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J/ Y , Charm and intermediate mass dimuons in Indium-Indium collisions

J/ Y , Charm and intermediate mass dimuons in Indium-Indium collisions. , not RHIC!. Results from recent data (year 2003) from SPS. Time is limited. I will focus on open charm+intermediate mass dimuons, first. then move to J/ y analysis, if time allowed.

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J/ Y , Charm and intermediate mass dimuons in Indium-Indium collisions

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  1. J/Y, Charm andintermediate mass dimuonsin Indium-Indium collisions , not RHIC! • Results from recent data (year 2003) from SPS • Time is limited. I will focus on open charm+intermediate mass dimuons, first. then move to J/y analysis, if time allowed Hiroaki Ohnishi, RIKEN/JAPAN For the NA60 collaboration XXXV International Symposiumon Multiparticle Dynamics 2005 KROMĚŘÍŽ, CZECH REPUBLIC, August 9-15, 2005

  2. This talk focuson this aspect! Search for the QCD phase transition QCD predicts that strongly interacting matter, above a critical temperature, undergoes a phase transition to a state where thequarks and gluons are no longer confinedin hadrons, and chiral symmetry is restored Such a phase transition should be seen through dilepton signals: • the suppression of strongly bound heavy quarkonium statesdissolved when certain critical thresholds are exceeded • the production of thermal dimuons • changes in the r spectral function(mass shifts, broadening, disappearance)when chiral symmetry restoration is approached

  3. Intermediate mass dimuon measurement from p-A to Pb-Pb • NA50 was able to describe the IMR dimuon spectra in p-A collisions as a sum of Drell-Yan and Open Charm contributions (but: charm production cross-section higher than the “world average”) NA38/NA50 proton-nucleus data

  4. Intermediate mass dimuon measurement from p-A to Pb-Pb • NA50 was able to describe the IMR dimuon spectra in p-A collisions as a sum of Drell-Yan and Open Charm contributions (but: charm production cross-section higher than the “world average”) • The yield of intermediate mass dimuons measured in heavy-ion collisions exceeds the sum of expected sources (Charm and DY) NA50 Pb-Pbcentral collisions NA38/NA50 proton-nucleus data

  5. Explanation of intermediate mass dimuon • The intermediate mass dimuon yields in heavy-ion collisions can bereproduced by • by scaling upthe Open Charmcontribution by up to a factor of 3 • by adding thermal radiation from a quark-gluon-plasma • To identify the source of enhancement, we need to separate D meson decays and prompt dimuons We need to measure secondary vertices with ~ 50 mm precision

  6. MWPC’s m ~ 1m Muon Spectrometer Iron wall Hadron absorber Toroidal Magnet Target area m beam Trigger Hodoscopes Dipole field2.5 T ZDC TARGET BOX MUON FILTER BEAM BEAMTRACKER VERTEX TELESCOPE IC not to scale NA60 detector concept Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorberand match them to the muon measured in the spectrometer Matching in coordinate and in momentum space  • Improved dimuon mass resolution • Origin of muons can be accurately determined Prompt dimuon Displaced dimuon 12 tracking planes made with Rad-hard silicon pixel detector OR

  7. Set A (low Muon magnet current) • Good acceptance at low mass • Used for LMR and IMR analysis • Set B (high muon magnet current) • Good resolution at high mass • Used for J/ analysis Data set • Two muon spectrometer settings • 5-week long run in 2003In-In @ 158 GeV/nucleon Events/50 MeV • Centrality selection using • beam spectator energyin the ZDC • or charged multiplicityin the vertex • spectrometer Raw +- invariant mass spectrum mµµ (GeV/c2) • ~ 4×1012 ions on target • ~ 2×108 dimuon triggers collected

  8. J/Weighted Offset ()  100 J/ Weighted Offset ()  100 Offset resolution (m) Offset resolution (m) J/ • To eliminate the momentum dependence of the offset resolution, we use the offset weighted by the error matrix of the fit: for single muons for dimuons Muon track offset resolution • Offset resolution is evaluated with prompt dimuon (J/y) ~ 40–50 m

  9. Background subtraction • Combinatorialbackground • Significantly reduced by the track matching procedure • Nevertheless, still the dominant dimuon source for m < 2 GeV/c2 Cannot use • NA60 acceptance quiteasymmetric Nback= 2√N++N-- • Mixed event technique developed  accurate to 1–2% • Fake matchesbackground: muon matched to awrongvertex telescope track • Evaluated with mixed eventscomplicated butrigorous approach

  10. Real data ! Low mass dimuons w h f Intermediate J/y Background subtraction: resulting mass distribution Detail will be discussedfollowing presentationby M. Floris Data integrated over centrality (Matching 2 < 1.5) This talk focuses on + (if possible)

  11. Intermediate mass dimuon analysis

  12. NA60 Signal analysis: simulated sources • Charm andDrell-Yancontributions are calculated by overlaying Pythia events on real data(using CTEQ6M PDFs with EKS98 nuclear modifications and mc=1.3 GeV/c2)The fake matches in the MC events are subtracted as in the real data • Relative normalizations: • for DY: K-factor of 1.8; to reproduce DY cross-sections of NA3 and NA50 • for charm: we use the cross-section needed to reproduce the NA50 p-A dimuon data(a factor 2 higher than the “world average” of direct charm measurements) • Absolute normalization:The expected DY contribution, as a function of the collision centrality, is obtained from the number of observed J/ events and the  suppression pattern A 10% systematical error is assigned to this normalization The fits to mass and weighted offset spectra are reported in terms ofthe DY and Open Charm scaling factors needed to describe the data

  13. IMR mass dimuons analysisa la NA50 • Procedure: Fix the Charm and DY contributions to the expected yields and see if their Sum describes the measured Data The expected Charm and DY yields, plus 10%, cannot explain the measured data An excess is clearly present !

  14. Pb-Pb S-U NA38+NA50 p-A Question: Is it compatible with the NA50 observation? • Procedure: Try to describe the measured mass spectrum by leaving the Charm normalization as a free parameter NA50 would require a factor 3.5 of Charm enhancement incentral Pb-Pb collisions… Answer: Yes, leaving the Charm yield free describes the In-In data, with ~ 2 times more charm than needed by the NA50 p-A data

  15. Question: Is this validated by the offsets information? • Procedure: Fix the prompt contribution to the expected DY yield and see if the offset distribution can be described with enhanced Charm Answer: No, Charm is too flat to describe the remaining spectrum…… we need more prompts!

  16. Question: How many more prompts do we need? • Procedure: Leave both contributions freeand see if we can describe the offset distribution Answer: A good fit requires two times more prompts than the expected Drell-Yan yield

  17. Question: Is the prompt yield sensitive to the Charm level? • Procedure: Change the Charm contribution by a factor of 2and see how that affects the level of prompts If we decrease the Charm yield to 0.55,the level of the Prompts contribution changes from 1.91 ± 0.11 to 2.08 ± 0.07 If we increase the Charm yield by a factor of 2, the description of the data deteriorates significantly Answer: No, we always need two times more prompts than the expected Drell-Yan, within 10% (the Charm contribution is too small to make a difference)

  18. Question: What is the mass shape of the excess? • Procedure: Fix the DY and Charm contributions to their expected yields and see how the excess, relative to DY or Charm, depends on the dimuon mass Answer: The mass spectrum of the excess dimuons is steeper than DY and flatter than Open Charm

  19. very preliminary very preliminary Centrality dependence of the Excess = Data - DY - Charm The yield of excess dimuons increases faster than linearly with Nparticipants If the excess dimuons are due to a hard process, they should have the same centrality dependence as the expected sources (DY + Charm). Not excluded by the data, at this time.

  20. Summary IMR dimuons • There is an excess of intermediate mass dimuons in Indium-Indium collisions • The offset distribution requires a factor 2 more prompts than expected from DY  The excess is not due to open charm enhancement • The excess grows faster than linearly with the number of participants • Results are very robust with respect to variations of the matching 2 cut: changingthe Signal / Background ratio by a factor of 2 changes the results by less than 10% The excess cannot be due to a bias in the background subtraction

  21. J/Y suppression

  22. NA38/NA50 Projectile J/y L Target Survival probability ofthe J/y: exp(-rLsabs) J/y normal nuclear absorption curve J/Y production in p-A to Pb-Pb • The study of J/y production in p-A collisions at 200, 400 and 450 GeV, by NA3, NA38, NA50 and NA51, gives a “J/y absorption cross-section in normal nuclear matter” of 4.18 ± 0.35 mb. • In p-A, light-ion, the data follow this normal nuclear absorption which scales with “the length of nuclear matter crossed by the (pre-resonant) J/y”, L. • peripheral Pb-Pb collisions also follows L scaling • In the more central Pb-Pb collisions the L scaling is broken and an “anomalous suppression” sets in

  23. The J/Y standard analysis Background without matching 6500 data set no centrality selection • Combinatorial background from  and K decays estimated from like-sign pairs(less than 3% under the J/y) • Signal mass shapes from Monte Carlo: • PYTHIA and GRV 94 LO parton densities • GEANT 3.21 for detector simulation • reconstructed as the measured data • Acceptances from Monte Carlo simulation: • for J/y : 12.4 % (6500 A); 13.8 % (4000 A) • for DY : 13.2 % (6500 A); 14.1 % (4000 A) • (in mass window 2.9–4.5 GeV) J/y Charm y’ DY A multi-step fit (max likelihood) is performed: a) M > 4.2 GeV : normalize the 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)

  24. Centrality dependence (standard analysis) An “anomalous suppression” is present in the Indium-Indium data The small statistics of high mass dimuons limits the number of centrality bins

  25. EZDC (TeV) Direct J/Y analysis • Idea: directly compare the measured J/ sample (only matched dimuons), as a function of centrality, with the yield expected from the normal nuclear absorption • The integrated ratio Measured / Expected is imposed to be the same as in the standard analysis

  26. Comparison with previous results S, In and Pb data points do not overlap in the L variable: the physics behind the “anomalous” J/ suppression does not depend on L The In-In and Pb-Pb J/y suppression patterns are in fair agreement as a function of the Npart variable

  27. Direct J/ sample: comparison with theoretical models It is important to emphasize that these models were previously tuned on the p-A, S-U and Pb-Pb suppression patterns obtained by NA38 and NA50 We consider models for which we have predictions specifically made for In-In collisions: J/y absorption by produced hadrons (comovers) Capella and Ferreiro, hep-ph/0505032; J/y suppression in the QGP and hadronic phases including thermal regeneration and in-medium properties of open charm and charmonium statesGrandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545; Phys.Rev.Lett. 92 (2004) 212301; hep-ph/0403204 cc suppression by deconfined partons when geometrical percolation sets inDigal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

  28. Suppression by produced hadrons (“comovers”) The model takes into account nuclear absorption and comovers interaction with sco = 0.65 mb (Capella-Ferreiro) In-In @ 158 GeV J/y / NColl nuclear absorption comover + nuclear absorption (E. Ferreiro, private communication) NA60 In-In 158 GeVpreliminary Pb-Pb @ 158 GeV The smeared form (dashed line) is obtained taking into account the resolution on NPart, due to our experimental resolution

  29. QGP + hadrons + regeneration + in-medium effects The model simultaneously takes into account dissociation and regeneration processes in both QGP and hadron gas (Grandchamp, Rapp, Brown) In-In @ 158 GeV fixed thermalization time fixed thermalization time centrality dependent thermalization time BmmsJ/y/sDY centrality dependent thermalization time Nuclear Absorption Suppression + Regeneration QGP+hadronic suppression Regeneration Number of participants NA60 In-In 158 GeVpreliminary The smeared form (dashed line) is obtained taking into account the resolution on NPart, due to our experimental resolution Pb-Pb @ 158 GeV

  30. NA60 In-In 158 GeVpreliminary NA60 In-In 158 GeVpreliminary The measured data show a similar pattern but the anomalous suppression sets in atNpart ~ 90 Suppression due to a percolation phase transition Model based on percolation (Digal-Fortunato-Satz) Sharp onset (due to the disappearance of the cc meson) at Npart ~ 125 for Pb-Pb and ~ 140 for In-In The dashed line includes thesmearing due to the ZDC resolution Pb-Pb @ 158 GeV

  31. Summary IMR dimuons • There is an excess of intermediate mass dimuons in Indium-Indium collisions • The offset distribution requires a factor 2 more prompts than expected from DY  The excess is not due to open charm enhancement • The excess grows faster than linearly with the number of participants • The J/y shows an anomalous suppression already in Indium-Indium • The suppression is centrality dependent and sets in at ~ 90 Npart J/Y suppression

  32. Our measured dimuon spectra consist of: correctly matched signal signal muons from the spectrometer are associated with their tracks in the Ver.Tel. wrongly matched signal (fakes)at least one of the muons is matched to an alien track correctly matched combinatorial pairsmuons from ,K decays are associated with their tracks or with the tracks of their parent mesons association between the ,K decay muon and an alien track All these types of backgroundare subtracted byEvent Mixing(in narrow bins in centrality for each target) wrongly matched combinatorials (fakes) Background Subtraction: method

  33. mixed event event 1 event 2 Background Subtraction: method (offsets) The “mixed” background sample (fake matches and combinatorial) must reproduce the offsets of the measured events: therefore, the offsets of the single muons (from different events) selected for mixing must be replicated in the “mixed” event. (All masses)

  34. NA60 In-In NA60 In-In NA50 Pb-Pb NA50 Pb-Pb Comparison with previous results very preliminary Bjorken energy density, estimated from VENUS

  35. Specific questions that remain open • 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? L, Npart, energy density? - 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 cc feed-down on the observed J/y suppression pattern? Study the nuclear dependence of cc production in p-A collisions - Study the nuclear dependence of cc production in p-A collisions

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