1. SPS charm(onium) and bottom(onium) measurements E. Scomparin INFN Torino (Italy) Introduction Past heavy quark and quarkonium measurements: NA38/NA50 (Helios-3) Present heavy quark and quarkonium measurements: NA60 What remains to be learned ? Conclusions

2. Heavy quarkonia Matsui and Satz prediction (1986) at the origin of the whole field No experiment was explicitly intended for charmonia detection Even NA38 (proposed in March 1985) was aiming at the study of thermal dimuon production Experimental facts Relatively small cross section (@ s=20 GeV, B µµ  J/ ~10 nb) J/µµ channel relatively clean Need large luminosities and a very selective trigger NA38 happened to be in a very good situation to study charmonium (its ancestor, NA10, studied high mass Drell-Yan and  production)

3. Charmonium production: nuclear collisions at fixed target Study carried out by NA38/NA50/NA60 at the SPS from 1986 until today Basic facts Essentially the same experiment, although with very significant upgrades Large set of results with very good statistics (Lots of) systems studied, including: p-p, p-d, p-Be, p-C, p-Al, p-Cu, p-Ag, p-W, p-Pb, p-U, O-Cu, O-U, S-U, In-In, Pb-Pb Similar (but not identical) energy/kinematical domain between various data sets Very significant contributions (in a slightly higher energy range) by: E866 HERA-B The question to be answered by studying charmonium in heavy-ion collisions at the SPS Is (at least part of the) suppression of charmonia that we observe in the data NOT due to usual hadronic processes ?

4. The NA38/NA50/NA60 experiments Based on the same muon spectrometer (inherited by NA10) no apparatus-dependent systematics Many updates in the target region, in parallel with the availability of radiation hard detectors NA50 MUON FILTER BEAM TRACKER TARGET BOX VERTEX TELESCOPE Dipole field 2.5 T BEAM IC not on scale NA60

5. pA collisions: the reference Glauber fit to B µµ  J/ at 400-450 GeV  J/ abs = 4.48  0.42 mb Main problem: extrapolation to 158 GeV/c S-U data (200 GeV) should not be used (absorption sources different wrt pA might be present) Obtain normalization ( J/ pp ) at 200 GeV using only pA data assuming  J/ abs does not depend on s High statistics 400/450 data:  J/ / DY ratios Obtain  J/ abs = 4.18  0.35 mb

6. Expected (J/)/DY at 158 GeV As it is well known, NA50 uses Drell-Yan as a reference process to study J/ suppression Is (J/)/DY equivalent to J/ cross section per N-N collision ?  Yes, Drell-Yan A-dependence measured  DY = 0.995  0.016 (stat.)  0.019 (syst.) Start from  J/ pp / DY pp @ 450 GeV (1.4% error) Rescale to 200 GeV J/  see previous page (7.8% error, SU not used) DY  LO calculation (2.5 % error) Rescale to 158 GeV J/  fit a la Schuler to measured J/ cross sections (1.5% error) DY  LO calculation (negligible error) Use Glauber (with neutron halo) to calculate centrality dependence of expected  J/ / DY Include experimental smearing on centrality determination (E T, E ZDC, N ch ) Direct measurement of  J/ / DY at 158 GeV would significantly decrease such errors (NA60)

7.  J/ / DY in Pb-Pb collisions at 158 GeV Final NA50 set of data Old reference (include S-U in the determination) New reference (only p-A collisions are used)

8. Compatibility of data sets Older data sets considered not as reliable as recent ones 1996 high statistic data: biased by reinteractions (thick target)

9. Study of various centrality estimators Pattern consistent with E T -based analysis Departure from normal nuclear absorption at mid-centrality Suppression increases with centrality

10. What about S-U ? Absorption curve calculated using p-A data only S-U data found to be in agreement (once rescaling are performed) with p-A extrapolation Peripheral Pb-Pb collisions No indication for a sizeable extra-absorption in S-U wrt p-A

11. Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems Study collisions between other systems, such as Indium-Indium Which is the variable driving the suppression? Is the anomalous suppression also present in lighter nuclear systems? Study the nuclear dependence of  c production in p-A What is the impact of the  c feed-down on the observed J/ suppression pattern? New charmonium studies : NA60 Study J/ production in p-A collisions at 158 GeV What is the normal nuclear absorption cross-section at the energy of the heavy ion data? S-U In-In Pb-Pb N part L (fm) pure Glauber calculation

12. NA60: In-In collisions 5-week long run in 2003 – In-In @ 158 GeV/nucleon ~ 4×10 12 ions on target ~ 2×10 8 dimuon triggers collected Set A Set B Raw  +  - invariant mass spectrum m µµ (GeV/c 2 ) Events/50 MeV Centrality selection: use spectator energy in the ZDC charged multiplicity in the vertex spectrometer Two muon spectrometer settings Set A (low ACM current) Good acceptance at low mass Used for LMR and IMR analysis Set B (high ACM current) Good resolution at high mass Used for J/ suppression, together with set A

13. Multi-step fit a) M > 4.2 GeV : normalize DY b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed) c) 2.9 < M < 4.2 GeV: get the J/y yield (with DY & charm fixed) Combinatorial background from  and K decays estimated from the measured like-sign pairs (<3% contribution under the J/) Signal mass shapes from MC PYTHIA and GRV 94 LO p.d.f. GEANT 3.21 for detector simulation reconstructed as the measured data Acceptances from Monte Carlo simulation: J/ : 12.4 % (setB); 13.8 % (setA) DY : 13.2 % (setB); 14.1 % (setA) (in mass window 2.9–4.5 GeV) J/  ’’ DY Background Charm without matching 6500 data set no centrality selection The J/ / DY analysis (NA50-like)

14. Normal absorption curve based on the NA50 results Uncertainty (~ 8%) at 158 GeV dominated by the extrapolation from the 400 and 450 GeV data Comparison with NA38/NA50 results “anomalous suppression” present in Indium-Indium How to get a more accurate suppression pattern ? Do not use Drell-Yan

15. Study of the J/ centrality distribution Compare the centrality distribution of the measured J/ sample with the distribution expected in case of pure nuclear absorption Main advantage  Much smaller statistical errors Main drawback  No intrinsic normalization, if absolute cross sections are not known Use matched J/ sample E ZDC (GeV) dN J/  /dE ZDC ε vertex dimuon > 99.5 % ε vertex Inefficiencies introduced by the cuts, used in the event selection, affect in a negligible way the J/ sample (or are not centrality dependent) Work in progress to obtain d J/ /dE ZDC

16. Comparison with expected yield Data are compared with a calculated J/ centrality distribution Use  J/ abs = 4.18  0.35 mb Onset of anomalous suppression in the range 80 < N part < 100 Saturation at large N part Nuclear absorption Ratio (Measured / Expected) normalized to the standard analysis (~7% error) E ZDC (TeV)

17. Comparison with previous results The S-U, In-In and Pb-Pb data points do not overlap in the L variable The J/ suppression patterns are in fair agreement in the N part variable S-U most central point ?

18. Other variables related to centrality NA50 Pb-Pb NA60 In-In NA50 Pb-Pb NA60 In-In A more significant comparison requires Pb-Pb points with reduced errors very preliminary Bjorken energy density, estimated using VENUS Work in progress inside NA50 to have a non-DY analysis for the 2000 data  Results expected soon

19. Comparison with theoretical models Good accuracy of NA60 data  quantitative comparisons possible J/ absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032 J/ suppression in the QGP and hadronic phases (including thermal regeneration and in-medium properties of open charm and charmonium states) Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; hep-ph/0403204  c suppression by deconfined partons when geometrical percolation sets in Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547. Consider models formulating specific predictions for In-In collisions previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50

20. Comparison with theoretical models Satz, Digal, Fortunato Rapp, Grandchamp, Brown Capella, Ferreiro No quantitative agreement with any model

21. One more model Maximum hadronic absorption (Hagedorn gas) not enough to reproduce In-In and Pb-Pb L. Maiani @ QM2005

22. Summary on charmonium at the SPS Anomalous J/ suppression Established fact in Pb-Pb (NA50) and, more recently, in In-In (NA60) Not present in S-U collisions (NA38) Onset around N part = 100 Does S-U show an incompatibility with Pb-Pb and In-In ? Other interesting results Suppression concentrated at low p T in PbPb (see NA50 @ QM05) Anomalous ’ suppression identical in S-U and Pb-Pb (vs L) Already sets in for peripheral S-U collisions (see NA50 @QM05) News to be exepcted in the near future NA50: non-DY analysis  more meaningful comparison with NA60 NA60: use full statistics for analysis  ~ factor 2 more No final word from theory on the interpretation of the results SPS+RHIC systematics  great opportunity

23. Can SPS go beyond charmonium ? NA50 measured  A-dependence in p-A at 450 GeV   = 0.98  0.08  production not accessible in A-A at present SPS, s too low

24. Bottomonium in A-A at the SPS ? In the framework of the upgrade of CERN machines the SPS+ concept is presently under discussion  Availability of ~1 TeV protons from ~2014 onwards Pb ions at ~ 400 GeV/nucleon (s ~ 28 GeV) Study J/ suppression vs. s (not possible at present SPS energies) Study suppression of  states (depends on available luminosity) J/   (2S)  c (1P) J/    (3S)  b (2P)  (2S)  b (1P)  (1S) Various possibilities: Needs NA60 upgrade  first discussions are now taking place

25. Heavy quark production Relatively comfortable cross section ( tot ~ 20 µb @ s=20 GeV) However D 0  K Difficult to single out in the high hadronic multiplicity (attempt by NA49,no signal, nucl-ex/0507031) D 0  µX Full reconstruction of the decay topology impossible Important background (combinatorial+Drell-Yan) Negligible contribution in the low-mass region Sizeable contribution in the intermediate mass region First studies by NA50, important progress with NA60 Pb-Pb  N part  =110 Pb-Pb  N part  =381

26. p-A shape analysis: m, y, p T, cos spectra Dimuon differential distributions in the region –0.5<y CM <0.5, cos CS <0.5 consistent with a superposition of Drell-Yan + open charm M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443 Absolute cross sections found to be consistent with direct measurements of open charm production

27. Extrapolation to A-A collisions Assumption: DY and open charm behave as hard processes  A scaling Pb-Pb  N part  =110 Pb-Pb  N part  =381 Excess of dimuon yield: Data/Sources ~1.3 in S-U, ~1.7 in Pb-Pb Smoothly growing with centrality Nature of the excess Enhancement of known sources New sources appear M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

28. Enhancement of known sources Excess not compatible with background shape Compatible with an an enhancement of open charm (m,p T spectra) Factor 3 enhancement in central Pb-Pb M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443

29. Thermal production? Good description of the mass spectra in the two approaches for central Pb-Pb events R. Rapp and E. Shuryak, Phys. Lett. B473(2000) 13 Explicit introduction of a QGP phase Initial temperature: T i =192 MeV Critical temperature: T c =175 MeV Fireball lifetime: 14 fm/c (increasing to T i =221 MeV still good agreement) L. Capelli et al.,NA50, Nucl. Phys. A698(2002) 539c Only way to solve the puzzle: discriminate between prompt and displaced dimuon sources

30. hadron absorber Muon Other and trackingMuon trigger magnetic field Iron wall NA50 spectrometer 2.5 T dipole magnet Matching in coordinate and momentum space targets beam tracker vertex tracker or ! NA60: detector concept Improved dimuon mass resolution Origin of muons can be accurately determined

31. Muon matching Vary the cut on the matching  2 Muons from muon spectrometer Vertex spectrometer tracks Compare slopes and momenta Define a matching  2 Re-fit matched tracks With this procedure Combinatorial background can be reduced A certain level of fake matches is present (new kind of background) improve the signal/background ratio

32. Vertex resolution  z ~ 200 µm along beam axis Good target ID (down to very peripheral events) Dispersion between beam track and VT vertex Vertex resolution (assuming  BT =20  m) 10 20 30 0  (  m) Number of tracks  x ~  y ~ 10- 20 µm in the transverse direction (by comparing beam impact point on the target and reconstructed interaction point)

33. Offset resolution J/  Weighted Offset (  )  100 Offset resolution (  m) Resolution of the impact parameter of the track at the vertex (offset) 40 – 50 µm (studied using J/ events)  vertex   impact < c (D + : 312 m, D o : 123 m) Prompt dimuons can be separated from open charm decays Define weighted offset  to eliminate momentum dependence of offset resolution (offset wighted by error matrix of the fit)

34. Weighted offset distribution of the expected sources Prompt contribution  average of the J/ and  measured offsets Open charm contribution  MC distribution, after smearing

35. Background subtraction Combinatorial background Dominant dimuon source for m  <2 GeV/c 2 Mixed event technique developed  accurate to ~ 1% NA60 acceptance quite asymmetric  Cannot use Fake matches background also rejected with a mixed event approach Less important in the intermediate mass region 1% error in the comb. background estimate 10% error on the signal

36. IMR: is an excess present ? Answer: Yes, an excess in the IMR is clearly present (same order of magnitude of the NA50 result) NA50 norm. Open charm and Drell-Yan generated with PYTHIA Drell-Yan normalization fixed using the high mass region Open charm normalization: use  NA50 p-A result (better control of systematics related to  channel)  World-average cc cross section (based on direct charm measurements) (differ by a factor ~ 2) World-aver. norm. data prompt charm prompt+charm Excess

37. Is the excess compatible with the NA50 observation? Can we describe the measured mass spectrum by leaving the open charm normalization as a free parameter, as done by NA50? ~ 2 in terms of NA50 p-A normalization Answer: Yes, we can describe the In-In data with a “charm enhancement” factor around 2 in “NA50 units” (to be compared with ~ 3 for PbPb in NA50) Results of fits reported in terms of DY and open charm scaling factors needed to describe the data

38. Check NA50 hypothesis using muon offsets Fix the prompt contribution to the expected DY Can the offset distribution be described with an enhanced charm yield? Dimuon weighted offsets Answer: No, the fit fails Charm is too flat to describe the remaining spectrum… Kinematical domain 1.2 < M < 2.7 GeV/c 2 0 < y CM < 1 |cos| < 0.5

39. Alternative options Dimuon weighted offsets Try to describe the offset distribution leaving both contributions free Answer: Two times more prompts than the expected Drell-Yan provides a good fit (and the charm yield is as expected from the NA50 p-A dimuon data)

40. Is the prompt yield sensitive to the charm level? Fix the charm contribution to either of the two references, and see how the level of prompts changes Answer: No, both options require two times more prompts than the expected Drell-Yan ! (the charm contribution is too small to make a difference) Dimuon weighted offsets “world average” “NA50 p-A  ”

41. Mass shape of the excess The mass spectrum of the excess dimuons is steeper than DY (and flatter than Open Charm) Fix the DY and Charm contributions to expected yields

42. Relative excess: (Data – Sources) / Sources Excess per participant: (Data – Sources) / N part Faster than linear increase with N part Centrality dependence of the excess very preliminary

43. Summary on open charm at the SPS Serious study much delayed with respect to charmonia investigations First generation experiments Excess in the intermediate mass region Connession with open charm possible (NA50) Could not be proved Second generation experiment (NA60) Equipped with accurate vertex detector Present understanding: open charm yield in A-A follows N coll scaling What next ? Update NA60 results (full statistics, more accurate alignment) Run NA60 with PbPb (after 2010) If the IMR excess is not charm, then what can it be ?

44. Conclusions Long and fascinating history (started 19 years ago!) Many interesting results, both recent and (relatively) ancient Still interesting now, when higher energy domains are opening up ? Surely yes! Finding a consistent description of phenomena occurring in various energy ranges is an important challenge, that deserves being investigated Future of heavy-ions at SPS ? Still not defined, but Heavy-ions can be available once LHC has been commissioned SPS+ will be built in case LHC luminosity upgrade is approved Some of us are starting to think about a new dimuon experiment at SPS Encouragement, suggestions, participation are very welcome !