Charmonium production in proton-proton collisions and in collisions of lead nuclei at CERN and comparison with Brookhaven data

A review of experimental data on charmoniumproduction that were obtained in fixed-target experiments at the SPS synchrotron and in proton-proton collisions and in collisions of lead nuclei in beams of the Large Hadron Collider (LHC) at CERN (Switzerland) is presented. A comparison with data obtained at the Brookhaven National Laboratory (USA) from experiments at the Relativistic Heavy Ion Collider (RHIC) is performed. Measurement of the suppression of J/ψ-meson production as a possible signal of the production of quark-gluon plasmawas proposed back in 1986 by T. Matsui and H. Satz. An anomalous suppression of J/ψ-meson production was discovered by the NA50 Collaboration at SPS (CERN) in central collisions of lead nuclei at the c.m. collision energy of 158 GeV per nucleon. Data obtained at the c.m. energy of 200 GeV per nucleon in the PHENIX experiment at RHIC indicate that, depending on multiplicity, the suppression of J/ψ-meson production at this energy approximately corresponds to the suppression of J/ψ-meson production in collisions of lead nuclei at the SPS accelerator. Theoretical models that take into account the regeneration of J/ψ mesons describe better RHIC experimental data. The measurement of charmonium production in proton-proton collisions and in collisions of lead nuclei in LHC beams revealed the importance of taking into account the regeneration process. At the LHC energies, it is also necessary to take into account the contribution of B-meson decays. Future measurements of charmonium production at the LHC to a higher statistical precision and over an extended energy region would be of importance for obtaining deeper insight into the mechanism of charmonium production and for studying the properties of matter at high energy density and temperature.     

Charmonium-state production in heavy-ion collisions

An overview of existing experimental data on the production of charmonium states (J/ψ and ψ′) at the superproton synchrotron (SPS, CERN) and the relativistic heavy ion collider (RHIC, Brookhaven National Laboratory, United States) is given. The production of J/ψ mesons shows an anomalous suppression discovered by the NA50 Collaboration (CERN) in collisions of lead nuclei at an energy of 158 GeV per nucleon and confirmed in the NA60 experiment (CERN) in collisions of indium nuclei at the same energy. The suppression of J/ψ production depends on interaction centrality and becomes anomalous at about 122 participant nucleons in PbPb collisions and at about 86 participant nucleons in InIn collisions. The experimental data in question are compared with the predictions of existing theoretical models. None of the models is able to simultaneously describe data on PbPb and InIn collisions. Data obtained in the PHENIX experiment at RHIC from measurements of J/ψ production in AuAu and CuCu collisions at an energy of 200 GeV (in the nucleon-nucleon c.m. frame) indicate that the suppression of J/ψ production at such energies approximately corresponds to the suppression of J/ψ production in PbPb collisions at SPS. Theoretical models that describe SPS data on PbPb collisions predict a stronger suppression of J/ψ production. Models that take into account J/ψ-meson regeneration better describe experimental data obtained at RHIC. Measurement of cross sections for charmonium and bottomonium production at the Large Hadron Collider (LHC, CERN) would make it possible to study the properties of matter and to explore the mechanism of quarkonium production at ultrahigh energy densities and temperature and high transverse momenta, as well as to investigate the effect of the regeneration and suppression of quarkonium production as the energy increases.     

High-mass dimuon resonances in Pb-Pb collisions at 5.5 TeV in CMS

The measurement of the charmonium (J/ψ, ψ′) and bottomonium (γ, γ′, γ″) resonances and Z ⁰ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of J/ψ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the γ family, only possible at LHC energies. The Z ⁰ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through q-q fusion, it is an excellent probe of the nuclear modification of quark distribution functions. This paper reports the capabilities of the CMS detector to study quarkonium and Z ⁰ production in Pb-Pb collisions at 5.5 TeV, through the dimuon decay channel.     

Quarkonium suppression: Gluonic dissociation vs. colour screening

We evaluate the suppressionof J/ψ production in an equilibrating quark gluon plasma for two competing mechanisms: Debye screening of colour interaction and dissociation due to energetic gluons. Results are obtained for S + S and Au + Au collisions at RHIC and LHC energies. At RHIC energies the gluonic dissociation of the charmonium is found to be equally important for both the systems while the screening of the interaction plays a significant role only for the larger systems. At LHC energies the Debye mechanism is found to dominate both the systems. While considering the suppression of directly produced T at LHC energies, we find that only the gluonic dissociation mechanism comes into play for the initial conditions taken from the self screened parton cascade model in these studies.     

(Non)thermal aspects of charmonium production and a new look at J/ψ suppression

To investigate a recent proposal that J/ψ production in ultra-relativistic nuclear collisions is of thermal origin we have reanalyzed the data from the NA38/50 Collaboration within a thermal model including charm. Comparison of the calculated with measured yields demonstrates the non-thermal origin of hidden charm production at SPS energy. However, the ratio ψ′/(J/ψ) exhibits, in central nucleus-nucleus collisions, thermal features which lead us to a new interpretation of open charm and charmonium production at SPS energy. Implications for RHIC and LHC energy measurements will be discussed.     

Can the RHIC <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> puzzle(s) be settled at LHC?

One observes strong suppression effects for hard probes, e.g. the production of J/ψ or high-p _T particles, in nucleus–nucleus (AA) collisions at RHIC. Surprisingly, the magnitude of the suppression is quite similar to that at SPS. In order to establish whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate shadowing for gluons in the Glauber–Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated by energy-momentum conservation effects. Furthermore, final-state suppression due to interaction with comoving matter (hadronic or pre-hadronic) has been shown to describe the data at SPS. We extend this model by including the backward reaction channel, i.e. recombination of open charm, which is estimated directly from pp data at RHIC. Strong suppression of charmonium both in pA and AA collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization of heavy quarks.     

LHC capabilities for quarkonium

The measurement of the charmonium and bottomonium resonances in nucleus-nucleus collisions provides crucial information on high-density QCD matter. First, the suppression of quarkonia production is generally agreed to be one of the most direct probes of quark-gluon plasma formation. The observation of anomalous J/ψ suppression at the CERN-SPS and at RHIC is well established but the clarification of some important remaining questions requires equivalent studies of the ϒ family, will be possible at the LHC. Second, the production of heavy-quarks proceeds mainly via gluon-gluon fusion processes and, as such, is sensitive to saturation of the gluon density at low-x in the nucleus. Measured departures from the expected vacuum quarkonia cross-sections in Pb+Pb collisions at the LHC will thus provide valuable information not only on the thermodynamical state of the produced partonic medium, but also on the initial-state modifications of the nuclear parton distribution functions. The capabilities of the LHC detectors (ALICE, ATLAS and CMS) to study quarkonia production in Pb+Pb collisions at 5.5 TeV per nucleon pair are discussed.     

Production of secondaries in high-energy heavy-ion collisions

In the framework of the quark-gluon string model, we calculate the inclusive spectra of secondaries produced in heavy-ion collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. We demonstrate that the mechanism of secondary production changed drastically in the energy interval √s = 20–60 GeV and that it is in agreement with qualitative estimates of Glauber-Gribov theory. The results of numerical calculations at intermediate energies are in reasonable agreement with the data without change of the model parameters. At RHIC energies, numerically large inelastic screening correlations should be accounted for in calculations. The text was submitted by the authors in English.     

Latest results onJ/ψ anomalous suppression

The NA50 experiment deals with Pb-Pb collisions at 158 GeV/nucleon at the CERN SPS accelerator. TheJ/ψ production is studied through the muon decay channel, using the Drell-Yan dimuons as a reference. New results based on recent analyses, from data taken with improved experimental conditions and using different centrality estimators, are presented and compared to an update of those already obtained from previous data samples. The stepwise pattern of the anomalousJ/ψ suppression as a function of centrality, already present in these previous results, is confirmed. This observation could be a fingerprint of the theoretically predicted melting of charmonia resonances in a deconfined quark-gluon plasma. On leave of absence from York College, CUNY, New York USA On leave of absence from YerPhI, Yerevan, Armenia     

B-meson measurement via secondary J/ψ production in Pb-Pb collisions at ?{sNN } \sqrt {s_{NN} } = 5.5 TeV in the CMS

The production of heavy quarks in heavy-ion collisions is important because it is expected to give information about high-density QCD matter. One interesting channel of heavy quark measurements at LHC is the decay of B to J/ψ. The J/ψ mesons from this channel can be separated from the primary J/ψ (produced at collision vertex) by using the secondary vertex which is efficiently reconstructed with the CMS tracker. In this work, the generator level results for the primary and secondary J/ψ obtained through dimuon pairs are presented for pp and PbPb collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 5.5 TeV.     

Production of secondaries in high-energy <Emphasis Type="Italic">d</Emphasis>+Au collisions

In the framework of the quark–gluon string model we calculate the inclusive spectra of secondaries produced in d+Au collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. The results of numerical calculations at intermediate energies are in reasonable agreement with the data. At RHIC energies numerically large inelastic screening corrections (percolation effects) should be accounted for in the calculations. We extract these effects from the existing experimental data of RHIC on minimum-bias and central d+Au collisions. The predictions for p+Au interactions at LHC energy are also given.     

Measurement of two-particle correlations in pp collisions at √<Emphasis Type="Italic">s</Emphasis> = 900 GeV with the ALICE detector

ALICE is the experiment at the CERN Large Hadron Collider dedicated to study high-energy nuclear collisions which is also carrying out a proton-proton physics program, thanks to its wide phase-space coverage and good momentum and spatial resolution. We present first results on two-pion Bose-Einstein correlations in pp collisions at √s = 900 GeV measured with ALICE. An increase of the HBT radius with increasing event multiplicity is observed, in agreement with previous measurements. However, a strong decrease of the radius with increasing transverse momentum, as observed at RHIC and at Tevatron, is not evident in our analysis.     

ICPAQGP2001: Conference summary

I review recent progress in ultrarelativistic nucleus-nucleus collisions, and the connection of this field to modern QCD theory of deconfinement and/or chiral symmetry restoration. The talks at this Conference have shown a convergence of data and theory as far as the CERN SPS investigations at √s = 17 GeV are concerned; the parton-hadron phase boundary seems now located atT = 170 ± 10 MeV. New data from RHIC and direct photon production results from CERN have been shown that point out the field’s future direction: analysis of partonic matter atT > 200 MeV. Astrophysics analysis was shown to be linked crucially to further theoretical progress with non-perturbative QCD.     

High-p t and jet physics from RHIC to LHC

The observation of the strong suppression of high-p t hadrons in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL has motivated a large experimental program using hard probes to characterize the deconfined medium created. However, what can be denoted as “leading particle” physics accessible at RHIC presents some limitations which motivate at higher energy the study of much more penetrating objects: jets. The gain in center-of-mass energy expected at the Large Hadron Collider (LHC) at CERN will definitively improve our understanding on how the energy is lost in the system, opening a major new window of study: the physics of jets on an event-by-event basis. We will concentrate on the expected performance for jet reconstruction in ALICE using the EMCal calorimeter. (for the ALICE Collaboration) The text was submitted by the author in English.     

Pseudorapidity distributions of charged particles in Pb-Pb collisions at super proton synchrotron energies from the NA50 experiment

We present the measurements of charged particle pseudorapidity distributions dN_ch/dη performed by the NA50 experiment in Pb-Pb collisions at the CERN SPS. Measurements were done at incident energies of 40 GeV (√s = 8.77 GeV) and 158 GeV (√s = 17.3 GeV) per nucleon over a broad impact parameter range. The multiplicity distributions are studied as a function of centrality using the number of participating nucleons(N _part ), or the number of binary nucleon-nucleon collisions (N_coll). Their values at midrapidity exhibit a linear scaling withN _part at both energies. Particle yield increases approximately by a factor of 2 betweeny √s = 8.77 GeV and √s = 17.3 GeV.     

What do we really know about cold nuclear matter effects?

The measurement of charmonium suppression in relativistic heavy ion collisions is posited to be an unambiguous probe of the properties of the strongly interacting quark gluon plasma (sQGP). In hot and dense QCD matter Debye color screening prevents charm and anti-charm quark pairs from forming J/ψ mesons if the screening radius is smaller than the binding radius. However, one must have a clear understanding of the expected suppression in normal density QCD matter before interpreting any additional anomalous suppression. The PHENIX experiment has measured J/ψ production from colliding proton + proton and deuteron + gold beams at 200 GeV from the relativistic heavy ion collider (RHIC). The deuteron + gold data can be compared to the proton + proton baseline in order to establish the typical suppression in cold nuclear matter (CNM). For PHENIX, a suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial state low-x gluons in the gold nucleus. These results can be convoluted with the nuclear-environment-modified parton distribution functions, extracted from deep inelastic scattering (DIS) and Drell-Yan (DY) data, in order to estimate the J/ψ break up cross section in cold nuclear matter. One can also use a data driven method that does not rely on the assumption of the production mechanism, or PDF parameterization, to extrapolate to the heavy ion collision case. At this time both the predictions for CNM effect suppression in heavy ion collisions are somewhat ambiguous. Future results using the data acquired by the PHENIX experiment in run-6 (p + p) and run-8 (d + Au) will be vital for our understanding. These data, which are in the process of being analyzed, will provide a needed improvement in the statistical and systematic precision of constraints for CNM effects. These constraints must be improved in order to make firm conclusions concerning additional hot nuclear matter charmonium suppression in the sQGP.     

J/psi production in quark-gluon plasma

We study J/psi production at RHIC and LHC energies with both initial production at energies reached and the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) with regeneration. We solve the coupled set of transport equations for the J/psi distribution in phase space and the hydrodynamic equation for evolution of quark-gluon plasma. At RHIC, continuous regeneration is crucial for the J/psi momentum distribution while the elliptic flow is still dominated by initial production. At energies reached at the LHC energy, almost all the initially created J/psis are dissociated in the medium and regeneration dominates the J/psi properties.     

J/ψ azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158?GeV per nucleon

The J/ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/ψ mesons at SPS energies. Hence, the measurement of J/ψ elliptic anisotropy, quantified by the Fourier coefficient v ₂ of the J/ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/ψ suppression observed in Pb-Pb collisions. We present the measured J/ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v ₂ as a function of the collision centrality and of the J/ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p _T sub-samples. The extracted v ₂ values are significantly larger than zero for non-central collisions and are seen to increase with p _T.     

Inclusive J/ψ and χc meson production in pp collisions at RHIC

Inclusive cross sections for J/ψ and χ _c production in pp collisions were calculated in the k _t -factorization approach for the RHIC energy. Several mechanisms were considered. Radiative χ _c decays and direct color-singlet contributions constitute the dominant mechanisms of J/ψ production. The results are compared with recent RHIC data.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.