Suppression In Nuclear Collisions Research Articles

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.

RCP and RAA Measurements of Identified and Unidentified Charged Particles at High pT in Au + Au Collisions at sNN=7.7,11.5,19.6,27,39,and62.4GeV in STAR

Abstract The suppression of high p T hadrons in 200 GeV Au + Au collisions at RHIC has been seen as a signature for a partonic medium being formed. The evolution of this key QGP signature is a powerful tool for studying the QCD phase structure in the RHIC Beam Energy Scan. In these proceedings, we present measurements of identified π + , K + , and p, as well as unidentified charged particles in Au + Au collisions at s N N = 7.7 , 11.5 , 19.6 , 27 , 39 , and 62.4 GeV . We report nuclear modification factors R C P and R A A where published p + p references are available. These results offer insight into the s N N dependence of high p T suppression in nuclear collisions.

Direct Observation of Dijets in CentralAu+AuCollisions atsNN=200 GeV

The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum (pT) charged hadrons in Au+Au collisions at higher pT than reported previously. As (pT) is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter.

Time structure of anomalous J/ψ and ψ′ suppression in nuclear collisions

The data for the mean squared transverse momentum 〈p2t〉(Et) as function of transverse energy Et of J/ψ and ψ′ produced in Pb–Pb collisions at the CERN-SPS are analyzed and it is claimed that they contain information about the time structure of anomalous suppression. A transport equation which describes transverse motion of J/ψ and ψ′ in the absorptive medium is proposed and solved for a QGP and a comover scenario of suppression. While the comover approach accounts for the data fairly well without adjusting any parameter, the fit to the data within the QGP scenario requires to assume anomalous suppression to become effective rather late, 3–4 fm/c after the nuclear overlap.

On energy densities reached in heavy-ion collisions at the CERN SPS

We present a few estimates of energy densities reached in heavy-ion collisions at the CERN SPS. The estimates are based on data and models of proton-nucleus and nucleus-nucleus interactions. In all of these estimates the maximum energy density in central Pb+Pb interactions is larger than the critical energy density of about 0.7 GeV/fm^3 following from lattice gauge theory computations. In estimates which we consider as realistic the maximum energy density is about twice the critical value. In this way our analysis gives some support to claims that deconfined matter has been produced at the CERN SPS. Any definite statement requires a deeper understanding of formation times of partons and hadrons in nuclear collisions. We also compare our results with implicit energy estimates contained in earlier models of anomalous J/psi suppression in nuclear collisions.

J/ ψ production and suppression in nuclear collisions

In terms of a new QCD factorization formula for J/$\psi$ production, we calculate the J/$\psi$ suppression in nuclear collisions by including the multiple scattering between the pre-J/$\psi$ partonic states and the nuclear medium. We find agreement with all data on J/$\psi$ suppression in hadron-nucleus and nucleus-nucleus collisions, except a couple of points (the ``second drop'') at the highest $E_T$ bins of the new NA50 data.

Charmonium production off nuclei: From SPS to RHIC

Physics of charmonium suppression in nuclear collisions drastically changes between the energies of SPS and RHIC. Mechanisms suppressing charmonia at SPS are reviewed, neither of which is important at RHIC. On the other hand, coherence, or shadowing of c-quarks and gluons barely seen at SPS, become a dominant effect at RHIC providing a much stronger suppression. A onset of coherence at Fermilab energies explains the observed cross section ratio steeply falling at large Feynman x_F. In nuclear collisions variation of charmonium suppression with x_F suggests a sensitive probe for QGP.

Quarkonium feed-down and sequential suppression

About 40-50 % of the quarkonium ground states J/psi(1S) and Upsilon(1S) produced in hadronic collisions originate from the decay of higher excitations. In a hot medium, these higher states are dissociated at lower temperatures than the more tightly bound ground states, leading to a sequential suppression pattern. Using new finite temperature lattice results, we specify the in-medium potential between heavy quarks and determine the dissociation points of different quarkonium states. On the basis of recent CDF data on bottomonium production, we then obtain first predictions for sequential Upsilon suppression in nuclear collisions.

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number <n_g> of the prompt gluons are calculated perturbatively and the gluon-Psi inelastic cross section is estimated. The integrated cross sections for AB --> J/Psi (Psi')X for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter <n_g>\sigma(gPsi).

A quantitative reanalysis of charmonium suppression in nuclear collisions

We present a quantitative description of $J/\psi$ and $\psi^\prime$ suppression in proton-nucleus and nucleus-nucleus collisions at CERN energies. We use a conventional hadronic framework based on nuclear absorption plus final state interaction of the $J/\psi$ or $\psi^\prime$ with co-moving hadrons.

A quantitative analysis of charmonium suppression in nuclear collisions

Data from J/ψ and ψ′ production in p - A collisions are used to determine the cross section for absorption of pre-resonance charmonium in nuclear matter. The J/ψ suppression in O - Cu, O - U and S - U collisions is fully reproduced by the corresponding nuclear absorption, while Pb - Pb collisions show an additional suppression increasing with centrality. We study the onset of this change in terms of hadronic comover interactions and conclude that so far no conventional hadronic description can consistently account for all data. Deconfinement, starting at a critical point determined by central S - U collisions, is in accord with the observed suppression pattern.

J/gy suppression as evidence for high densities in nuclear collisions

We determine the hadron density necessary to account forJ/ψ suppression in nuclear collisions by absorption, including the effects of nuclear stopping andJ/ψ formation time. The suppression measured in central 200 GeV/A O-U and S-U collisions cannot be obtained from interactions of theJ/ψ with the primary nucleons alone. Absorption on primary and secondary hadrons can describe these data, however, provided the density of the secondaries is at least 0.8 fm−3, i.e., five times standard nuclear density.

Strangeness enhancement and strangeness suppression in nuclear collisions at 200 GeV/N

Data on yields of strange and negative hadrons in full phase space produced in proton-nucleus and central S+S collisions at 200 GeV/nucleon are reviewed. The total number of produced strange and nonstrange valence quark-antiquark pairs is determined on the basis of the hadron multiplicity at freezout. It is shown that the strangeness production factor λs is increased by a factor of 2 in central S+S collisions, whereas no increase is observed in proton-nucleus collisions.

φ-enhancement and J/ψ suppression in nuclear collisions by rescattering of secondary hadrons

We point out that recent indications from the NA38 Collaboration for an increase of the φ/(ω+ϱ 0) ratio in central 200 A GeV O+U and S+U collisions at CERN give evidence for an approach to chemical equilibrium in these systems. Under the assumption (which is supported by previous theoretical work) that the J/ψ suppression found in the same experiment under similar kinematic conditions is due to final state absorption via rescattering in the dense collision zone, we solve the rate equations for φ-production and -absorption via secondary collisions in the reaction zone. Eliminating unknown geometrical parameters of the collision zone by using the J/ψ suppression data, we obtain a quantitative description of the measured φ enhancement.

J/ψ suppression in nuclear collisions revisited

We study a particular type of final state interaction which gives a decrease of the parameter α, characterizing the mass number dependence, with increasing values of x F. Using this final state interaction for a produced J/ψ we compute, in the framework of the dual parton model, the E T-distributions of events with either J/ψ or Drell-Yan pair production in O-U collisions-which are in qualitative agreement with available data in the large E T region.

J/ψ suppression in nuclear collisions: Deconfinement or absorption?

The characteristic features of J/ψ suppression by deconfinement due to colour screening are compared to those obtained by combining absorption in very dense hadronic matter with initial state gluon scattering.

Quark-gluon plasma diagnostics and J/ψ suppression in nuclear collisions

The suitability of J/ψ suppression, observed in ultrarelativistic heavy ion collisions, for detecting quark-gluon plasma is examined.

J/ψ suppression by quark-gluon plasma formation

If high energy heavy ion collisions lead to the formation of a hot quark-gluon plasma, then colour screening prevents cc binding in the deconfined interior of the interaction region. To study this effect, the temperature dependence of the screening radius, as obtained from lattice QCD, is compared with the J/ψ radius calculated in charmomium models. The feasibility to detect this effect clearly in the dilepton mass spectrum is examined. It is concluded that J/ψ suppression in nuclear collisions should provide an unambiguous signature of quark-gluon plasma formation.