1. 1/21 Carlos Lourenço, 4 th International Workshop on Heavy Quarkonium, June 27–30 2006 Recent results from the CERN SPS on quarkonium production in p-nucleus and nucleus-nucleus collisions Summary Quarkonia resonances can be measured as nice peaks above a “flat” dilepton continuum; no problem with backgrounds or “particle identification”, if we have good mass resolution and vertexing capabilities (to clean event sample at the SPS; to evaluate beauty feed-down at LHC) But: J/  suppression... J/  enhancement... with respect to what? Before we can discuss “new physics” anomalies in nuclear collisions, it is crucial to define the “normal expected behaviour”, on the basis of measured p-nucleus and light-ion data And we must learn how to relate the normal behaviours for different energies and y windows

2. 2/21 Basic idea: in the presence of new physics (formation of a QCD medium with deconfined quarks and gluons) the centrality dependence of quarkonia production yields will be very significantly affected → we have a “signature” Prediction: above certain consecutive thresholds, the  ’, the  c and the J/  resonances (besides the Upsilon states) will “dissolve” in the formed medium → we have more than a simple signature; we have a “smoking gun”... However,... What happens to the charmonia states in the presence of “old physics”? Do we understand the basic properties of J/  and  ’ production in pp and p-A collisions? In A-A collisions, do we have a robust and well understood baseline with respect to which we can clearly and unambiguously identify patterns specific of the high density medium produced in high-energy nuclear collisions? What should we really expect in the absence of a deconfined QCD medium but accounting for all the other aspects surely existing in nuclear collisions? → We need accurate p-A data and a robust theory to extrapolate the p-A patterns to A-A expectations... Quarkonia studies in heavy-ion collisions: why? how?

3. 3/21 Measurements of J/  and  ’ production have been made in the last few years at the SPS by the NA50 and NA60 collaborations, in p-A and A-A collisions. Charmonia production yields have been presented either in relative terms, with respect to the yield of high-mass Drell-Yan dimuons, or as absolute production cross-sections per target nucleon. Results have also been obtained in what concerns p T distributions, centrality dependence of production yields, etc. NA50 collected p-A data at 400 and 450 GeV, with 5 or 6 different target nuclei. More than 3 000 000 J/  events in total. Charmonia studies at the CERN SPS J/  Pb-Pb 158 GeV p-Pb 400 GeV

4. 4/21 The J/  and  ’ are absorbed in p-nucleus collisions... NA50 p-A data collected in year 2000, with Be, Al, Cu, Ag, W and Pb targets The J/  and  ’ production cross-sections scale less than linearly with the number of target nucleons (contrary to what happens with high-mass Drell-Yan dimuons). p-Pb @ 400 GeV  J/  ~ 105 MeV ’’ J/  p-A 400 GeV Note: the  pA =  pp x A  parametrization leads to extrapolated  (J/  ) and  (  ’) pp values which are 10 to 20% higher than those obtained using the Glauber model

5. 5/21... as a function of the mass number and of L... L is the “path length” which the J/  and  ’ states traverse in the target nucleus, from the production point of the ccbar pair to the nuclear surface the “  L parametrization” exp(-  L  abs ) is a good approximation of the full Glauber calculation J/J/ L Projectile Target ’’ J/  p-A 400 GeV The solid lines are the result of Glauber calculations, assuming that the reduction of the production cross-section per target nucleon is due to final state absorption of the charmonia states in the cold nuclear matter it crosses.

6. 6/21... at 400 and at 450 GeV... The  abs values derived from  and  /DY are “identical”, indicating negligible (initial state) nuclear effects in Drell-Yan production at these energies and at mid-rapidity. From a global fit to the 400 and 450 GeV p-A data, NA50 determined the following absorption cross-sections:  abs (J/  = 4.5 ± 0.5 mb ;  abs (  ’) = 8.3 ± 0.9 mb from production cross-sections  abs (J/  = 4.2 ± 0.5 mb ;  abs (  ’) = 7.7 ± 0.9 mb from cross-section ratios (  /DY)  2 /ndf = 0.7  2 /ndf = 1.4

7. 7/21 The J/  production cross-sections measured in O-Cu, O-U and S-U are compatible with the Glauber extrapolation of the p-A data, keeping the same absorption cross-section, and scaling the curve down from 450 to 200 GeV. Pb-Pb 158 GeV But the J/  suppression pattern changes significantly for Pb-Pb collisions...... and is suppressed in Pb-Pb collisions...

8. 8/21 The J/  “central over peripheral ratio” strongly depends on p T (at the SPS)  Only the low p T J/  mesons get suppressed !... at low transverse momentum R i = (N J/  / N DY ) (E Ti ) (N J/  / N DY ) (E T1 )

9. 9/21 It seems that the J/  absorption, at mid-rapidity, becomes weaker with increasing collision energy, at least between SPS and RHIC energies The 158 GeV p-A data of NA60 will clarify if the trend continues to lower energies J/  0 mb 3 mb Low x 2 ~ 0.003 (shadowing region) PHENIX J/  production in p-A collisions vs. collision energy...

10. 10/21 p T (GeV/c) J/  The increase of  with p T seems to be identical at 400, 800 and 920 GeV (at mid-rapidity)  Maybe the increase of  from NA50 to E866 to HERA-B to PHENIX is due to the increase of the average p T of the J/  when  s increases...... vs. p T... NA50  s (GeV) pp pp 〈 p T 2 〉 pp (GeV/c) 2

11. 11/21  strongly decreases at high x F... Why is this so? Higher parton densities? If so, the J/  should be strongly absorbed in d-Au at RHIC energies; and it is not... E866... and vs. x F

12. 12/21 a R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 - K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599 Models (with variants): 1.0 0.9 0.8 xFxF -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 If you have enough models... one should describe the data... xFxF E86638.8 GeVBe/Fe/W E78938.8 GeVBe/C/Cu/W E77238.8 GeVH 2 /C/Ca/Fe/W NA5029.1 GeVBe/Al/Cu/Ag/W NA322.9 GeVH 2 /Pt E86638.8 GeVBe/Fe/W E78938.8 GeVBe/C/Cu/W E77238.8 GeVH 2 /C/Ca/Fe/W NA5029.1 GeVBe/Al/Cu/Ag/W NA322.9 GeVH 2 /Pt B&KB&K -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 0.9 0.8 0.7 HERA-B preliminary Vogt: final state absorption

13. 13/21 At RHIC energies, for charm production, the nuclear effects on the parton densities (according to EKS98) are just in the crossing from anti-shadowing to shadowing, and have a significant impact on the rapidity dependence of the measured absorption.  abs = 3 mb PHENIX No final state absorption  abs = 0 mb Such a y-dependent effect is not expected to be seen in the SPS p-A data Normal nuclear absorption of J/  production at RHIC

14. 14/21 NA50 measures dimuons within one unit of rapidity, at around mid-rapidity What’s known about the J/  dN/dy in SPS p-A collisions? NA50 p-A 450 GeV The J/  y distributions are not centered at 0, even for the p-Be collision system ! All five distributions are well described by Gaussians of mean y 0   0.2 and  = 0.85 Forcing y 0 = 0, the  2 /ndf increases from 1–3 to 20–50, depending on the data set (target) Why is the J/  rapidity distribution changing from pp to p-Be? Not because of nuclear effects on the PDFs... Pythia with EKS98 gives the same shape for pp and p-Be

15. 15/21  abs from PHENIX: after accounting for the nuclear effects on the PDFs (assuming EKS98)  abs from NA50: effective parameter, convoluting nuclear PDFs and final state absorption → The numerical values, 1–3 mb at RHIC and 4.2 mb at the SPS, are not directly comparable Is there really gluon anti-shadowing at SPS energies? If the EKS98 model is correct, then the absorption cross-section extracted from p-A data (collected at 400/450 GeV) is not directly applicable to A-A data (collected at 158 GeV). → We need to extract  abs from the p-A data collected by NA60 at 158 GeV (in progress) For now, we can make a rough estimate of the importance of this issue Nuclear effects on the PDFs and final state J/  absorption

16. 16/21 For p-Pb collisions, the EKS98 nuclear modification factor is 1.12 at 450 GeV and 1.06 at 158 GeV Nuclear effects on the PDFs and the J/  absorption (cont.) = 1.12 exp(-  L  abs,real )  p-Pb (450) 208  pp (450) = exp(-  L  abs,conv )  abs,conv = 4.2 mb   abs,real = 5.9 mb Assuming the same  abs at 158 as at 450 GeV: = 1.06 exp(-  L  abs,real ) = 0.71 (instead of 0.75)  p-Pb (158) 208  pp (158) → The final state absorption increases to compensate for the anti-shadowing... This is the  abs value directly comparable to the PHENIX values, 0–3 mb

17. 17/21 In A-A collisions, the shadowing or anti-shadowing effect is squared (two nuclei) and it should change with centrality... At the SPS, maybe the increased initial production yield (anti-shadowing) with centrality compensates for the higher  abs value, so that the “expected normal nuclear absorption” curve in Pb-Pb collisions remains approximately the same as used up to now... Questions: 1) Can the EKS98 model be trusted at the percent level for the gluon anti-shadowing? 2) How can the centrality dependence of the nuclear effects on the PDFs be fixed? “Give me two parameters and I can fit an elephant, give me three and I make its tail wiggle” [Eugene Wigner] 3) When will we have accurate measurements of open charm production in p-A or d-Au collisions to separate initial state from final state effects? Will it be done at RHIC? 4) How is all this affected by the feed-down sources, which have a higher  abs value? Nuclear effects on the PDFs: from p-A to A-A

18. 18/21 Influence of feed-down from higher states Approximate radii of the J/ ,  ’ and  c states: r(J/  ) = 0.25 fm; r(  ’) = 2 x r(J/  ); r(  c ) = 1.5 x r(J/  ) Geometrical cross-sections of the J/ ,  ’ and  c states:  geom (J/  ) = 1.96 mb;  geom (  ’) = 7.85 mb;  geom (  c ) = 4.42 mb NA50 data:  geom (  ’) = 7.7 ± 0.9 mb Assuming 60% / 30% / 10% as the fractions of direct J/  production and feed-downs from  c and  ’ decays... Equivalent to the fit with an effective  geom (J/  ) = 4.2 ± 0.5 mb It suggests that the J/ ,  ’ and  c states are formed immediately as such and interact with their asymptotic geometrical cross-section values...  2 /ndf = 1.0 coincidence?

19. 19/21 The  ’ suppression pattern in S-U and in Pb-Pb shows a significantly stronger drop than expected from the Glauber extrapolation of the p-A data  abs = 8 ± 1 mb  abs ~ 20 mb ’’ ’’ J/  The “change of slope” looks very abrupt... The  ’ is suppressed from p-nucleus to nucleus-nucleus

20. 20/21 Could it be because of melting in the QGP? Yes, it could be... But it is very unfortunate that the “drop” happens between p-A and S-U/Pb-Pb, when we change collision systems and energies, from 400/450 to 200/158 GeV. Poor statistics prevents the NA60 In-In data from defining the  ’ suppression pattern. If the extra (strong)  ’ suppression is due to the dissolution of the bound cc state by the QGP, Lattice QCD says that this would indicate that T c sits in the most peripheral S-U or Pb-Pb collisions at SPS energies... The  ’ suppression measurements deserve more attention... And have the advantage of not being affected by feed-down sources ’’ Extra  ’ suppression from p-nucleus to S-U and Pb-Pb

21. 21/21 Take-home messages A clear interpretation of the charmonia suppression results obtained in heavy-ion collisions requires a detailed understanding of charmonia production in “elementary” pp and p-nucleus collisions! Guidance from theory has been very important... but significant progress in the field comes from high accuracy measurements → RHIC experiments need accurate d-Au data to enter the charmonia suppression game in a robust way → The LHC heavy-ion program must invest in p-A runs Otherwise, we will say, in about five years from now: “Just when we were about to find the answer, we forgot the question”