Quarkonium Suppression Research Articles

Small system studies: A theory overview

There is now a substantial body of evidence that a deconfined quark-gluon plasma is created in ultrarelativistic collisions of heavy nuclei. Some key observables which are used to gauge the production of the quark-gluon plasma are the hadronic spectra and multiplicities across many species, azimuthal anisotropies in the production of various hadrons, jet quenching, photon/dilepton production, and heavy quarkonium suppression. A key question in the study of the quark-gluon plasma is what happens to these observables as one changes the system size. In particular, it is very interesting to study collisions of “small systems” such as pp, dA, pA, and e+e− since naively one does not expect to produce a QGP in these cases. One of the surprises from such studies was the existence of sizable azimuthal anisotropies in the hadron spectra in the highest multiplicity classes, which led to speculations that one can generate a QGP in such relatively rare events. This interpretation, however, is complicated by the fact that there can be multiple sources of azimuthal anisotropy. I will discuss our current understanding of the different sources of azimuthal anisotropies and the multiplicity windows in which each mechanism is expected to dominate. I will also provide a critical discussion of the reliability of hydrodynamic models applied to small systems. Finally, I will briefly discuss jet quenching and heavy quarkonium suppression in small systems and discuss whether or not these observables show any indication of the production of a small QGP droplet.

The fate of quarkonia in heavy-ion collisions at LHC energies: a unified description of the sequential suppression patterns

Measurements made at the LHC have shown that the production of the mathrm{J}/psi , psi mathrm{(2S)}, varUpsilon mathrm{(1S)} and varUpsilon mathrm{(2S)} quarkonia is suppressed in Pb–Pb collisions, with respect to the extrapolation of the pp production yields. The psi mathrm{(2S)} and varUpsilon mathrm{(2S)} states are more strongly suppressed than the ground states and the level of the suppression changes with the centrality of the collision. We show that the measured patterns can be reproduced by a simple model, where all quarkonia are treated in a unified way, starting from the recent realisation that, in pp collisions, the probability of quarkonium formation has a universal dependence on the binding-energy of the bound state. The hot-medium suppression effect is parametrized by a penalty factor in the binding energy, identical for all (S- and P-wave) charmonium and bottomonium states, including those that indirectly contribute to the measured results through feed-down decays. This single parameter, computed through a global fit of all available suppression patterns, fully determines the hierarchy of nuclear effects, for all states and centrality bins. The resulting faithful description of the data provides convincing evidence in favour of the conjecture of sequential quarkonium suppression induced by QGP formation.

Probing the Time Structure of the Quark-Gluon Plasma with Top Quarks.

The tiny droplets of quark gluon plasma (QGP) created in high-energy nuclear collisions experience fast expansion and cooling with a lifetime of a few fm/c. Despite the information provided by probes such as jet quenching and quarkonium suppression, and the excellent description by hydrodynamical models, direct access to the time evolution of the system remains elusive. We point out that the study of hadronically decaying W bosons, notably in events with a top-antitop quark pair, can provide key novel insight into the time structure of the QGP. This is because of a unique feature, namely a time delay between the moment of the collision and that when the W-boson decay products start interacting with the medium. Furthermore, the length of the time delay can be constrained by selecting specific reconstructed top-quark momenta. We carry out a MonteCarlo feasibility study and find that the LHC has the potential to bring first limited information on the time structure of the QGP. Substantially increased LHC heavy-ion luminosities or future higher-energy colliders would open opportunities for more extensive studies.

Is pQCD energy loss in trouble?

Current pQCD calculations for the energy loss of a hard parton moving through a medium of thermalized static scattering centres are inapplicable to the small colliding systems (such as p/d + A) that have, in recent years, hinted at the presence of tiny droplets of QGP through the presence of collective behaviour, strangeness enhancement and quarkonium suppression. The well-known DGLV, ASW, BDMPS-Z, AMY and HT calculations all exploit both the large separation distance (between scattering centre and radiation) and the large system approximations. We relax the large system assumption and recompute the energy loss in the DGLV formalism in order to address the glaring lack of theoretical control over small-system energy loss. Alarmingly, we find that the correction terms dominate at large energies, resulting in ∼ 100% negative correction, calling into question the validity of the large formation time assumption used in all pQCD-based energy loss calculations. Our results demand a complete overhaul of energy-loss calculations for all system sizes.

High statistics study of in-medium S- and P-wave quarkonium states in lattice Non-relativistic QCD

Many measurements of quarkonium suppression at the LHC, e.g. the nuclear modification factor RAA of J/Ψ, are well described by a multitude of different models. Thus pinpointing the underlying physics aspects is difficult and guidance based on first principles is needed. Here we present the current status of our ongoing high precision study of in-medium spectral properties of both bottomonium and charmonium based on NRQCD on the lattice. This effective field theory allows us to capture the physics of quarkonium without modeling assumptions in a thermal QCD medium. In our study a first principles and realistic description of the QCD medium is provided by state-of-the-art lattices of the HotQCD collaboration at almost physical pion mass. Our updated results corroborate a picture of sequential modification of states with respect to their vacuum binding energy. Using a novel low-gain variant of the Bayesian BR method for reconstructing spectral functions we find that remnant features of the Upsilon may survive up to T ∼ 400MeV, while the χb signal disappears around T ∼ 270MeV. The cc‾ analysis hints at melting of χc below T ∼ 190MeV while some J/Ψ remnant feature might survive up to T ∼ 245MeV. An improved understanding of the numerical artifacts in the Bayesian approach and the availability of increased statistics have made possible a first quantitative study of the in-medium ground state masses, which tend to lower values as T increases, consistent with lattice potential based studies.

Bottomonium suppression in heavy-ion collisions

The thermal suppression of heavy quark bound states represents an ideal observable for determining if one has produced a quark-gluon plasma in ultrarelativistic heavy-ion collisions. In recent years, a paradigm shift has taken place in the theory of quarkonium suppression due to new first principles calculations of the thermal widths of these states. These thermal widths are large, e.g. O(20–100 MeV) for the ϒ(1S), and cause in-medium suppression of the states at temperatures below their traditionally defined disassociation temperatures. In order to apply the newly developed understanding to phenomenology, however, one must make detailed 3+1d dissipative hydrodynamical models of the plasma including the effects of finite shear viscosity. These effects include not only the modification of the time evolution of the temperature of the system, flow, etc., but also non-equilibrium modifications of the heavy quark potential itself. In this proceedings contribution, we briefly review the setup for these model calculations and present comparisons of theory with data from RHIC 200 GeV/nucleon Au-Au collisions, LHC 2.76 TeV/nucleon Pb-Pb, and LHC 5.02 TeV/nucleon Pb-Pb collisions as a function of number of participants, rapidity, and transverse momentum.

PHENIX measurements of open and hidden heavy flavor in p+p, p+Al, and p/d/3He+Au collisions across a wide range of rapidity

Despite intense theoretical and experimental investigation, the physical mechanisms governing the suppression of bound quark-antiquark states in nuclear collisions are not yet fully understood. While color screening in a plasma phase is expected to play a role, there are numerous other possible suppression mechanisms that do not require deconfinement, as well as effects on the heavy quark initial state in the nucleus which can also play a role. To study these effects, the PHENIX collaboration has used the flexibility of the RHIC accelerator complex to observe the evolution of open heavy flavor and quarkonia dynamics as both the projectile and target nuclei size are varied. Open heavy flavor in small collision systems can serve as the baseline for interpreting quarkonia production in the nuclear environment, and comparisons of the ψ(2S) with the ψ(1S) show that in rapidity regions with relatively high hadron density, the larger 2S state is preferentially more suppressed than the more tightly bound ψ(1S). This suggests that late-stage mechanisms may be at least partially responsible for quarkonia suppression in nuclear collisions. In this talk, we will present results on excited-state quarkonia in p+p, p+Al, and p/d/3He+Au collisions and open heavy flavor in small systems, and discuss how these measurements impact our understanding of heavy quark behavior in the quark-gluon plasma.

A generalization to all system sizes of DGLV energy loss in the quark-gluon plasma

In the last few years, experiments with small colliding systems (such as p/d + A) have yielded some evidence for the presence of small droplets of quark-gluon plasma (QGP). The collective behavior, strangeness enhancement and quarkonium suppression that have been seen beg the question: What of the energy loss? However, in order to answer such a question, one must apply existing perturbative Quantum Chromodynamical (pQCD) energy loss calculations to systems that are much smaller than the systems in which pQCD energy loss calculations were originally so successful, a process that is not currently allowed for in any pQCD energy loss calculation. We relax the large system assumption in the well-known DGLV formalism (an assumption that is also utilized in the ASW, BDMPS-Z, AM and HT calculations), but find, alarmingly, that the correction terms dominate at large energies, resulting in ∼ 100% negative correction, calling into question the validity of the large formation time assumption used in all pQCD-based energy loss calculations. Our results demand a complete overhaul of energy-loss calculations for all system sizes.

Fixed target project AFTER at the LHC beams for heavy ion and hadron physics

High intensity proton and lead ion beams at the LHC collider allow one to use the beam halo by placing a fixed target or a bent crystal for beam extraction. The particle energy in this case is just half that at the RHIC collider, but the luminosity exceeds the collider luminosity many times. It is also possible to install a polarized target in the extracted beam. The project AFTER is aimed at investigation of rare processes, polarization phenomena, determination of the parameters required for analysis of cosmic rays and neutrino astrophysics, detailed investigation of quarkonia production and suppression depending on the phase transition of matter to the quark-gluon phase.

Overview of ATLAS results

The heavy-ion programme in the ATLAS experiment at the Large Hadron Collider aims to probe and characterise hot and dense matter created in relativistic lead-lead collisions. Moreover, smaller collision systems involving nuclei and hadrons are of interest to disentangle initial- from final-state effects. This report presents new results based on lead-lead and proton-proton data collected at sNN=5.02 TeV in 2015, including measurements of bulk collectivity, charged-particle production, electroweak bosons, photon-jet correlations, and quarkonium suppression. First attempts to measure electromagnetic processes in ultra-peripheral collisions are also discussed.

Suppression versus enhancement of heavy quarkonia in pA collisions

We describe production of heavy quarkonia in pA collisions within the dipole approach, assuming dominance of the perturbative color-singlet mechanism (CSM) in the $p_T$-integrated cross section. Although accounting for a nonzero heavy $Q$-$\bar Q$ separation is a higher twist correction, usually neglected, we found it to be the dominant source of nuclear effects, significantly exceeding the effects of leading twist gluon shadowing and energy loss. Moreover, this contribution turns out to be the most reliably predicted, relying on the precise measurements of the dipole cross section at HERA. The nuclear suppression of quarkonia has been anticipated to become stronger with energy, because the dipole cross section steeply rises. However, the measured nuclear effects remain essentially unchanged within the energy range from RHIC to the LHC. A novel production mechanism is proposed, which enhances the charmonium yield. Nuclear effects for the production of $J/\psi$, $\psi(2S)$, $\Upsilon(1S)$ and $\Upsilon(2S)$ are calculated, in agreement with data from RHIC and LHC. The dipole description offers a unique explanation for the observed significant nuclear suppression of $\psi(2S)$ to $J/\psi$ ratio, related to the nontrivial features of the $\psi(2S)$ wave function.

Heavy-ion collisions in a fixed target mode at the LHC beams

The interaction of high-energy LHC beams with fixed target, including polarized nuclei targets, can expand the range of fundamental physical investigations accessible at CERN. High-intensity beam of protons and lead ions accumulated in the LHC collider allows to apply the gas-target system like the LHCb SMOG or HERMES systems. It is also possible to use the beam halo by placing in the halo the fixed target in the form of thin ribbon or use a bend crystal to extract the beam. In the extracted beam it is possible to install a polarized target. Using the proton and ion beams of the LHC with fixed targets, the data in the energy interval between maximum energy of the SPS and the nominal RHIC energy in p-A and A-A collisions could be obtained. The fixed target mode allows the intensive study of rare processes, the study of polarization phenomena, the measurements of the parameters needed to analyze the data of cosmic rays and neutrino astrophysics, detailed study of the processes of quarkonia production and suppression. The high statistics data on quarkonium production at these energies will give the possibility to clarify the mechanism of production, to investigate the importance of recombination process and the energy dependence on the phase transition of nuclear matter to the quark-gluon phase. Also the physical program includes the study of the Drell-Yan process, D-meson production, flow and spin physics.

Effective Field Theories for heavy probes in a hot QCD plasma and in the early universe

There are many interesting problems in heavy-ion collisions and in cosmology that involve the interaction of a heavy particle with a medium. An example is the dissociation of heavy quarkonium seen in heavy-ion collisions. This was believed to be due to the screening of chromoelectric fields that prevents the heavy quarks from binding, however in the last years several perturbative and lattice computations have pointed out to the possibility that dissociation is due to the finite lifetime of a quarkonium state inside the medium. Regarding cosmology, the study of the behavior of heavy Majorana neutrinos in a hot medium is important to understand if this model can explain the origin of dark matter and the baryon asymmetry. A very convenient way of studying these problems is with the use of non-relativistic effective field theories (EFTs), this allows to make the computations in a more systematic way by defining a more suitable power counting and making it more difficult to miss necessary resummations. In this proceedings I will review the most important results obtained by applying the EFT formalism to the study of quarkonium suppression and Majorana neutrinos, I will also discuss how combining an EFT called potential non-relativistic QCD (pNRQCD) with concepts coming from the field of open quantum systems it is possible to understand how the population of the different quarkonium states evolve with time inside a thermal medium.

Entropic destruction of heavy quarkonium in the quark-gluon plasma

The excitations of a bound state immersed in a strongly coupled system are often delocalized and characterized by a large entropy, so that the state is strongly entangled with the rest of the statistical system. If this entropy S increases with the separation r between the constituents of the bound state, S=S(r), then the resulting entropicforceF=T∂S/∂r (T is temperature) can drive the dissociation process. Lattice QCD indicates a large amount of entropy associated with the heavy quark pair in strongly coupled quark-gluon plasma. This entropy S(r) peaks at temperatures 0.9Tc≤T≤1.5Tc (Tc is the deconfinement temperature) and grows with the inter-quark distance r. This peak in the holographic description arises because the heavy quark pair acts as an eyewitness to the black hole formation in the bulk – the process that describes the deconfinement transition. In terms of the boundary theory, this entropy likely emerges from the entanglement of a “long string” connecting the quark and antiquark with the rest of the system. We argue that the entropic mechanism results in an anomalously strong quarkonium suppression in the temperature range near Tc. This entropic destruction may thus explain why the experimentally measured quarkonium nuclear modification factor at RHIC (lower energy density) is smaller than at LHC (higher energy density), possibly resolving the “quarkonium suppression puzzle” – all of the previously known mechanisms of quarkonium dissociation operate more effectively at higher energy densities, and this contradicts the data.

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.

Quarkonia in high energy nuclear collisions

We first review the cold and hot nuclear matter effects on quarkonium production in high energy collisions, then discuss three kinds of models to describe the quarkonium suppression and regeneration: the sequential dissociation, the statistical production and the transport approach, and finally make comparisons between the models and the experimental data from heavy ion collisions at SPS, RHIC and LHC energies.

Unified description of charmonium suppression in a quark-gluon plasma medium at RHIC and LHC energies

Recent experimental and theoretical studies suggest that the quarkonium suppression in a thermal QCD medium created in heavy ion collisions is a complex interplay of various physical processes. In this article we put together most of these processes in a unified way to calculate the charmonium survival probability (nuclear modification factor) at energies available at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments. We include shadowing as the dominant cold-nuclear-matter effect. Further, gluonic dissociation and collision damping are included, which provide width to the spectral function of charmonia in a thermal medium and cause the dissociation of charmonium along with the usual color screening. We include color screening by using our recently proposed modified Chu--Matsui model. Furthermore, we incorporate the recombination of uncorrelated charm and anticharm quarks for the regeneration of charmonium over the entire temporal evolution of the QGP medium. Finally, we do a feed-down correction from the excited states to calculate the survival probability of charmonium. We find that our unified model suitably and simultaneously describes the experimental nuclear modification data of $J/\ensuremath{\psi}$ at RHIC and LHC.

Quarkonia suppression in PbPb collisions atsNN=2.76TeV

We estimate the modification of quarkonia yields due to different processes in the medium produced in PbPb collisions at LHC energy. The quarkonia and heavy flavor cross sections calculated up to next-to-leading order (NLO) are used in the study. Shadowing corrections are obtained with the NLO EPS09 parametrization. A kinetic model is employed which incorporates quarkonia suppression inside a QGP, suppression due to hadronic comovers, and regeneration from charm pairs. The quarkonia dissociation cross section due to gluon collisions has been considered and the regeneration rate has been obtained using the principle of detailed balance. The modification in quarkonia yields due to collisions with hadronic comovers has been estimated assuming that the comovers are pions. The manifestations of these effects on the nuclear modification factors for both $J/\ensuremath{\psi}$ and $\mathrm{\ensuremath{\Upsilon}}$ in different kinematic regions has been demonstrated for PbPb collisions at $\sqrt{{s}_{{}_{NN}}}=2.76$ TeV in comparison with the measurements. Both the suppression and regeneration due to a deconfined medium strongly affect the low and intermediate ${p}_{T}$ range. The large observed suppression of $J/\ensuremath{\psi}$ at ${p}_{T}g10\text{GeV}/c$ exceeds the estimates of suppression by gluon dissociation.

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.

Dilepton production from RHIC to the LHC

The goal of ultrarelativistic heavy-ion collisions at RHIC and the LHC is to study the properties of the quark-gluon plasma (QGP), a phase of matter with partonic degrees of freedom. Electromagnetic radiation, in form of photons or lepton pairs, is a penetrating probe that allows the investigation of the full time evolution and dynamics of the produced matter as it does not undergo strong interaction in the final state. The dilepton spectrum is extremely rich in physics sources: Thermal black-body radiation is of particular interest as it carries information about the QGP temperature. Modifications of the spectral functions of light vector mesons are linked to the potential restoration of chiral symmetry in the QGP phase. Correlated lepton pairs from semi-leptonic charm and beauty decays provide additional information about the heavy-quark energy loss. Finally, the suppression of quarkonia in the QGP give access to an independent temperature measurement. In this proceedings, dilepton results from RHIC are reviewed and the status as well as prospects of low-mass dilepton measurements at the LHC are given.