open questions in quarkonium and electromagnetic probes

Open Questions in Quarkonium

and Electromagnetic Probes

Carlos LourencoCERN-PH, CH-1211 Geneva 23, Swizterland

AbstractIn my (“not a summary”) talk at the Hard Probes 2006 conference, I gave “a

personal and surely biased view on only a few of the many open questions onquarkonium and electromagnetic probes”. Some of the points reported in that talkare exposed in this paper, having in mind the most important of all the openquestions: do we have, today, from experimental data on electromagnetic probesand quarkonium production, convincing evidence that shows, beyond reasonabledoubt, the existence of “new physics” in high-energy heavy-ion collisions?

The first experimental measurements of photon and dilepton production inhigh-energy heavy-ion collisions took place 20 years ago, at the CERN SPS,with Oxygen ions accelerated to 200 GeV per nucleon. Many difficulties makethese measurements particularly challenging. In particular, the productioncross-sections of such particles as the J/ψ and ψ′ are very low, the branchingratios of the decay channels ρ, ω, φ → l+l− are very small, the backgroundsfrom π0 → γγ and other hadronic decays are very high, etc. Under these condi-tions, it is quite remarkable that significant physics progress actually resultedfrom the first generation of SPS experiments (none of the AGS heavy-ion ex-periments measured dileptons or photons). It is important to keep in mindthat these experiments were built in a very short time and, most of them ifnot all, used existing detectors with only minimal additions or modifications.For instance, the NA38 experiment was proposed to the SPS committee in1985 (to search for thermal dimuons) and was taking data one year later, withnewly built beam detectors, an active target system and an electromagneticcalorimeter, added to the muon spectrometer used by NA10 since 1980. Nowthat most people in our field work in experiments that cost in excess of 100million Swiss francs, it might be worth recalling that the 17 m long muonspectrometer built by NA10 worked in the NA10 experiment for 5 years andin the NA38, NA50 and NA60 “heavy-ion experiments” for 18 years! Surely avery good deal. Already in this first generation of SPS heavy-ion experiments,using beams of Oxygen and Sulphur ions, several observations indicated the

Nuclear Physics A 783 (2007) 451c–460c

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presence of “anomalies” in the measured data. Until 1995, many people arguedthat the drop of the ratio between the J/ψ and the “Drell-Yan continuum”production cross-sections, from p-U to O-U to S-U collisions and from periph-eral to central O-U and S-U collisions, as measured by NA38, constituted aclear signature of QGP formation, following the prediction published in 1986,in what is nowadays arguably the most famous paper of our field [1] (citedalmost 1000 times by now). In parallel, an excess in the production of lowmass dielectrons was found by the CERES collaboration and interpreted asa possible indication that chiral symmetry was (approximately) restored inthe matter produced in S-Au collisions at SPS energies. Also the continuumdimuons with masses below the J/ψ peak were under scrutiny, by NA38 andHelios-3. Both experiments found an excess in the production yield of suchdimuons in Sulphur induced collisions, with respect to the yields expectedfrom a superposition of Drell-Yan dimuons and muon pairs from simultaneoussemi-muonic decays of (correlated) pairs of D mesons. These observations weretaken seriously because the same analyses could reproduce the data collectedin proton-nucleus collisions, with normalisations compatible with the expec-tations available at the time. Possible explanations of this “intermediate massregion dimuon excess” included thermal dimuons (maybe emitted from theQGP phase) and modifications of the pair correlations of the D mesons whiletraversing the medium produced in the nuclear collisions. Between 1994 and2000, a second important step took place at the SPS, with several data takingperiods with Pb ions, at 158 GeV and lower energies. The new experimentsconfirmed the observation of excess production of low and intermediate massdileptons, now in Pb induced collisions. The situation changed, however, inthe case of the production and suppression of charmonium states, in particularthanks to a much better understanding of the proton-nucleus reference data.In the case of the J/ψ, we now think that the “normal nuclear absorption”already present in p-A collisions, and nicely reproduced by a simple modelbased on the Glauber formalism with a single “absorption cross section” pa-rameter, σabs, can also account for the “suppression pattern” measured in S-Ucollisions, with the same σabs value, indicating that the J/ψ is not sensitiveto the medium formed in such collisions, contrary to what happens in Pb-Pb collisions, where an extra and significant suppression is seen, beyond theexpected “normal nuclear absorption curve”, for the most central collisions(Fig. 1-left). In the case of the ψ′, on the other hand, there is a very signifi-cant extra suppression observed already in S-U collisions, with respect to thep-A reference, and no significant change is visible between the S-U and Pb-Pbsuppression patterns (Fig. 1-right).

It can be easily argued that one of the most important “discoveries” of the first10 years of heavy-ion experimentation at the CERN SPS, and not only in whatconcerns hard and electromagnetic probes, is that it is extremely important,indeed crucial, to have very robust data collected in more elementary collisionsystems, such as proton-proton and proton-nucleus interactions, collected by

C. Lourenço / Nuclear Physics A 783 (2007) 451c–460c452c

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Fig. 1. Left: The J/ψ suppression pattern as measured by NA38 and NA50, in p-A,S-U and Pb-Pb collisions, as a function of L, the distance of nuclear matter traversedby the charmonium, compared to the “normal nuclear absorption” band resultingfrom the Glauber fit to the p-A data [2]. The p-A and S-U data points, as well asthe curves, have been rescaled to the conditions (energy and rapidity window) ofthe Pb-Pb data. Right: The suppression of the ψ′ as measured by NA38 and NA50in p-A, S-U and Pb-Pb collisions [3], compared to the normal nuclear absorptioncurve calculated using the Glauber formalism [4].

the same experiments, at the same collision energies, and in the same phasespace acceptance windows, as the heavy-ion measurements. Only after havinga solid “expected baseline”, provided by a good understanding of the relevantphysics processes and based, in particular, on detailed analyses of p-A data,we can realistically hope to identify patterns in the high-energy heavy-ion datathat will clearly and convincingly signal the presence of “new physics” in thematter produced in those interactions. It is important to recognise the effortmade at RHIC in this respect, with pp and d-Au runs that have provided ex-tremely valuable information to understand the observations made in Au-Aucollisions. Unfortunately, for certain physics topics, such as J/ψ production,the presently available d-Au statistics is very limited; a high-statistics d-Aurun should take place at RHIC at the earliest possible date. At the SPS en-ergies, the NA50 experiment has recently provided a very detailed analysis ofJ/ψ and ψ′ production in p-A collisions [2], at 450 and 400 GeV, using 5 and6 different nuclear targets, respectively, both in terms of absolute productioncross sections (Fig. 2) and in terms of ratios between charmonia and Drell-Yanyields (Fig. 1). The results, corresponding to 16 different data sets collectedbetween 1996 and 2000, in different experimental conditions, give a ratherconsistent picture of the J/ψ and ψ′ “normal nuclear absorptions”, with ab-

sorption cross sections σJ/ψabs = 4.5 ± 0.5 mb and σψ′

abs = 8.3 ± 0.9 mb from the

production cross-sections, and σJ/ψabs = 4.2 ± 0.5 mb and σψ′

abs = 7.7 ± 0.9 mbfrom the cross-section ratios. However, it is important to keep in mind thata significant fraction of the observed J/ψ mesons result from decays of ψ′

and χc mesons, that the ψ′ has a higher σabs value (at least at mid-rapidity),

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Fig. 2. The J/ψ and ψ′ production cross sections measured in p-A collisions byNA50, as a function of the mass number of the target nuclei (left), and of L (right).The lines represent fits to three commonly used parameterizations of the “normalnuclear absorption”.

and that this is likely to also be the case for the χc, a larger and more weaklybound charmonium state than the J/ψ. If we assume that 60 % of the observedJ/ψ mesons are directly produced, 30% result from χc decays and 10 % fromψ′ decays, and if, furthermore, we calculate σabs geometrically, as πr2, withrJ/ψ = 0.25 fm, rψ′ = 2 × rJ/ψ and rχc = 1.5 × rJ/ψ, we can redo the Glaubercalculation, now without leaving σabs as a free parameter, and we obtain anequally good description of the J/ψ data (Fig. 3-left), with a reduced χ2 of

1.0 (by chance?). Besides, the value of σψ′abs from this very simple model is

7.85 mb, in perfect agreement with the NA50 measurements (by chance?).The values of σabs mentioned here are “effective” values, corresponding to aconvolution of the final state absorption which affects the produced cc stateswhile traversing the nuclear matter, with initial state effects, such as the nu-clear effects on the gluon distribution functions. According to the well-knownEKS98 model of nuclear effects on the parton distribution functions [5], charmproduction at SPS energies, and at mid-rapidity, is in the anti-shadowing re-gion, leading to an initial state cc production enhanced in p-Pb collisions, say,with respect to the case when nuclear effects are neglected (Fig. 3-right). Avery simple calculation indicates that the “convoluted” σabs value, 4.2 mb,becomes around 6 mb if we explicitly take into account the anti-shadowingeffect, using the EKS98 model. This value is more directly comparable to thevalues obtained from the d-Au data of PHENIX, between 0 and 3 mb, whichwere also obtained after taking into account the (EKS98) nuclear effects onthe PDFs [7]. The observation that the σabs values at SPS and RHIC energiesare considerably different raises the question of whether σabs varies within theenergy range covered by the fixed-target experiments. A detailed analysis ofall the available data, with the Glauber formalism, including nuclear effectson the PDFs and properly accounting for the feed-down sources, is beyond


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Fig. 3. Left: NA50 p-A values of the J/ψ over Drell-Yan cross-section ratio, as afunction of A, compared to a Glauber calculation taking into account the feed-downcontributions from higher cc states. Right: Changes induced on the cc (and bb)cross-sections, in p-Pb and Pb-Pb collisions, by the nuclear modifications of thePDFs, at mid-rapidity [6].

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Fig. 4. The nuclear-dependence α parameter as a function of xF (left) and of pT

(middle), for J/ψ mesons measured at three different collision energies, in p-A colli-sions [8]. The right panel shows that 〈p2

T〉 increases with the size of the target nucleiand with the energy of the collision [8].

the scope of this paper. Let us simply state that the (initial and final state)nuclear absorption of the J/ψ, if parameterized with the Aα expression, leadsto values of α that, at mid-rapidity, increase with collision energy (Fig. 4-left). It is very interesting to notice that the values of α measured at threesignificantly different (fixed-target) energies, at mid-rapidity, show a perfectscaling as a function of pT (Fig. 4-middle). We may conjecture, then, thatthe observed increase of α with energy is actually due to a convolution be-tween the increase of α with pT and the increase of the average pT of the J/ψwith the collision energy (Fig. 4-right). From the previous lines, and also con-sidering that the level of (anti-)shadowing changes with the collision energy(Fig. 3-right), it seems natural to deduce that the σabs value at 158 GeV, theenergy of the Pb-Pb collisions, will be somewhat higher than the value de-


termined at 400/450 GeV. A stronger “normal nuclear absorption” will leadto a decrease of the “extra suppression” presently seen in the Pb-Pb J/ψdata. We will know more once the NA60 collaboration extracts σabs from thep-A data collected at 158 GeV. At this moment, however, in the absence ofthat measurement, we cannot really argue that we have a robust referencebaseline at the energy of the heavy-ion data. Hence, the “anomalous” aspectof the J/ψ suppression pattern seen in heavy-ion collisions has not yet beenconvincingly demonstrated, and requires, in particular, a more detailed under-standing of charmonia production in elementary pp and p-A collisions. Thestriking “change of slope” of the ψ′ suppression pattern (Fig. 1-right) lookseven more “anomalous” but, again, it occurs between the p-A and the S-U/Pb-Pb data points, collected with different collision systems and at significantlydifferent energies. A convincing demonstration of “new physics” would requirethat the anomaly happens within a consistent set of data points (ideally, a sin-gle collision system, from peripheral to central collisions). Unfortunately, poorstatistics prevents NA60 from significantly contributing to the understand-ing of the ψ′ suppression pattern. At this moment, and the situation is notlikely to change in the coming years, we cannot say what is the nature of the“anomaly” seen in the ψ′ suppression pattern between the p-A and the S-U/Pb-Pb colliding systems; maybe the extra suppression is due to the matterproduced in the heavy-ion collisions; maybe that matter is in the QGP phaseand “melts” the cc bound states; maybe this indicates that the critical en-ergy density is reached already in the most peripheral S-U or Pb-Pb collisionsat the SPS energies; maybe. . . There is much more experimental informationconcerning the suppression of the J/ψ production yield, with the NA50 Pb-Pb pattern recently being complemented by PHENIX data taken at 10 timeshigher centre of mass energies [7] and by NA60 data taken with a smallercolliding system, In-In [9]. However, the present statistical uncertainties ofthe PHENIX measurements severely limit the insights gained from comparingthe results obtained at both energies. Figure 5-left is an attempt to integratein a single plot, as a function of Npart the available mid-rapidity suppression

patterns. The PHENIX results are RAA, i.e. NJ/ψAu−Au(ci) / [NJ/ψ

pp × Ncoll(ci)](and similar for d-Au), where ci represents the centrality bin. The NA38 andNA50 data points are the same as shown in Fig. 1-left, but normalised to thepp value, R

J/ψAA (ci) / RJ/ψ

pp , where RJ/ψ stands for B × σJ/ψ / σDY . The NA60points were obtained by an approximate procedure, convoluting the In-In ratio“measured/expected” [9] with the Pb-Pb “normal nuclear absorption curve”,NJ/ψ(ci)/G

J/ψ(ci) × GJ/ψ / DY (ci) / GJ/ψ / DYpp . In this representation, the differ-

ent data sets are directly comparable, since Drell-Yan production scales withNcoll. It is also not simple to compare the SPS heavy-ion patterns. The S-U en-ergy is different, it is a very asymmetric collision system, the Uranium nucleusis not spherical, and, besides, the centrality can only be determined from theET measurement, while the In-In centrality distribution is determined fromthe EZDC variable. The comparison between the In-In and Pb-Pb suppression


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Fig. 5. Comparison between the J/ψ suppression patterns observed at SPS andRHIC energies (left), as a function of Npart, and observed in In-In and Pb-Pb colli-sions at the SPS (right), as a function of the energy density.

patterns should suffer from less systematic uncertainties (same energy, sym-metric collisions, spherical nuclei), especially if we take the Pb-Pb values fromthe NA50 EZDC analysis rather than from the ET analysis; but we must keepin mind that NA50 reports the suppression pattern in terms of the J/ψ / DYcross-section ratio while NA60 (with much less high-mass Drell-Yan statis-tics and a perfect vertexing efficiency even in the most peripheral collisions)reports the measured J/ψ yield itself, directly normalised by the calculated“expected” nuclear absorption. These two procedures surely have very differ-ent sources of systematic uncertainties. Figure 5-right [10] compares the In-Inand Pb-Pb suppression patterns, as a function of the energy density, estimatedusing a calculation based on the VENUS event generator. The observed pat-tern is consistent with a double step function (smeared by the resolution ofthe EZDC measurement), as would be expected in case the ψ′ and χc stateswould be completely melted in the medium, above two different thresholds inenergy density, leaving only the (more strongly bound) directly produced J/ψmesons, around 60 % of the yield expected in the absence of anomalies [11].However, there is a big difference between “the measurements are compatiblewith. . . ” and “the measurements show, beyond reasonable doubt, that. . . ”.

Concerning the intermediate mass region dimuons, there has been extremelysignificant progress provided by the NA60 experiment. The analysis reportedat this conference [12] convincingly shows (thanks to the very good vertexingaccuracy of the data) that the excess dimuons seen in In-In collisions, withrespect to the superposition of Drell-Yan dimuons and simultaneous semi-muonic decays of D meson pairs, is of a prompt nature (Fig. 6-left). It isworth underlining that this eliminates the long-standing alternative that thesignal excess would, in fact, be due to a small fraction of un-subtracted back-ground from pion and kaon decays. It now remains to be understood what is


Fig. 6. Left: Demonstration that the intermediate mass dimuon excess is of promptorigin [12]. Right: Ratio between the double-scattering and the single-scatteringcontributions to the Drell-Yan yield, versus dilepton mass, for p-W and Pb-Pbcollisions, in the kinematical conditions of the NA60 experiment [13].

the physics process responsible for this source of prompt dimuons. Maybe weare seeing thermal dimuons shining from a thermal medium (but, hadronic ordeconfined?). However, once again, in order to clearly disclose any putativenew phenomena, specific of nuclear collisions, we need to understand in detailthe physics processes already contributing in the more “elementary” collisions.The two physics processes that are expected to give sizeable yields of dimuonsbetween the φ and the J/ψ peaks are the dimuons from qq annihilation (Drell-Yan) and the simultaneous semi-muonic decays of correlated charmed hadronpairs. Unfortunately, the Drell-Yan yields are not theoretically calculable in areliable way for such low values of the dimuon mass, where higher order ef-fects may be more important than at higher masses. Furthermore, even if theperturbative QCD calculations would be consistent with the proton-protonprompt dimuon continuum down to masses below 1.5 GeV, this could verywell no longer be the case for proton-nucleus and nucleus-nucleus collisions,where contributions from interactions involving extra partons from the collid-ing hadrons may lead to a significant increase of the low-mass dimuon yieldwith respect to a linear scaling of the yield in elementary nucleon-nucleon in-teractions [13] (Fig. 6-right). Without having this possible source of promptdimuons under control, through a detailed analysis of proton-nucleus data,there is little hope that we can find clear evidence for thermal dimuon pro-duction in heavy-ion data. Also the understanding of the open charm baselineis under question, given that the charm production cross section deduced fromthe measurements made by many experiments (Fig. 7-left) is two times smallerthan the value required to describe the dimuon data collected by NA50 in p-A collisions (Fig. 7-right). This indicates that there seems to be something“anomalous” already in the intermediate mass dimuons produced in the p-Acollisions studied by NA50.

Reliable baseline measurements, made in elementary collisions, are also indis-pensable to correctly interpret photon and low mass dilepton production. Inboth cases, a large background from hadron decays needs to be removed so


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Pythia [14] using three different sets of parton densities [6]. The value that bestdescribes the NA50 p-A dimuon data [15] is a factor of two higher than expectedfrom the interpolation of the direct measurements made by other experiments. In-terestingly, the value recently measured by PHENIX [16], at much higher energies,is in good agreement with the extrapolation of the fixed-target data.

that we access a rare signal, which then needs to be understood as a convo-lution of several physics sources, maybe including thermal radiation from theQGP and/or from a hot hadron gas [17]. In the case of low mass dilepton pro-duction, the exceptional increase in statistics between the CERES Pb-Au dataand the NA60 In-In data implies that the significance of the measurements isnow determined by systematic uncertainties, related to experimental aspectslike efficiencies and acceptances (increasingly difficult to keep under controlas we move down in mass and pT) and to the definition of the hadronic decay“cocktail”, given our limited knowledge of kinematical distributions, ρ/ω in-terference effects, the ω dimuon branching ratio, the ω form factor, the ρ massline shape, and other “inputs” needed to define the “expected sources” refer-ence. We must also keep in mind that there are “normal nuclear effects” at playin low mass dilepton production, such as the increase of the φ/ω cross-sectionratio in p-A collisions [18]. How much of the φ enhancement seen in heavy-ioncollisions remains “anomalous” after accounting for a correct extrapolation ofthe p-A observations? And how can we conciliate our basic understanding oflow mass dilepton production in “elementary” collisions with E325’s claim [19]that there are already “nuclear matter modifications” of the properties of theρ, ω and φ mesons produced in p-Cu collisions at 12 GeV?

In summary, while it is certainly true that there has been very considerableprogress in the recent years regarding the diversity and quality of the mea-surements related to quarkonium and electromagnetic probes production inhigh-energy heavy-ion collisions, it remains a field with many open questions.Some of them might be given satisfactory answers once the data samplesrecently collected at the SPS and at RHIC are fully analysed. Others will


presumably remain unanswered and would easily justify significant upgradesof existing experiments or even the construction of new ones, provided theallocated integrated beam time (and human resources) would properly matchthe investment required to build the new hardware. In any case, the presentsituation, after 20 years of efforts by many hundreds of physicists, remainsunsatisfactory: we do not yet have convincing evidence, from experimentaldata on quarkonium or electromagnetic probes, that shows beyond reasonabledoubt the existence of “new physics” in high-energy heavy-ion collisions.

This paper benefited from valuable discussions with several people, including,in particular, David d’Enterria, Goncalo Borges, Helena Santos, Helmut Satz,Hermine Wohri, Louis Kluberg, Mike Leitch, Pietro Faccioli, Ramona Vogt,Raphael Granier de Cassagnac and Roberta Arnaldi.

References

[1] T. Matsui and H. Satz, Phys. Lett. B178 (1986) 416.[2] G. Borges, PhD Thesis, IST, Lisbon, Portugal, 2005.

G. Borges et al. (NA50 Coll.), Eur. Phys. J. C43 (2005) 161.B. Alessandro et al. (NA50 Coll.), CERN-PH-EP/2006-018, EPJC, in print.B. Alessandro et al. (NA50 Coll.), Eur. Phys. J. C39 (2005) 335.

[3] H. Santos, PhD Thesis, IST, Lisbon, Portugal, 2004.B. Alessandro et al. (NA50 Coll.), CERN-PH-EP/2006-019, EPJC, in print.

[4] D. Kharzeev, C. Lourenco, M. Nardi and H. Satz, Z. Phys. C74 (1997) 307.[5] K.J. Eskola, V.J. Kolhinen and C. Salgado, Eur. Phys. J. C9 (1999) 61.[6] C. Lourenco and H.K. Wohri, Phys. Rep. 433 (2006) 127.[7] S.S. Adler et al. (PHENIX Coll.), Phys. Rev. Lett. 96 (2006) 012304.

A. Bickley et al. (PHENIX Coll.), these proceedings.[8] M.J. Leitch et al. (E866 Coll.), Phys. Rev. Lett. 84 (2000) 3256.

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open questions in quarkonium and electromagnetic probes

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