spectrum depends on the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors., Comment: 10 pages, 7 figures; computation of the effective area is defined more rigorously, figures are updated accordingly. Added paragraph on page 7, discussing the impact of inhomogeneities in transverse density profile. Added paragraph in the conclusion on page 9, on fitting the invariant mass spectrum with a power function

We employ a non-equilibrium Quantum Chromodynamics (QCD) kinetic description to study the kinetic and chemical equilibration of the Quark-Gluon Plasma (QGP) at weak coupling. Based on our numerical framework, which explicitly includes all leading order processes involving light flavor degrees of freedom, we investigate the thermalization process of homogeneous and isotropic plasmas far-from equilibrium and determine the relevant time scales for kinetic and chemical equilibration. We further simulate the longitudinally expanding pre-equilibrium plasma created in ultrarelativistic heavy-ion collisions at zero and non-zero density of the conserved charges and study its microscopic and macroscopic evolution towards equilibrium., Comment: 32+10 pages, 32 figures, also see our companion paper: Equilibration of the Quark-Gluon Plasma at finite net-baryon density in QCD kinetic theory

We explore the out-of-equilibrium dynamics of the Quark-Gluon Plasma at zero and finite net-baryon density based on an effective kinetic theory of Quantum Chromo Dynamics (QCD). By investigating the isotropization of the longitudinal pressure, we determine the relevant time and temperature scales for the onset of viscous hydrodynamics, and quantify the dependence on the chemical composition of the QGP. By extrapolating our results to realistic coupling strength, we discuss phenomenological consequences regarding the role of the pre-equilibrium phase at different collision energies., Comment: 7 pages, 3 figures, see our companion paper: Equilibration of weakly coupled QCD plasmas

We solve a leading-order QCD kinetic theory with light quarks and gluon degrees of freedom to study the non-equilibrium dynamics of the quark-gluon plasma (QGP). By including both elastic and inelastic scatterings for quarks and gluon, the model is proficient to describe kinetic and chemical equilibration of the QGP, and thus connects the initial (semi-) hard production of partons at early times with the hydrodynamic description of a near-thermalized quark-gluon plasma after the first fm/c of the collision. Within this approach, we investigate the time scales and mechanisms for kinetic and chemical equilibration of the QGP at zero and non-zero net-baryon density and elaborate on the connections to jet quenching physics and hydrodynamics., Comment: 5 pages, 10 figures; Proceeding on 10th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probe 2020) (Austin, TX, USA), May 31- June 5, 2020

The production of the $X(3872)$ particle in heavy-ion collisions has been contemplated as an alternative probe of its internal structure. To investigate this conjecture, we perform transport calculations of the $X(3872)$ through the fireball formed in nuclear collisions at the LHC. Within a kinetic-rate equation approach as previously used for charmonia, the formation and dissociation of the $X(3872)$ is controlled by two transport parameters, i.e., its inelastic reaction rate and thermal-equilibrium limit in the evolving hot QCD medium. While the equilibrium limit is controlled by the charm production cross section in primordial nucleon-nucleon collisions (together with the spectra of charm states in the medium), the structure information is encoded in the reaction rate. We study how different scenarios for the rate affect the centrality dependence and transverse-momentum ($p_T$) spectra of the $X(3872)$. Larger reaction rates associated with the loosely bound molecule structure imply that it is formed later in the fireball evolution than the tetraquark and thus its final yields are generally smaller by around a factor of two, which is qualitatively different from most coalescence model calculations to date. The $p_T$ spectra provide further information as the later decoupling time within the molecular scenario leads to harder spectra caused by the blue-shift from the expanding fireball.

The in-medium color potential is a fundamental quantity for understanding the properties of the strongly coupled quark-gluon plasma (sQGP). Open and hidden heavy-flavor (HF) production in ultrarelativistic heavy-ion collisions (URHICs) has been found to be a sensitive probe of this potential. Here we utilize a previously developed quarkonium transport approach in combination with insights from open HF diffusion to extract the color-singlet potential from experimental results on $\Upsilon$ production in URHICs. Starting from a parameterized trial potential, we evaluate the $\Upsilon$ transport parameters and conduct systematic fits to available data for the centrality dependence of ground and excited states at RHIC and the LHC. The best fits and their statistical significance are converted into a temperature-dependent potential. Including nonperturbative effects in the dissociation rate guided from open HF phenomenology, we extract a rather strongly coupled potential with substantial remnants of the long-range confining force in the QGP., Comment: 8 pages, 15 figures; version accepted for publication in PLB

We study charmonium production in proton-nucleus ($p$-A) collisions focusing on final-state effects caused by the formation of an expanding medium. Toward this end, we utilize a rate equation approach within a fireball model as previously employed for a wide range of heavy-ion collisions, adapted to the small systems in $p$-A collisions. The initial geometry of the fireball is taken from a Monte-Carlo event generator where initial anisotropies are caused by fluctuations. We calculate the centrality and transverse-momentum dependent nuclear modification factor ($R_{p{\rm A}}$) as well as elliptic flow ($v_2$) for both $J/\psi$ and $\psi(2S)$ and compare them to experimental data from RHIC and the LHC. While the $R_{p{\rm A}}$s show an overall fair agreement with most of the data, the large $v_2$ values observed in $p$-Pb collisions at the LHC cannot be accounted for in our approach. While the former finding generally supports the formation of a near thermalized QCD medium in small systems, the discrepancy in the $v_2$ suggests that its large observed values are unlikely to be due to the final-state collectivity of the fireball alone., Comment: 8 pages, 15 figures; version accepted for publication in JHEP

The production of ground-state and excited bottomonia in ultrarelativistic heavy-ion collisions is investigated within a kinetic-rate equation approach including regeneration. We augment our previous calculations by an improved treatment of medium effects, with temperature-dependent binding energies and pertinent reaction rates, $B$-meson resonance states in the equilibrium limit near the hadronization temperature, and a lattice-QCD based equation of state for the bulk medium. In addition to the centrality dependence of the bottomonium yields we compute their transverse-momentum ($p_T$) spectra and elliptic flow with momentum-dependent reaction rates and a regeneration component based on $b$-quark spectra from a nonperturbative transport model of heavy-quark diffusion. The latter has noticeable consequences for the shape of the bottomonium $p_T$ spectra. We quantify how uncertainties in the various modeling components affect the predictions for observables. Based on this we argue that the $\Upsilon(1S)$ suppression is a promising observable for mapping out the in-medium properties of the QCD force, while $\Upsilon(2S)$ production can help to quantify the role of regeneration from partially thermalized $b$ quarks., Comment: 26 pages, 52 figures

The objective of this paper is to assess the current theoretical understanding of the extensive set of quarkonium observables (for both charmonia and bottomonia) that have been attained in ultrarelativistic heavy-ion collisions over two orders of magnitude in center-of-mass energy. We briefly lay out and compare the currently employed theoretical frameworks and their underlying transport coefficients, and then analyze excitation functions of quarkonium yields to characterize the nature of the varying production mechanisms. We argue that an overall coherent picture of suppression and regeneration mechanisms emerges which enables to deduce insights on the properties of the in-medium QCD force from SPS via RHIC to LHC, and forms a basis for future quantitative studies., Comment: 8 pages, 10 figures; contribution to proceedings of XXVIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2017), Chicago (IL, USA), Feb. 05-11, 2017

We study bottomonium production in heavy-ion collisions using a transport model which utilizes kinetic-rate and Boltzmann equations to calculate the energy, centrality and transverse-momentum ($p_T$) dependence of the yields. Both gluo-dissociation and inelastic parton-induced break-up including interference effects are improved over previous work by using in-medium binding energies from a thermodynamic $T$-matrix approach. A coalescence model with bottom-quark spectra from Langevin simulations is implemented to account for thermal off-equilibrium effects in the $p_T$-spectra of the regeneration contribution. We also update the equation of state for the bulk medium by extracting it from lattice-QCD results. A systematic analysis of bottomonium observables is conducted in comparison to RHIC and LHC data. In particular, the off-equilibrium bottom-quark spectra are found to play an important role in the bottomonium $p_T$ spectra., Comment: 4 pages, 6 figures; Proceeding on XXVIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2017) (Chicago, IL, USA), February 6-11, 2017

We employ a time-dependent Schr\"odinger equation to study the evolution of a $c\bar c$ dipole in a quark-gluon plasma (QGP). Medium effects on the heavy-quark potential in the QGP are found to significantly affect the timescales of the internal evolution of the dipole. Color-screening can enhance the overlap of the expanding wavepackage with excited states at high temperature, while it is reduced at lower temperatures where the dipole favors the formation of the charmonium ground state. We investigate the consequences of this mechanism on the double ratio of charmonium nuclear modification factors, $R_{AA}^{\psi(2S)}/R_{AA}^{J/\psi}$, in heavy-ion collisions. The impact of the transition mechanisms on this ratio turns out to be rather sensitive to the attractive strength of the potential, and to its temperature dependence., Comment: 4 pages, 4 figures, proceeding for Hard Probe 2016

We calculate the production of $\psi(2S)$ and the pertinent double ratio of its nuclear modification factor ($R_{\rm AA}$) over that of the $J/\psi$ in Pb-Pb collisions at the LHC. Based on a transport model with temperature dependent reaction rates, a sequential regeneration pattern emerges: the larger $\psi(2S)$ width, relative to the $J/\psi$, around and below the critical temperature, implies that most of the $\psi(2S)$ states are regenerated later in the evolution of the fireball. This has noticeable consequences for the transverse-momentum ($p_T$) spectra of the regenerated charmonia. While the total yield of $\psi(2S)$ meson remains smaller than those of $J/\psi$'s, their harder $p_T$ spectra can produce a double ratio above unity for a $p_T>3$GeV cut, as applied by the CMS collaboration. A significant uncertainty in our calculations is associated with the values of the temperature where most of the $\psi(2S)$ regeneration occurs, the quantitative temperature dependence of its inelastic width., Comment: 4 pages, 4 figures; Proceeding on Strangeness in Quark Matter 2016 (Berkeley, CA, USA), June 27-July 1, 2016

We investigate the production of psi(2S) in nuclear collisions at RHIC and LHC energies. We first address charmonium production in 200 GeV d-Au collisions at RHIC; the strong suppression of psi' mesons observed in these reactions indicates mechanisms beyond initial cold nuclear matter effects. We find that a more complete treatment of hadronic dissociation reactions leads to appreciable psi' suppression in the hadronic medium of an expanding fireball background for d-Au collisions. When implementing the updated hadronic reaction rates into a fireball for 2.76 TeV Pb-Pb collisions at LHC, the regeneration of psi' mesons occurs significantly later than for J/psi's. Despite a smaller total number of regenerated psi', the stronger radial flow at their time of production induces a marked enhancement of their R_{AA} relative to J/psi's in a momentum range p_t \simeq 3-6 GeV. We explore the consequences and uncertainties of this "sequential regeneration" mechanism on the R_{AA} double ratio and find that it can reproduce the trends observed in recent CMS data., Comment: 7 pages, 7 figures