www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 261 (2007) 1058–1060

NIMBBeam Interactions

with Materials & Atoms

Results from RHIC and a look to the future

Saskia Mioduszewski *

Cyclotron Institute, MS #3366, Texas A&M University, College Station, TX 77843-3366, United States

Available online 10 May 2007

Abstract

After 6 years of RHIC operation, much has been learned about the matter created in ultra-relativistic heavy-ion collisions. We haveevidence that the matter is strongly interacting and highly collective, behaving like a liquid. We also have evidence that the matter is verydense, with estimates of the energy density exceeding expectation for a phase transition. However, the results obtained thus far have ledto further questions about the matter created at RHIC. Future investigations include (i) why the high-pT suppression of heavy flavor iscomparable to the suppression of mesons containing only light quarks, (ii) whether the J/W regenerates through combination of c and �cquarks, (iii) how chiral symmetry restoration is realized, and (iv) whether we are seeing saturation of gluons in the initial-state nuclearwave-function. This paper discusses the fate of the J/W in the medium at RHIC (suppression versus regeneration).� 2007 Published by Elsevier B.V.

PACS: 25.75.�q

Keywords: Quark gluon plasma; Relativistic heavy-ion collisions

One of the main goals in relativistic heavy-ion collisionsis to understand nuclear matter at extreme temperaturesand densities. According to lattice QCD calculations [1],a phase transition is predicted at critical temperatureTc � 170 MeV or critical energy density ec � 0.7 GeV/fm3

(approximately five times the density of normal nuclearmatter). RHIC was proposed in 1983 to help achieve thisgoal. The relativistic heavy-ion collider (RHIC) at Brook-haven National Laboratory has provided high-energyheavy-ion collisions since the year 2000.

After the first 6 runs, much has been learned about thematter created in these collisions. We have observed largecollectivity in the particles emitted from the heavy-ion col-lisions, which can only be achieved by early thermalization[2]. In fact, the matter produced is not only liquid-like butnearly an ideal fluid [3], described well by hydrodynamicswith no viscosity. We have also observed a very large, fac-tor of 5, suppression of hadrons at large transversemomenta (pT > 2 GeV/c) [4–6]. This can be described by

0168-583X/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.nimb.2007.04.302

* Tel.: +1 979 845 1411; fax: +1 979 845 1899.E-mail address: mio@comp.tamu.edu

calculations of parton energy loss in the matter created[7] and has led to the conclusion that the matter createdat RHIC is very dense, more than 100 times the densityof normal nuclear matter [8].

The J/W (charmonium) was long considered one of themost promising direct probes of deconfinement. Accordingto theoretical predictions in 1986 [9], the produced charm–anticharm pair will not be able to form a J/W bound statein the QGP, if a sufficiently high temperature is reachedwhere the screening radius is smaller than the bindingradius of the J/W resonant state. The ‘‘Debye’’ screeningradius is the distance at which the color charges of twoquarks are screened from another, and the confinementforce is not able to hold the quarks together. Such a sup-pression was observed in Pb + Pb collisions by the NA50[10] experiment at CERN (

psNN � 17 GeV). Fig. 1 shows

this data along with data from NA38 and more recent datafrom NA60 [11].

At RHIC, the predicted suppression of J/W due toscreening in the Quark Gluon Plasma may be confoundedby the large initial production of (anti)charm at

psNN =

200 GeV, in conjunction with their possible thermalization

Fig. 1. J/W suppression from NA38, NA50 [10], and NA60 [11] for heavy-ion collisions at

psNN � 17 GeV. Measured J/W yields are divided by

normal nuclear absorption.

Fig. 2. J/W yields scaled by Nbinary as a function of Npart (or centrality)measured by the PHENIX collaboration [16]. An absence of nucleareffects would result in an RAA (nuclear modification factor) of 1. Thesuppression is compared to a calculation of only normal nuclearsuppression [17].

S. Mioduszewski / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1058–1060 1059

in the created medium [12]. If charm quarks do thermalizein RHIC collisions, then the coalescence of c�c pairs couldlead to an enhancement of J/W, rather than a suppression[13]. In the first measurement of the J/W yields in Au + Aucollisions at RHIC [14], it was difficult to differentiatebetween a modest J/W suppression (less suppression thancalculations describing the NA50 data predict for RHIC)and an interplay between a suppression and a recombina-tion of the J/W [15]. Fig. 2 shows more recent higher-statis-tics preliminary data on J/W yields as a function of Npart

(or centrality) for Au + Au and Cu + Cu collisions, also

Fig. 3. Left panel: The PHENIX data is compared to calculations of J/W suppanel: Comparisons to calculations including quark recombination [19–22] or

measured by the PHENIX collaboration [16], scaled bythe number of binary collisions Nbinary. Since primordi-ally-produced J/W are produced in hard processes, anabsence of any modification to the yields due to initialand/or final nuclear effects would result in a scaling ofthe yields with Nbinary. These data are compared to a calcu-lation including only normal nuclear suppression [17].Fig. 3 shows the same data with comparisons to calcula-tions of suppression due to the medium that can alsodescribe the NA50 suppression [18–20] (left panel), and

pression calculations [18–20] also able to describe the NA50 data. Rightdetailed transport of J/W in the medium [23].

Fig. 4. A theoretical calculation [24] showing (left panel) the relative sources of J/W at RHIC: surviving primordial J/W taking into account thesuppression versus the J/W formed through recombination of charm–anticharm pairs. The right panel is the same calculation of upsilon production atRHIC.

1060 S. Mioduszewski / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1058–1060

those that include quark recombination [19–22] or detailedtransport of the J/W in the medium [23] (right panel). Themost central data points seem to indicate that the suppres-sion is larger than normal nuclear suppression (Fig. 2), butnot as strong as predicted by models that contain norecombination of charm–anticharm quarks (Fig. 3, leftpanel). Model calculations including both suppressionand regeneration of J/W can describe the data reasonablywell (Fig. 3, right panel). With counteracting effects, it isnow a challenge to disentangle the suppression from theregeneration.

While the question of whether charm quarks thermalizeor not is still somewhat of an open question, bottomquarks are much less likely to thermalize and much lessabundantly produced because they are much heavier.Therefore bottomonium (Y) may indeed be the key to dis-entangling the effects of suppression and regeneration inthe measured J/W yields [24]. Fig. 4 shows a theoretical cal-culation predicting the upsilon yields at RHIC. The rightpanel shows that, unlike charmonium, bottomonium yieldswill be dominated by primordial production because regen-eration of bottomonium is negligible.

In summary, one of the open questions at RHIC is thefate of the J/W. Future runs at RHIC will be very importantfor the measurements of charmonium and bottomonium.

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