Suppression In Heavy Ion Collisions Research Articles

Correction to: Bottomonium suppression in heavy-ion collisions and the in-medium strong force

Correction to: Bottomonium suppression in heavy-ion collisions and the in-medium strong force

Bottomonium suppression in heavy-ion collisions and the in-medium strong force

Bottomonium suppression in heavy-ion collisions and the in-medium strong force

Measurement of Sequential ϒ Suppression in Au+Au Collisions at sqrt[s_{NN}]=200 GeV with the STAR Experiment.

We report on measurements of sequential ϒ suppression in Au+Au collisions at sqrt[s_{NN}]=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) through both the dielectron and dimuon decay channels. In the 0%-60% centrality class, the nuclear modification factors (R_{AA}), which quantify the level of yield suppression in heavy-ion collisions compared to p+p collisions, for ϒ(1S) and ϒ(2S) are 0.40±0.03(stat)±0.03(sys)±0.09(norm) and 0.26±0.08(stat)±0.02(sys)±0.06(norm), respectively, while the upper limit of the ϒ(3S) R_{AA} is 0.17 at a 95%confidence level. This provides experimental evidence that the ϒ(3S) is significantly more suppressed than the ϒ(1S) at RHIC. The level of suppression for ϒ(1S) is comparable to that observed at the much higher collision energy at the Large Hadron Collider. These results point to the creation of a medium at RHIC whose temperature is sufficiently high to strongly suppress excited ϒ states.

Cone-Size Dependence of Jet Suppression in Heavy-Ion Collisions.

The strong suppression of high-p_{T} jets in heavy-ion collisions is a result of elastic and inelastic energy loss suffered by the jet multiprong collection of color charges that are resolved by medium interactions. Hence, quenching effects depend on the fluctuations of the jet substructure that are probed by the cone-size dependence of the spectrum. In this Letter, we present the first complete, analytic calculation of the inclusive R-dependent jet spectrum in PbPb collisions at LHC energies, including resummation of energy loss effects from hard, vacuumlike emissions occurring in the medium and modeling of soft energy flow and recovery at the jet cone. Both the geometry of the collision and the local medium properties, such as the temperature and fluid velocity, are given by a hydrodynamic evolution of the medium, leaving only the coupling constant in the medium as a free parameter. The calculation yields a good description of the centrality and p_{T} dependence of jet suppression for R=0.4 together with a mild cone-size dependence, which is in agreement with recent experimental results. Gauging the theoretical uncertainties, we find that the largest sensitivity resides in the leading logarithmic approximation of the phase space of resolved splittings, which can be improved systematically, while nonperturbative modeling of the soft-gluon sector is of relatively minor importance up to large cone sizes.

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.

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.

Interplaying mechanisms behind inclusive jet RAA and extraction of jet energy loss distributions

The observed inclusive jet suppression in heavy-ion collisions at LHC has a very weak pT dependence over a large range of pT=50–1000GeV and is almost independent of the colliding energy, though the initial energy density of the bulk medium has increased from s=2.76 to 5.02TeV by about 20%. This interesting phenomenon is investigated in the linear Boltzmann transport (LBT) model for jet propagation in an event-by-event 3+1D hydro background. We show that the pT dependence of jet RAA is determined by the initial spectrum in p+p collisions and pT dependence of jet energy loss. Furthermore, jet energy loss distributions for inclusive jet and γ-jet at both LHC energies are extracted directly from experimental data through the state-of-art Bayesian analysis. The averaged jet energy loss has a weak pT dependence and the scaled jet energy loss distributions have a large width, both of which are consistent with the LBT simulations and indicate that jet quenching is caused by only a few out-of-cone jet medium scatterings.

Experimental review of the Υ(1S, 2S, 3S) physics at e+e− colliders and the LHC

The three lowest-lying $\Upsilon$ states, i.e. $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$, composed of $b\bar b$ pairs and below the $B\bar B$ threshold, provide a good platform for the researches of hadronic physics and physics beyond the Standard Model. They can be produced directly in $e^+e^-$ colliding experiments, such as CLEO, Babar, and Belle, with low continuum backgrounds. In these experiments, many measurements of the exclusive $\Upsilon(1S)$ and $\Upsilon(2S)$ decays into light hadrons, which shed light on the "80\% rule" for the Okubo-Zweig-Iizuka suppressed decays in the bottomonium sector, were carried out. Meanwhile, many studies of the charmonium and bottomonium productions in $\Upsilon(1S,2S,3S)$ decays were performed, to distinguish different Quantum Chromodynamics (QCD) models. Besides, exotic states and new physics were also extensively explored in $\Upsilon(1S,2S,3S)$ decays at CLEO, BaBar, and Belle. The $\Upsilon(1S,2S,3S)$ states can also be produced in $pp$ collisions and in collisions involving heavy ions. The precision measurements of their cross sections and polarizations at the large hadron collider (LHC), especially in the CMS, ATLAS, and LHCb experiments, help to understand $\Upsilon$ production mechanisms in $pp$ collisions. The observation of the sequential $\Upsilon$ suppression in heavy ion collisions at CMS is of great importance for verifying the quark-gluon plasma predicted by QCD. In this article, we review the experimental results on $\Upsilon(1S,2S,3S)$ at $e^+e^-$ colliders and the LHC, and summarize their prospects at Belle II and the LHC.

Bottomonia suppression in heavy-ion collisions from anti–de Sitter space/conformal field theory

We compute for the first time the suppression of bottomonia in a strongly coupled quark-gluon plasma (QGP) and compare the results to those from a weakly coupled QGP. Using imaginary time techniques we numerically determine the real and imaginary parts of the ground-state binding energy of the bottomonia in one potential computed from anti--de Sitter space/conformal field theory (AdS/CFT) and another computed from perturbative quantum chromodynamics. We confirm the strong-coupling binding energies by independently deriving the meson spectrum in AdS/CFT using semiclassical, rotating open strings. We then use these binding energies in a suppression model to determine the $\mathrm{\ensuremath{\Upsilon}}$(1S) nuclear modification factor in ${\sqrt{s}}_{NN}=2.76$-TeV $\mathrm{Pb}+\mathrm{Pb}$ collisions. AdS/CFT significantly overpredicts the suppression compared to data, although the predictive power of our calculation is limited by its significant sensitivity to the exact details of the suppression model.

Equation of States and Charmonium Suppression in Heavy-Ion Collisions

The present article is the follow-up of our work Bottomonium suppression in quasi-particle model, where we have extended the study for charmonium states using quasi-particle model in terms of quasi-gluons and quasi quarks/antiquarks as an equation of state. By employing medium modification to a heavy quark potential thermodynamic observables, viz., pressure, energy density, speed of sound, etc. have been calculated which nicely fit with the lattice equation of state for gluon, massless, and as well massive flavored plasma. For obtaining the thermodynamic observables we employed the debye mass in the quasi particle picture. We extended the quasi-particle model to calculate charmonium suppression in an expanding, dissipative strongly interacting QGP medium (SIQGP). We obtained the suppression pattern for charmonium states with respect to the number of participants at mid-rapidity and compared it with the experimental data (CMS JHEP) and (CMS PAS) at LHC energy (Pb+Pb collisions, sNN = 2.76 TeV).

Interplaying mechanisms behind single inclusive jet suppression in heavy-ion collisions

The suppression factor for single inclusive jets in Pb+Pb collisions at the Large Hadron Collider (LHC) has a weak dependence on the transverse momentum $p_T$ and remains almost the same at two colliding energies, $\sqrt{s}=2.76$ and 5.02 TeV, though the central rapidity density of bulk hadrons increases by about 20\%. This phenomenon is investigated within the Linear Boltzmann Transport (LBT) model, which includes elastic and inelastic processes based on perturbative QCD for both jet shower and recoil medium partons as they propagate through a quark-gluon plasma (QGP). With the dynamic evolution of the QGP given by the 3+1D CLVisc hydrodynamic model with event-by-event fully fluctuating initial conditions, single inclusive jet suppression in Pb+Pb collisions from LBT agrees well with experimental data. The weak $\sqrt{s}$ and $p_T$-dependence of the jet suppression factor at LHC are found to result directly from the $\sqrt{s}$-dependence of the initial jet $p_T$ spectra and slow $p_T$-dependence of the jet energy loss. Contributions from jet-induced medium response, influence of radial expansion, both of which depend on jet-cone size, and jet flavor composition all conjoin to give a slow $p_T$-dependence of jet energy loss and the single jet suppression factor $R_{\rm AA}$, their dependence on $\sqrt{s}$ and jet-cone size. Single inclusive jet suppression at $\sqrt{s}=200$ GeV is also predicted that actually decreases slightly with $p_T$ in the $p_T<50$ GeV/$c$ range because of the steeper initial jet spectra though the $p_T$-dependence of the jet energy loss is weaker than that at LHC.

Simultaneous description of hadron and jet suppression in heavy-ion collisions

We present a global fit to all data on the suppression of high-energy jets and high-energy hadrons in the most central heavy-ion collisions at the CERN Large Hadron Collider (LHC) for two different collision energies, within a hybrid strong-weak coupling quenching model. Even though the measured suppression factors for hadrons and jets differ significantly from one another and appear to asymptote to different values in the high-energy limit, we obtain a simultaneous description of all these data after constraining the value of a single model parameter. We use our model to investigate the origin of the difference between the observed suppression of jets and hadrons and relate it, quantitatively, to the observed modification of the jet fragmentation function in jets that have been modified by passage through the medium produced in heavy-ion collisions. In particular, the observed increase in the fraction of hard fragments in medium-modified jets, which indicates that jets with the fewest hardest fragments lose the least energy, corresponds quantitatively to the observed difference between the suppression of hadrons and jets. We argue that a harder fragmentation pattern for jets with a given energy after quenching is a generic feature of any mechanism for the interaction between jets and the medium that they traverse that yields a larger suppression for wider jets. We also compare the results of our global fit to LHC data to measurements of the suppression of high-energy hadrons in BNL Relativistic Heavy Ion Collider (RHIC) collisions, and find that with its parameter chosen to fit the LHC data, our model is inconsistent with the RHIC data at the $3\ensuremath{\sigma}$ level, suggesting that hard probes interact more strongly with the less hot quark-gluon plasma produced at RHIC.

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.

Parton Energy Loss and Charmonia Suppression in Heavy-ion Collisions

Parton Energy Loss and Charmonia Suppression in Heavy-ion Collisions

Cold nuclear matter effects on jet suppression in heavy-ion collisions

These proceedings discuss the sensitivity of the nuclear modification factor RAA of fully reconstructed jets to cold nuclear matter effects. To test the parton energy loss interpretation of the observed RAA of ∼0.3, obtained from reconstructed jets in Pb–Pb collisions at = 2.76 TeV, we measured the inclusive jet production in minimum bias p–Pb collisions at = 5.02 TeV for resolution parameters R=0.2 and 0.4. The reconstructed jets incorporate the neutral and charged energy component and cover a momentum range = 20-90 GeV. The comparison of the jet yield to PYTHIA simulations shows no significant depletion in the measured jet cross section attributed to cold nuclear matter effects present in p–Pb collisions.

Sequential Bottomonium Production at High Temperatures

Bottomonium production in heavy ion collisions is modified compared with any simple extrapolation from elementary collisions. This modification is most likely caused by the presence of a deconfined system of quarks and gluons for times of several fm/c. In such a medium, bottomonium can be destroyed, but the constituent bottom quarks will likely stay spatially correlated due to small mean free paths in this system. With these facts in mind, we describe bottomonium formation with a coupled set of equations. A rate equation describes the destruction of $\Upsilon(1S)$ particles, while a Langevin equation describes how the bottom quarks stay correlated for a sufficiently long time so that recombination into bottomonia is possible. We show that within this approach it is possible to understand the magnitude of $\Upsilon(1S)$ suppression in heavy ion collisions and the larger suppression of the $\Upsilon(2S)$ state, implying that the reduction in the ratio of $\Upsilon(1S)/\Upsilon(2S)$ yield in heavy ion collision does not necessarily correspond to sequential melting picture.

Quarkonium suppression in heavy-ion collisions from coherent energy loss in cold nuclear matter

The effect of parton energy loss in cold nuclear matter on the suppression of quarkonia (J/ψ, ϒ) in heavy-ion collisions is investigated, by extrapolating a model based on coherent radiative energy loss recently shown to describe successfully J/ψ and ϒ suppression in proton-nucleus collisions. Model predictions in heavy-ion collisions at RHIC (Au-Au, Cu-Cu, and Cu-Au) and LHC (Pb-Pb) show a sizable suppression arising from the sole effect of energy loss in cold matter. This effect should thus be considered in order to get a reliable baseline for cold nuclear matter effects in quarkonium suppression in heavy-ion collisions, in view of disentangling hot from cold nuclear effects.

NLO Monte Carlo predictions for heavy-quark production at the LHC: pp collisions in ALICE

Next-to-leading order (NLO) QCD predictions for the production of heavy quarks in proton-proton collisions are presented within three different approaches to quark mass, resummation and fragmentation effects. In particular, new NLO and parton shower simulations with POWHEG are performed in the ALICE kinematic regime at three different centre-of-mass energies, including scale and parton density variations, in order to establish a reliable baseline for future detailed studies of heavy-quark suppression in heavy-ion collisions. Very good agreement of POWHEG is found with FONLL, in particular for centrally produced D^0, D^+ and D^*+ mesons and electrons from charm and bottom quark decays, but also with the generally somewhat higher GM-VFNS predictions within the theoretical uncertainties. The latter are dominated by scale rather than quark mass variations. Parton density uncertainties for charm and bottom quark production are computed here with POWHEG for the first time and shown to be dominant in the forward regime, e.g. for muons coming from heavy-flavour decays. The fragmentation into D_s^+ mesons seems to require further tuning within the NLO Monte Carlo approach.

Open and closed heavy-flavour suppression in heavy-ion collisions with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure quarkonia in the dimuon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Quarkonia are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/ψ (from b-hadron decays), prompt J/ψ, inclusive ψ(2S), and the first three ϒ states in PbPb collisions at = 2.76 TeV. A suppression of non-prompt J/ψ, which is sensitive to the in-medium b-quark energy loss, has been measured at relatively high pT (6.5 < pT < 30 GeV/c) in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. For prompt J/ψ in the same kinematic range, a strong, centrality-dependent suppression is observed. Such strong suppression at high pT has previously not been observed at RHIC. At mid-rapidity and high pT, ψ(2S) show an even stronger suppression than J/ψ. Furthermore, CMS has measured the suppression of the three ϒ states, separately, down to pT =0 GeV/c. A clear ordering of the suppression with binding energy is observed, as expected from sequential melting of quarkonium states.

Measurement of beam energy dependent nuclear modification factors at STAR

The nuclear modification factors RAA and RCP have been used to measure medium-induced suppression in heavy-ion collisions at = 200 GeV which was among the earliest evidence for the existence of a strongly interacting medium called a quark-gluon plasma (QGP). Nuclear modification factors for asymmetric collisions (RdA) have measured the Cronin Effect, an enhancement of high transverse momentum particle yields in deuteron-gold collisions relative to proton-proton collisions. A similar enhancement is observed in data presented in these proceedings and competes with the quenching caused by partonic energy loss in the QGP. In these proceedings we will present charged-hadron RCP at mid-rapidity for = 7.7 – 62.4 GeV as well as identified π+, K+, and p RCP. Comparisons to HIJING motivate possible methods for disentangling competing modifications to nuclear transverse momentum spectra.