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Recent results from the CERN SPS on quarkonium production in p-nucleus and nucleus-nucleus collisions. Summary
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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/y suppression... J/y 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 Carlos Lourenço, 4th International Workshop on Heavy Quarkonium, June 27–30 2006
Quarkonia studies in heavy-ion collisions: why? how? 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 y’, the cc and the J/y 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/y and y’ 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...
J/y Pb-Pb 158 GeV p-Pb 400 GeV Charmonia studies at the CERN SPS Measurements of J/y and y’ 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 beenpresented 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 pT 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/y events in total.
p-A 400 GeV J/y p-Pb @ 400 GeV sJ/y ~ 105 MeV y’ The J/y and y’ are absorbed in p-nucleus collisions ... The J/y and y’ production cross-sections scale less than linearly with the number of target nucleons (contrary to what happens with high-mass Drell-Yan dimuons). NA50 p-A data collected in year 2000,with Be, Al, Cu, Ag, W and Pb targets Note: the spA = spp x Aa parametrization leads to extrapolated s(J/y) and s(y’) pp values which are 10 to 20% higher than those obtained using the Glauber model
Projectile J/y L Target p-A 400 GeV J/y y’ ... as a function of the mass number and of L ... L is the “path length” which the J/y and y’ states traverse in the target nucleus, from the production point of the ccbar pair to the nuclear surface the “r L parametrization” exp(-r L sabs) is a good approximation of the full Glauber calculation 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.
... at 400 and at 450 GeV ... From a global fit to the 400 and 450 GeV p-A data, NA50 determined the following absorption cross-sections: sabs(J/y) = 4.5 ± 0.5 mb ; sabs(y’) = 8.3 ± 0.9 mb from production cross-sections sabs(J/y) = 4.2 ± 0.5 mb ; sabs(y’) = 7.7 ± 0.9 mb from cross-section ratios (y/DY) c2/ndf = 0.7 c2/ndf = 1.4 The sabs values derived from y and y/DY are “identical”, indicating negligible (initial state) nuclear effects in Drell-Yan production at these energies and at mid-rapidity.
Pb-Pb 158 GeV ... and is suppressed in Pb-Pb collisions ... The J/y production cross-sections measured in O-Cu, O-U and S-U arecompatible 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. But the J/y suppression pattern changes significantly for Pb-Pb collisions...
(NJ/y / NDY) (ETi) Ri = (NJ/y / NDY) (ET1) ... at low transverse momentum The J/y “central over peripheral ratio”strongly depends on pT (at the SPS) Only the low pT J/y mesons get suppressed !
PHENIX J/y Low x2 ~ 0.003 (shadowing region) 0 mb 3 mb J/y production in p-A collisions vs. collision energy ... It seems that the J/y 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
pp 〈pT2〉pp(GeV/c)2 pp J/y s (GeV) pT (GeV/c) ... vs. pT ... NA50 The increase of a with pT seems to be identical at 400, 800 and 920 GeV (at mid-rapidity) Maybe the increase of a from NA50 to E866 to HERA-B to PHENIX is due to the increase of the average pT of the J/y when s increases...
E866 ... and vs. xF astrongly decreases at high xF ... Why is this so? Higher parton densities? If so, the J/y should be strongly absorbed in d-Au at RHIC energies; and it is not...
1.0 a 0.9 0.8 Models (with variants): • - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539 • K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599 xF -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 B&K 1.0 Vogt: final state absorption 0.9 HERA-Bpreliminary E866 38.8 GeV Be/Fe/W E789 38.8 GeV Be/C/Cu/W E772 38.8 GeV H2/C/Ca/Fe/W NA50 29.1 GeV Be/Al/Cu/Ag/W NA3 22.9 GeV H2/Pt 0.8 xF 0.7 -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...
Normal nuclear absorption of J/y production at RHIC 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. PHENIX No final stateabsorptionsabs = 0 mb sabs = 3 mb Such a y-dependent effect is not expected to be seen in the SPS p-A data
NA50 p-A 450 GeV Pythia with EKS98 gives the same shape for pp and p-Be What’s known about the J/y dN/dy in SPS p-A collisions? The J/y y distributions are not centered at 0,even for the p-Be collision system ! NA50 measures dimuons within one unitof rapidity, at around mid-rapidity All five distributions are well described by Gaussians of mean y0 -0.2 and s = 0.85 Forcing y0 = 0, the c2/ndf increases from 1–3 to 20–50, depending on the data set (target) Why is the J/y rapidity distribution changing from pp to p-Be? Not because of nuclear effects on the PDFs...
Nuclear effects on the PDFs and final state J/y absorption sabs from PHENIX: after accounting for the nuclear effects on the PDFs (assuming EKS98) sabs 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 sabs 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
exp(-r L sabs,conv) sp-Pb(450) sp-Pb(158) = 208 spp(450) 208 spp(158) = 1.12 exp(-r L sabs,real) Nuclear effects on the PDFs and the J/y absorption (cont.) For p-Pb collisions, the EKS98 nuclear modification factor is 1.12 at 450 GeV and 1.06 at 158 GeV sabs,conv= 4.2 mbsabs,real= 5.9 mb → The final state absorption increases to compensate for the anti-shadowing... This is the sabs value directly comparableto the PHENIX values, 0–3 mb Assuming the same sabs at 158 as at 450 GeV: = 1.06 exp(-r L sabs,real)= 0.71 (instead of 0.75)
Nuclear effects on the PDFs: from p-A to A-A 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 sabs 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 sabs value?
c2/ndf = 1.0 Influence of feed-down from higher states Approximate radii of the J/y, y’ and cc states: r(J/y) = 0.25 fm; r(y’) = 2 x r(J/y); r(cc) = 1.5 x r(J/y) Geometrical cross-sections of the J/y, y’ and cc states: sgeom(J/y) = 1.96 mb; sgeom(y’) = 7.85 mb; sgeom(cc) = 4.42 mb NA50 data: sgeom(y’) = 7.7 ± 0.9 mb Assuming 60% / 30% / 10% as thefractions of direct J/y production andfeed-downs from cc and y’ decays... Equivalent to the fit with an effectivesgeom(J/y) = 4.2 ± 0.5 mb It suggests that the J/y, y’ and ccstates are formed immediately assuch and interact with their asymptoticgeometrical cross-section values... coincidence? coincidence?
The y’ is suppressed from p-nucleus to nucleus-nucleus The y’ 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 J/y y’ y’ sabs = 8 ± 1 mb sabs ~ 20 mb The “change of slope” looks very abrupt...
Extra y’ suppression from p-nucleus to S-U and Pb-Pb 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 y’ suppression pattern. If the extra (strong) y’ suppression isdue to the dissolution of the bound ccstate by the QGP, Lattice QCD saysthat this would indicate that Tc sitsin the most peripheral S-U or Pb-Pbcollisions at SPS energies... The y’ suppression measurements deserve more attention... And have the advantage of not being affected by feed-down sources y’
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”
