1.  Meson Production in In-In Collisions and Highlights from NA60 Michele Floris 1 for the NA60 Collaboration Strangeness in Quark Matter 2007 1 University and INFN, Cagliari, Italy

2. June 28, 2007Strangeness in Quark Matter2 Outline  The NA60 Experiment Detector Concept  Phi Meson Production in In-In Collisions Analysis details p T, y and decay angular distributions  ratio  Highlights from NA60 In medium modification of the  Intermediate mass range excess: prompt or charm? Centrality dependence of J/  suppression

3. June 28, 2007Strangeness in Quark Matter3 The NA60 Experiment Fixed target dimuon experiment at the CERN SPS Apparatus composed of 4 main detectors 17m The vertex region (2 detectors): Zero degree calorimeter (centrality measurements) Muon Spectrometer Origin of muons can be accurately determined Improved dimuon mass resolution (~20 MeV/c 2 at  instead of 80 MeV/c 2 ) Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles and momentum) to the tracks measured in the spectrometer High luminosity  experiment: possible with radiation tolerant detectors and high speed DAQ 2.5 T dipole magnet hadron absorber targets beam tracker vertex tracker muon trigger and tracking (NA50) magnetic field >10m<1m

4. June 28, 2007Strangeness in Quark Matter4 Data Sample  InIn collisions at 158 AGeV Incident beam energy 5 weeks in Oct.-Nov. 2003 ~ 4 ∙ 10 12 ions delivered ~ 230 million dimuon triggers  Data analysis Select events with only one reconstructed vertex in target region (avoid re-interactions) Match muon tracks from Muon Spectrometer with charged tracks from Vertex Tracker (candidates selected using weighted distance squared  matching  2 ) Subtract Background  Two data samples Different current settings in the Muon Spectrometer magnet Different acceptances High current setting suppresses LMR

5. June 28, 2007Strangeness in Quark Matter5  Meson Production: Motivation  Strangeness production in Heavy Ion Collisions  Mass and Width changes of the  in the medium Differerences between  →  KK and  →   K meson modification in the medium  p T dependence of K suppression  Two channels have been studied in 158 AGeV PbPb :     Muons not influenced by the medium  Previous SPS measurements: NA50  Acceptance limited to high p T   KK  Better mass resolution  No physical BG  Previous SPS measurements: NA49  Broad p T coverage, but dominated by low p T   puzzles: Absence of in-matter modifications of width in KK Discrepancy between absolute yields Discrepancy between T slope: radial flow (NA49) or no radial flow (NA50)?  Measurements from NA60 in In-In collisions NA60 measures the  channel with good p T coverage (0-2.6 GeV) Rapidity, decay angular distribution, p T and  ratio

6. Spectra: Analysis Procedure We select the events on the  peak and use two side mass windows to estimate the p T,y and decay angle distribution of the continuum under the peak background total  Acceptance would require correction with 2D matrices: p T vs y and decay angle vs p T After tuning MC to data (iterative procedure)  1D acceptance correction Systematic error: variation of analysis cuts and parameters 5 centrality bins 4000 A data set only

7. June 28, 2007Strangeness in Quark Matter7 Rapidity Distribution Width estimated with a Gaussian fit Constant within errors Agreement with previous measurements in other colliding systems at the same energy  gaus All centralities  = 1.13 ± 0.06 ± 0.05 reflected  gauss

8. June 28, 2007Strangeness in Quark Matter8 Decay Angular Distributions p projectile p target z axis CS p µ+ y x Viewed from  rest frame Collins-Soper p projectile p target z axis GJ p µ+ y Viewed from  rest frame Gottfried-Jackson p projectile p target z axis Hel p µ+ x Helicity  y The angular distribution of the positive muon can be measured with respect to 3 different quantization axes 1.Collins – Soper 2.Gottfried – Jackson 3.Helicity Angular distributions fitted with the function: polarization Provides information on the production mechanism Previous measurements: ACCMOR (h-Be) and Sixel et al. (K - - p /  - - p)  Non negligible  (GJ Frame) Heavy Ion  Global polarization? The 3 frames are identical for p T  0 First measurement in HI collisions at the SPS We studied centrality and p T dependence of  in the 3 frames Further developments: azimuthal distributions, study wrt the reaction plane

9. June 28, 2007Strangeness in Quark Matter9 Helicity Distribution All centralities  = 0.1 ± 0.1 ± 0.1  = 0, independent of centrality Analysis repeated at p T 1GeV. No evidence for  ≠ 0. p T > 0.2 GeV/c

10. June 28, 2007Strangeness in Quark Matter10 Gottfried – Jackson Distribution All centralities  = 0. ± 0.1 ± 0.04  = 0, independent of centrality Analysis repeated at p T 1GeV. No evidence for  ≠ 0. p T > 0.2 GeV/c

11. June 28, 2007Strangeness in Quark Matter11 Collins – Soper Distribution All centralities  = -0.2 ± 0.2 ± 0.2 Hint for  < 0 in peripheral events? Not significant (2  ). Acceptance limits fit range  Large errors on  Analysis repeated at p T 1GeV. No evidence for  ≠ 0. p T > 0.2 GeV/c

12. June 28, 2007Strangeness in Quark Matter12 Spectra fitted with the function: m T Distribution Depends on the fit range in presence of radial flow  Effective temperature Centrality dependence stronger at low p T Linear mass dependence at low p T

13. June 28, 2007Strangeness in Quark Matter13 T slope : Fit Range Dependence NA60 In-In (p T < 1.6 GeV/c) NA49 Pb-Pb NA50 Pb-Pb NA60 In-In (p T > 1.1 GeV/c) NA49 Pb-Pb NA50 Pb-Pb Low p T (NA49): Agreement with NA49 when the fit is performed in the same range High p T (NA50): Lower T absolute values, flatter rise with centrality. No agreement with NA50  Difference between NA50 and NA49 was not due to different decay channel  Hint for the presence of radial flow → Blast Wave analysis

14. June 28, 2007Strangeness in Quark Matter14  ratio: Analysis Procedure  ratio extracted from a fit of the mass distribution with expected sources: MC Data 2-body and Dalitz decays of low mass mesons Open charm continuum (low level) Parameters allowed to vary:  and the continuum  region in central bins parameterised to reproduce the NA60 excess data. Little dependence on the parameterisation (~ 5%) p T > 1 GeV/c

15. June 28, 2007Strangeness in Quark Matter15  ratio p T > 1 GeV/c Full p T and y  yield increases from peripheral to central collision by a factor ~ 3 (Consistent with previous measurement) Absolute yield measurement in progress

16. June 28, 2007Strangeness in Quark Matter16  NA38/NA50 was able to describe the IMR dimuon spectra in p-A (Al, Cu, Ag, W) collisions at 450 GeV as the sum of Drell-Yan and Open Charm contributions.  However, the yield observed by NA50 in heavy-ion collisions (S-U, Pb-Pb) exceeds the sum of DY and Open Charm decays, extrapolated from the p-A data (factor ~2 excess for central Pb-Pb).  The study of this excess was one of the main objectives of the NA60 experiment at SPS. NA38/NA50 proton-nucleus data central collisions M (GeV/c 2 ) IMR Excess: Previous Measurements

17. June 28, 2007Strangeness in Quark Matter17 NA60 Measurement of the IMR excess NA60 can separate the prompt and open charm contribution on a statistical basis by measuring the dimuon offset with respect to the primary vertex Single muon weighted offset Dimuon weighted offset To eliminate the momentum dependence of the offset resolution, we use the muon offset weighted by the error matrix of the fit: J/  muons Offset resolution ≈ ~40  m, < c  (D + : 312  m, D o : 123  m)

18. June 28, 2007Strangeness in Quark Matter18 Sources (open charm and Drell Yan) simulated using Pythia Monte Carlo dimuons reconstructed on top of a real event IMR: Expected Sources Analysis of the mass spectra in the range 1.16 GeV/c 2 < M  < 2.56 GeV/c 2 Coverage in Collins-Sopper angle: | cos  CS | < 0.5 Analysis repeated for the 2 samples (4000 A and 6500 A) and for different cuts on the matching  2 (  2 match < 1.5 and  2 match < 3.0) Relative normalization: Drell Yan: Reproduce high mass cross section measured by NA3 and NA50 Open Charm: Cross section which reproduces the NA50 p-A dimuon data  Yield of expected sources in units of expected cross section in the following

19. Fit range 4000 A,  2 match <1.5 Fit of the mass spectra with prompts fixed to Drell-Yan (within 10%) shows that the dimuon yield in IMR is higher than expected 4000 A,  2 match <1.5 6500 A,  2 match <1.5 Fit range 6500 A,  2 match <1.5 Data integrated in collision centralities and in p T and the fit to the offset spectra shows that the excess is prompt.

20. June 28, 2007Strangeness in Quark Matter20  ~2.4 times more prompts are required than what Drell-Yan provides.  The two data sets, with different systematics, are consistent with each other  Obtained Charm contribution is lower than extrapolation from NA50 p-A data. Statistics is not enough for its study vs centrality and p T, it will be fixed to 0.7  0.15 (average of 4 and 6.5 kA data) 4000 A,  2 match < 3 6500 A,  2 match < 3 4000 A,  2 match <1.5 6500 A,  2 match <1.5 Offset fits with free prompt and charm

21. June 28, 2007Strangeness in Quark Matter21 Our statistics is not enough to study the differentially in centrality. But bulk of existing measurements is in agreement with expectation of its scaling with number of binary collisions, characteristic for hard process:  = 1 in In further analysis the normalization factor for will be fixed to 0.7  0.15 (wrt extrapolation from NA50 pA data) leading to 9.5±2  b/nucleon H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180 cc Cross Section

22. June 28, 2007Strangeness in Quark Matter22 NA60 H.Woehri and C.Lourenco, Phys.Rep. 433 (2006) 127-180 Effect of nuclear modification of PDFs  CC : comparison with other measurements

23. June 28, 2007Strangeness in Quark Matter23 The excess acceptance correction is done differentially in M and p T Assuming: flat cos  CS distribution for decay angle and rapidity distribution similar to Drell-Yan (  y ~1) Once the excess is corrected for acceptance, the two data sets can be summed up Systematic errors account for uncertainty in Drell-Yan and Charm normalization factors Excess corrected for acceptance Define the excess as Signal – [ Drell-Yan (1  ± 0.1) + Open Charm (0.7±0.15) ]

24. June 28, 2007Strangeness in Quark Matter24 Centrality and p T dependence of excess Excess/N participants (arb. scale) Excess already present in peripheral collisions, scales faster than N Part Excess stronger at low p T

25. June 28, 2007Strangeness in Quark Matter25 Fit in 0.5<P T <2 GeV/c Fit in P T <2.5 GeV/c p T Spectra of the excess T EFF is rather low compared both to the Drell-Yan and to the Low Mass Region (T ~ 250 MeV) Drell Yan

26. June 28, 2007Strangeness in Quark Matter26 Summary   Meson Production T slope increases with centrality and depends on fit range → hint for radial flow  Compatible with NA49 Absolute yields measurement in progress  IMR excess Excess is prompt Open charm yield agrees with NA50 p-A Excess is qualitatively different from Drell-Yan

27. June 28, 2007Strangeness in Quark Matter27 Lisbon CERN Bern Torino Yerevan Cagliari Lyon Clermont BNL Riken Stony Brook Palaiseau Heidelberg BNL 56 people 13 institutes 8 countries R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen, B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord, N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço, J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri The NA60 Collaboration http://na60.cern.ch/

28. BACKUP

29. June 28, 2007Strangeness in Quark Matter29  ratio p T > 1 GeV/c Full phase space Compared to NA50  : NA50 data have a common m T > 1.5 GeV/c 2 cut Extrapolated to p T > 1 GeV/c using T slope measured by NA50 Ambiguity: need to assume  to extract  (Arbitrary rescaling of NA50 data) NA60 NA50 (Arb. Rescaled)

30. June 28, 2007Strangeness in Quark Matter30 p T Distribution peripheral central Spectra fitted with the function: to extract the T slope Depends on the fit range!

31. June 28, 2007Strangeness in Quark Matter31 Acceptance would require correction with 2D matrices: p T vs y and decay angle vs p T. After tuning MC to data (iterative procedure)  1D acceptance correction Systematic error: variation of analysis cuts and parameters:  2 cut of Matched Dimuon Fake subtraction method Side windows Offset Mass window width

32. Vertex resolution (in the transverse plane) The interaction vertex is identified with a resolution of 10–20  m accuracy in the transverse plane Dispersion between beam track and VT vertex Vertex resolution (deconvoluting  BT =20  m) 10 20 30 0  (  m) Number of tracks Beam Tracker measurement vs. vertex reconstructed with Vertex Tracker BT The BT measurement (  = 20  m at the target) allows us to control the vertexing resolution and systematics

33. June 28, 2007Strangeness in Quark Matter33  Charm and Drell-Yan contributions are obtained by overlaying real event data on dimuons generated by Pythia 6.326 (CTEQ6L PDFs with EKS98 nuclear modifications. m c =1.5 GeV/c 2. k T =0.8 for Drell-Yan and 1 for Charm) The fake matches in the MC events are subtracted as in the real data, by event mixing.  Relative normalizations:  Drell-Yan: K-factor of 1.9. Reproduces In-In data at M>4 GeV/c 2 and cross sections measured by NA3 and NA50 ( J. Badier et al. (NA3 Coll.), Z.Phys.,C26: (1985) 489. M.C. Abreu et al. (NA50 Coll.), Phys. Lett. B410 (1997) 337).  Charm: = 13.6  b/nucleon. Obtained from the cross section describing NA50 p-A dimuon data at 450 GeV by its rescaling to 158 GeV using Pythia. Note: this is factor ~2 higher than the extrapolation from the “world average” cross section (H.Woehri and C.Lourenco, J.Phys. G30 (2004) 315) Possible explanation: both NA60 and NA50 detect dimuons only in |cos  |<0.5, while shows very strong rise at large cos   Our full phase space acceptance for charm is very sensitive to the correctness of kinematic distribution from Pythia NA60 Signal Analysis: simulated sources

34. June 28, 2007Strangeness in Quark Matter34 Absolute normalization: the expected Drell-Yan contribution, as a function of the collision centrality, is obtained from the number of observed J/  events and the  suppression pattern:  Data is split in 12 bins in collision centrality (number of participants obtained from the measured charged multiplicity in the Vertex Tracker).  In each bin the number of J/  events is extracted and corrected for the anomalous suppression (E.Scomparin, proceedings of Quark Matter 2006, Shanghai)  Expected number of events Drell-Yan events at 2.9<M<4.1 GeV/c 2 is extracted from  DY accounting for the nuclear absorption of the J/  A 10% systematical error (mostly due to the uncertainty of the J/  nuclear absorption cross section) is assigned to this normalization. Signal shapes used to fit the dimuon weighted offset distributions are: prompt : mixture of J/  and  data (open charm contamination is < 1%) charm: Monte Carlo smeared by amount needed for J/  and  MC to reproduce data The fits to mass and weighted offset spectra are reported in terms of the DY and Open Charm scaling factors needed to describe the data NA60 Signal analysis: simulated sources

35. June 28, 2007Strangeness in Quark Matter35 A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with  2 < 3 are selected (standard analysis). Fake matches are subtracted by a mixed-events technique and an overlay MC method (only for signal pairs, see below) Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Tracker (VT) is done using the weighted distance (  2 ) in slopes and inverse momenta. For each candidate a global fit through the MS and VT is performed, to improve kinematics. Muon track matching

36. June 28, 2007Strangeness in Quark Matter36 Combinatorial Background CB (uncorrelated muon pairs coming from  and K decays) is estimated with an Event Mixing technique Take muons from different events and calculate their invariant mass. Takes account of charge asymmetry, correlations between the two muons (induced by magnetic field sextant subdivision: detector geometry), trigger conditions Apparatus triggers both opposite sign (     ) and like sign (         ) pairs. Quality of CB is assessed comparing LS spectra. Accuracy ~1% over several orders of magnitude! Fakes in CB also subtracted!

37. June 28, 2007Strangeness in Quark Matter37 Fake Matches  “Fake Matches” are those tracks where a muon track from the Muon Spectrometer is matched to the wrong track from the Vertex Tracker  Fake matches of the signal pairs (<10% of CB) can be obtained in two different ways: Overlay MC Superimpose MC signal dimuons onto real events. Reconstruct and flag fake matches. Choose MC input such as to reproduce the data. Start with hadron decay cocktail + continuum; improve by iteration. Event mixing More rigorous, but more complicated. Less statistics hadron absorber muon trigger and tracking target fake correct

38. June 28, 2007Strangeness in Quark Matter38   = 23 MeV  fake = 110 MeV Example of Overlay MC: the  Fakes calculation with Overlay MC and Mixing method agree in absolute level and shape within 5%!

39. June 28, 2007Strangeness in Quark Matter39 Fakes/CB < 10 % For the first time  and  peaks clearly visible in dilepton channel (23 MeV mass resolution at the   also visible    Clean Spectrum