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SPS charm(onium) and bottom(onium) measurements

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 . Heavy quarkonia.

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SPS charm(onium) and bottom(onium) measurements

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  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, Bµµ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 • 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 ? • 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

  4. NA50 Dipole field2.5 T NA60 TARGET BOX MUON FILTER BEAM BEAMTRACKER VERTEX TELESCOPE IC not on scale 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

  5. pA collisions: the reference • Glauber fit to Bµµ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/DYpp@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 (ET, EZDC, Nch) 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 New reference (only p-A collisions are used) Old reference (include S-U in the determination)

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

  12. Events/50 MeV Set A Set B Raw +- invariant mass spectrum mµµ (GeV/c2) NA60: In-In collisions • 5-week long run in 2003 – In-In @ 158 GeV/nucleon • ~ 4×1012 ions on target • ~ 2×108 dimuon triggers collected • 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 • Centrality selection: use • spectator energy in the ZDC • charged multiplicity in the vertex • spectrometer

  13. Background without matching 6500 data set no centrality selection J/y Charm y’ DY The J/ / DY analysis (NA50-like) • 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) • 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)

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

  15. dNJ/y/dEZDC EZDC (GeV) εvertex dimuon > 99.5 % εvertex 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 • Use matched J/ sample • Inefficiencies introduced by the cuts, • used in the event selection, affect in • a negligible way the J/ sample • (or are not centrality dependent) • Main advantage •  Much smaller statistical errors • Main drawback •  No intrinsic normalization, if • absolute cross sections are not • known Work in progress to obtain dJ//dEZDC

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

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

  18. NA60 In-In NA60 In-In NA50 Pb-Pb NA50 Pb-Pb Other variables related to centrality very preliminary Bjorken energy density, estimated using VENUS • A more significant comparison requires Pb-Pb points with reduced errors • 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 • 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 • 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 inDigal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

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

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

  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 Npart = 100 • Does S-U show an incompatibility with Pb-Pb and In-In ? • No final word from theory on the interpretation of the results • SPS+RHIC systematics  great opportunity • Other interesting results • Suppression concentrated at low pT 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

  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. (3S) b(2P) (2S) b(1P) (1S)  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) • Various possibilities: (2S) Study J/ suppressionvs. s (not possible atpresent SPS energies) c(1P) J/ Study suppression of states (depends on available luminosity) J/ Needs NA60 upgrade  first discussions are now taking place

  25. Pb-Pb Npart=110 Pb-Pb Npart=381 Heavy quark production • Relatively comfortable cross section (tot~ 20 µb @ s=20 GeV) • However • D0  K • Difficult to single out in the high hadronic multiplicity • (attempt by NA49,no signal, nucl-ex/0507031) • D0  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

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

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

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

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

  30. Muon trigger and tracking Iron wall magnetic field hadron absorber Muon Other or ! NA60: detectorconcept 2.5 T dipole magnet NA50 spectrometer beam tracker vertex tracker targets Matching in coordinate and momentum space • Improved dimuonmass resolution • Origin of muonscan be accurately determined

  31. Muon matching 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 Vary the cut on the matching 2

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

  33. J/ Weighted Offset ()  100 Offset resolution (m) Offset resolution 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, Do : 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/c2 • NA60 acceptance quite asymmetric Cannot use • Mixed eventtechnique developed  accurate to ~ 1% • 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. 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) IMR: is an excess present ? Excess Excess NA50 norm. World-aver. norm. data prompt charm prompt+charm • Answer: Yes, an excess in the IMR is clearly present • (same order of magnitude of the NA50 result)

  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 NA50p-A normalization Results of fits reported in terms of DY and open charm scaling factors needed to describe the data 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)

  38. Dimuon weighted offsets 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? Kinematical domain 1.2 < M < 2.7 GeV/c20 < yCM < 1|cos| < 0.5 Answer: No, the fit fails Charm is too flat to describe the remaining spectrum…

  39. Dimuon weighted offsets Alternative options • 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 “NA50 p-A mm” “world average” Dimuon weighted offsets Answer: No, both options require two times more prompts than the expected Drell-Yan ! (the charm contribution is too small to make a difference)

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

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

  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 Ncoll 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 !

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