Quarkonium Suppression Research Articles

Towards Experimental Confirmation of Quarkonia Melting in Quark–Gluon Plasma: A Review of Recent Measurements of Quarkonia Production in Relativistic Heavy-Ion Collisions

The dissociation, or “melting”, of heavy quarkonia states due to color charge screening is a predicted signature of quark–gluon plasma (QGP) formation, with a quarkonium state predicted to dissociate when the temperature of the medium is higher than the binding energy of the quarkonium state. A conclusive experimental observation of quarkonium melting coupled with a detailed theoretical understanding of the melting mechanism would enable the use of quarkonia states as temperature probes of the QGP, a long-sought goal in the field of relativistic heavy-ion collisions. However, the interpretation of quarkonia suppression measurements in heavy-ion collisions is complicated by numerous other cold nuclear matter effects that also result in the dissociation of bound quarkonia states. A comprehensive understanding of these cold nuclear matter effects is therefore needed in order to correctly interpret quarkonia production measurements in heavy-ion collisions and to observe the melting of quarkonium states experimentally. In this review, recent measurements of quarkonia production in pA and AA collisions and their state-of-the-art theoretical interpretations will be discussed, as well as the future measurements needed to further the knowledge of cold nuclear matter effects and realize a measurement of quarkonia melting in heavy-ion collisions.

An Effective Field Theory Study of Medium Heavy Quark Evolution

The evolution of hard probes in a medium is a complex multiscale problem that significantly benefits from the use of Effective Field Theories (EFTs). Within the EFT framework, we aim to define a series of EFTs in a way that addresses each energy scale individually in separate steps. However, studying hard probes in a medium presents challenges. This is because an EFT is typically constructed by formulating the most general Lagrangian compatible with the problem’s symmetries. Nevertheless, medium effects may not always be encoded adequately in an effective action. In this paper, we construct an EFT that is valid for studying the evolution of a heavy quark in a QCD plasma containing few other heavy quarks, where degrees of freedom with an energy of the order of the temperature scale are integrated out. Through this example, we explicitly demonstrate how to handle the doubling of degrees that arise in non-equilibrium field theory. As a result, we derive a Fokker–Planck equation using only symmetry and power counting arguments. The methods introduced in this paper will pave the way for future developments in the study of quarkonium suppression.

Studies of the relative suppression of excited quarkonium states with CMS

One of the findings of the LHC heavy ion program is the observation of stronger suppression of the excited quarkonium states compared to the ground states in lead-lead (PbPb) and proton-lead (pPb) collisions. Such differences among the states may imply dissociation effects occurring at late stages, after the evolution of heavy quark pairs into well-defined physical states. The variety of binding energies within the quarkonia families offers an experimental tool to characterize the phenomena at play. Measuring the excited states is crucial as they represent significant feed-down contributions to the production of the ground states and must be accounted for in the interpretation of the data. We present studies of the relative suppression of quarkonia in pPb and PbPb collisions performed by CMS. For the first time, the nuclear modification factor has been extended to the strongly suppressed Υ(3S) state. Moreover, nuclear modification factors as well as excited-to-ground state cross section ratios are measured as a function of particle transverse momentum and rapidity, and event activity. The results are compared with several model calculations incorporating initial- and final-state effects.

Monte Carlo simulations of Upsilon meson production in p + p at = 500 GeV

Quarkonium production in proton-proton collisions serves as an essential baseline for studying the effect of quarkonium suppression in proton-nucleus and nucleus-nucleus collisions. This paper presents the main characteristics of Upsilon mesons production in proton-proton collisions at studied with two Monte Carlo event generators: Pythia and Herwig and the results compared. Main aim of these simulations is to explore the dependence of normalised Upsilon meson yield on normalised event multiplicity and the influence of the feed-down effect on Upsilon meson production. The simulations show that the normalised Upsilon yield rises stronger than linear with regards to the normalised charged particle multiplicity. A mass ordering is found for Upsilons originating from feed-down and it suggests that the observed strong increase depends on the scale of the original hard process producing the quarkonium state.

Quarkonium suppression in the open quantum system approach

Quarkonium suppression in relativistic heavy ion collisions has been studied experimentally for decades to probe the properties of the quark-gluon plasma. For this purpose, complete theoretical understanding of the time evolution of quarkonium inside the quark-gluon plasma is needed but challenging. Here I review recent progress in applying the open quantum system framework to describe the real-time dynamics of quarkonium, with a focus on the chromoelectric field correlators of the plasma that control the dynamics.

Simple model to include initial-state and hot-medium effects in the computation of quarkonium nuclear modification factor

Quarkonium suppression is one of the more useful observables to obtain information about the hot medium created in ultrarelativistic heavy-ion collisions. In this manuscript, we discuss a simple way to implement both the initial-state effects and the hot-medium evolution, and to compute the quarkonium nuclear modification factor if the survival probability for a bound state at a given energy density is known. Based on the Glauber model, the temperature of the evolving medium and the centrality dependence of the nuclear modification factor will be analysed.

Phenomenological study of quarkonium suppression and the impact of the energy gap between singlets and octets

The study of heavy quarkonium suppression in heavy-ion collisions represents an important source of information about the properties of the quark-gluon plasma produced in such collisions. In a previous paper, we have considered how to model the evolution of a quarkonium in such a way that the solution of the resulting equations evolves toward the correct thermal equilibrium distribution for a homogeneous and static medium. We found that it is crucial to take into account the energy gap between singlet and octet configurations when the temperature is not much greater than this energy gap. In this paper, we explore in more detail the phenomenological consequences of this observation in the more realistic situation of an expanding system. We consider two different scenarios, based on the same approximation scheme, but on different choices of parameters. In the first case, we rely on a Hard Thermal Loop approximation, while the second case is based on a recent determination of the static potential in lattice QCD. In both cases, we compute the decay width and the nuclear modification factor, both taking the energy gap into account and ignoring it. We find that the impact on the predictions is as large in the expanding medium as it is in the static case. Our conclusion is that this energy gap should be taken into account in phenomenological studies.

Bottomonium suppression in PbPb collisions at energies available at the CERN Large Hadron Collider

High energy collisions are the laboratories within our reach to study strongly interacting matter under extreme temperatures. In the present study, we use a quarkonia suppression scheme to explain the bottomonium production at the two energies available at the CERN Large Hadron Collider. We employ echo-qgp to model the $(3+1)$-dimensional relativistic viscous hydrodynamic evolution of the medium. Bottomonia produced in the early stage dissociates due to color screening, gluonic dissociation, and collisional damping in addition to shadowing as an initial state effect. In the color screening mechanism, the temperature from hydrodynamics is used to find the screening radii at each centrality and rapidity. The shadowing effect utilizes the parton distribution functions obtained from the CT14 global analysis and shadowing factors from EPPS16. A lattice QCD based equation of state from the Wuppertal-Budapest Collaboration has been used. The experimental values of pion $({\ensuremath{\pi}}^{+})$ spectra were used to constrain the initial conditions of the dynamics. The bottomonium suppression is determined as a function of centrality, transverse momentum, and rapidity for $\mathrm{\ensuremath{\Upsilon}}(1S)$ and $\mathrm{\ensuremath{\Upsilon}}(2S)$ states at the LHC energies of 2.76 and $5.02\phantom{\rule{4pt}{0ex}}\mathrm{TeV}$. We find a fairly good agreement between our theoretically calculated survival probability and the measured nuclear modification factor $({R}_{AA})$ at the two energies.

Open quantum systems for quarkonia

I review recent applications of the open quantum system framework in the understanding of quarkonium suppression in heavy-ion collisions, which has been used as a probe of the quark–gluon plasma for decades. The derivation of the Lindblad equations for quarkonium in both the quantum Brownian motion and the quantum optical limits and their semiclassical counterparts is explained. The hierarchy of time scales assumed in the derivation is justified from the separation of energy scales in nonrelativistic effective field theories of QCD. Physical implications of the open quantum system approach are also discussed. Finally, I list some open questions for future studies.

Energy loss of heavy quarkonia in hot QCD plasmas

We compute the energy loss of heavy quarkonia in high temperature QCD plasmas and investigate the energy loss effects on quarkonium suppression. Based on the effective vertex derived from the Bethe-Salpeter amplitude for quarkonium, the collisional and radiative energy loss are determined by quarkonium-gluon elastic scattering and the associated gluon-bremsstrahlung, respectively. In the energy regime $E<m_{\Upsilon}^2/T$ the collisional energy loss is dominant over the radiative one, and the total energy loss increases with the plasma temperature and the initial energy of quarkonium. Our numerical analysis indicates that the medium-induced energy loss of the $\Upsilon$(1S) results in stronger suppression at higher momentum, although the energy loss effects are found to be small compared with the previous estimates of quarkonium dissociation in heavy-ion collisions.

Bottomonium suppression and elliptic flow using Heavy Quarkonium Quantum Dynamics

We introduce a framework called Heavy Quarkonium Quantum Dynamics (HQQD) which can be used to compute the dynamical suppression of heavy quarkonia propagating in the quark-gluon plasma using real-time in-medium quantum evolution. Using HQQD we compute large sets of real-time solutions to the Schrödinger equation using a realistic in-medium complex-valued potential. We sample 2 million quarkonia wave packet trajectories and evolve them through the QGP using HQQD to obtain their survival probabilities. The computation is performed using three different HQQD model parameter sets in order to estimate our systematic uncertainty. After taking into account final state feed down we compare our results to existing experimental data for the suppression and elliptic flow of bottomonium states and find that HQQD predictions are good agreement with available data for RAA as a function of Npart and pT collected at sqrt{s_{mathrm{NN}}} = 5.02 TeV. In the case of v2 for the various states, we find that the path-length dependence of ϒ(1s) suppression results in quite small v2 for ϒ(1s). Our prediction for the integrated elliptic flow for ϒ(1s) in the 10−90% centrality class, which now includes an estimate of the systematic error, is v2[ϒ(1s)] = 0.003 ± 0.0007 ± {}_{0.0013}^{0.0006} . We also find that, due to their increased suppression, excited bottomonium states have a larger elliptic flow. Based on this observation we make predictions for v2[ϒ(2s)] and v2[ϒ(3s)] as a function of centrality and transverse momentum.

An Effective Theory of Quarkonia in QCD Matter

The problem of quarkonium production in heavy ion collisions presents a set of unique theoretical challenges – from the relevant production mechanism of J/ψs and ϒs to the relative significance of distinct cold and hot nuclear matter effects in the observed attenuation of quarkonia. In these proceedings we summarize recent work on the generalization of non-relativistic Quantum Chromodynamics (NRQCD) to include Glauber gluon interactions in strongly interacting matter. This new effective theory provides for the first time a universal microscopic description of the in-medium interaction of heavy quarkonia, consistently applicable to a range of phases such as cold nuclear matter, dense hadron gas, and quark-gluon plasma. It is an important step forward in understanding the common trends in proton-nucleus and nucleus-nucleus data on quarkonium suppression. We derive explicitly the lowest order (LO) and next-to-leading order (NLO) interaction terms in the Lagrangian and show the connection of the LO result to existing phenomenology.

Overview of quarkonium production studies at the STAR experiment

Quarkonium states can be used to study the properties of the quark-gluon plasma created in heavy-ion collisions. However, Cold Nuclear Matter (CNM) effects need to be taken into account when interpreting the quarkonium suppression observed in these collisions. In addition, the quarkonium production mechanism in elementary collisions need to be better understood. These proceedings contains an overview of recent quarkonium measurements with the STAR experiment. A comprehensive study of both J/ψ and Υ production is performed in different colliding systems (p+p, p+Au, Au+Au).

Suppression of quarkonia in PbPb collisions at = 5.02 TeV

We study different processes responsible for the modification of quarkonia yields in the medium produced in PbPb collisions at = 5.02 TeV. The quarkonia and heavy flavor cross sections are estimated using the measurements in pp collisions at LHC energies and shadowing corrections are obtained using the EPS09 parameterizations. A kinetic model is used which incorporates quarkonia suppression inside the quark gluon plasma (QGP), suppression due to hadronic comovers, and regeneration from recombination of heavy quark pairs. The quarkonia dissociation cross section due to gluon collisions, including both color-electric dipole and color-magnetic dipole transitions, has been employed. The regeneration rate is obtained using the principle of detailed balance. The effect of these processes on the nuclear modification factors for both J/ψ and in different ranges of transverse momentum pT and rapidity has been studied for PbPb collisions at = 5.02 TeV. The calculations are compared with the available results from LHC experiments. Both the suppression and regeneration due to a QGP are effective in the low and intermediate pT range. The large observed suppression of J/ψ at pT > 10 GeV/c is larger than the suppression expected due to gluon dissociation.

Heavy quarkonium suppression beyond the adiabatic limit

Many prior studies of in-medium quarkonium suppression have implicitly made use of an adiabatic approximation in which it was assumed that the heavy quark potential is a slowly varying function of time. In the adiabatic limit, one can separately determine the in-medium breakup rate and the medium time evolution, folding these together only at the end of the calculation. In this paper, we relax this assumption by solving the 3d Schrodinger equation in real-time in order to compute quarkonium suppression dynamically. We compare results obtained using the adiabatic approximation with real-time calculations for both harmonic oscillator and realistic complex heavy quark potentials. Using the latter, we find that, for the Upsilon(1s), the difference between the adiabatic approximation and full real-time evolution is at the few percent level, however, for the Upsilon(2s), we find that the correction can be as large as 18% in low temperature regions. For the J/Psi, we find a larger difference between the dynamical evolution and the adiabatic approximation, with the error reaching approximately 36%.

An effective theory of quarkonia in QCD matter

For heavy quarkonia of moderate energy, we generalize the relevant successful theory, non-relativistic Quantum Chromodynamics (NRQCD), to include interactions in nuclear matter. The new resulting theory, NRQCD with Glauber gluons, provides for the first time a universal microscopic description of the interaction of heavy quarkonia with a strongly interacting medium, consistently applicable to a range of phases, such as cold nuclear matter, dense hadron gas, and quark-gluon plasma. The effective field theory we present in this work is derived from first principles and is an important step forward in understanding the common trends in proton-nucleus and nucleus-nucleus data on quarkonium suppression.

Strongly coupled suppression in TeV Pb+Pb collisions

In a revolutionary paper, Matsui and Satz proposed using the suppression of quarkonia as a smoking gun signature of deconfinement in relativistic heavy ion collisions. The stunning success of using strong-coupling, AdS/CFT techniques to predict the viscosity to entropy density ratio extracted from RHIC and LHC heavy ion collision data using sophisticated 3+1D relativistic viscous hydrodynamics has prompted further investigation into the physics of a strongly-coupled plasma. We compute, for the first time, the suppression of bottomonia in a strongly coupled QGP, and compare the results to those from a weakly coupled QGP and to data. The complex binding energies which inform the thermal width and hence the RAA of are determined using imaginary time techniques.

Centrality and transverse momentum dependent suppression of Upsilon (1S) and Upsilon (2S) in p\u2013Pb and Pb\u2013Pb collisions at the CERN Large Hadron Collider

Deconfined QCD matter in heavy-ion collisions has been a topic of paramount interest for many years. Quarkonia suppression in heavy-ion collisions at the relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments indicate the quark-gluon plasma (QGP) formation in such collisions. Recent experiments at LHC have given indications of hot matter effect in asymmetric p–Pb nuclear collisions. Here, we employ a theoretical model to investigate the bottomonium suppression in Pb–Pb at sqrt{s_{NN}}=2.76, 5.02 TeV, and in p–Pb at sqrt{s_{NN}}=5.02 TeV center-of-mass energies under a QGP formation scenario. Our present formulation is based on an unified model consisting of suppression due to color screening, gluonic dissociation along with the collisional damping. Regeneration due to correlated Qbar{Q} pairs has also been taken into account in the current work. We obtain here the net bottomonium suppression in terms of survival probability under the combined effect of suppression plus regeneration in the deconfined QGP medium. We mainly concentrate here on the centrality, N_text {part} and transverse momentum, p_{T} dependence of Upsilon (1S) and Upsilon (2S) states suppression in Pb–Pb and p–Pb collisions at mid-rapidity. We compare our model predictions for Upsilon (1S) and Upsilon (2S) suppression with the corresponding experimental data obtained at the LHC energies. We find that the experimental observations on p_t and N_text {part} dependent suppression agree reasonably well with our model predictions.

Recent Quarkonium Studies from the PHENIX Experiment

Quarkonium suppression in nucleus-nucleus collisions is a powerful tool to probe the density and temperature of the medium created in heavy ion collisions. Forward rapidity measurements in p(d)+Au collisions are essential to understand how quarkonium states are affected by initial state effects, formation time, and local particle multiplicity. Earlier measurements in Au+Au collisions showed a stronger suppression of forward J/ψ compared to mid-rapidity results, indicating the possibility of a smaller contribution of regenerated quarkonium states at forward rapidity. These proceedings report on the latest quarkonium studies performed by the PHENIX collaboration in the rapidity range 1.2<|y|<2.2.

Bottomonium suppression at RHIC and LHC

The strong suppression of heavy quarkonia is a good indicator that one has generated a quark-gluon plasma (QGP) in an ultrarelativistic heavy ion collision. Recent advancements in first principles calculations of the heavy quark potential provide additional insight into this suppression and can be used to further pin down the in-medium properties of the QGP. Realistic 3+1d dissipative hydrodynamical models can be used to simulate the QGP and, with bottomonium as a probe, one can make inferences for the initial temperature of the QGP and other phenomenological parameters such as the shear viscosity to entropy density ratio. However, progressing to LHC sNN=5.02TeV Pb-Pb collisions, one expects regeneration to have an increasingly important impact on the suppression observables. In this proceedings, we present a brief overview to set up the model and then provide model results for bottomonium suppression and regeneration, for ultrarelativistic heavy-ion collision experiments at RHIC and LHC.