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NA60 results on the  spectral function in Indium-Indium collisions

NA60 results on the  spectral function in Indium-Indium collisions. J. Seixas NA60 Collaboration. Outline. Event sample Isolation of excess dimuons above hadron decays Mass spectra Shape analysis of mass spectra Comparison to theory Acceptance-corrected p T spectra Conclusions.

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NA60 results on the  spectral function in Indium-Indium collisions

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  1. NA60 results on the  spectral function in Indium-Indium collisions J. Seixas NA60 Collaboration Quark Matter 2006 November 14-20, Shanghai

  2. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  3. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  4. 2.5 T dipole magnet beam tracker vertex tracker targets hadron absorber Muon Other or ! Measuring dimuons in NA60: concept Muon trigger and tracking NA50 spectrometer Iron wall ACM magnet 40 cm >10 m Matching in coordinate and momentum space prompt • Origin of muons can be accurately determined • Improved dimuon mass resolution displaced J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  5. Event sample • 5-week long run in Oct.–Nov. 2003 • Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape • Present analysis: ~1/2 of total data J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  6. Main steps of the data analysis • Reconstruction of the event vertex within the segmented target • Matching of tracks from muon spectrometer and silicon vertex telescope • Assessment of fake matches (incorrect match between tracks in the muon spectrometer and silicon vertex telescope)by overlay MC and/or event mixing: agreement ~5% • Assessment of combinatorial background (pairing of uncorrelated muon pairs from  and K decays) by event mixing: accuracy ~1% J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  7. Mass spectrum • Net data sample: 360 000 events • Fakes / CB < 10 % • ω and  peaks clearly visible in dilepton channel; even μμ seen • Mass resolution:23 MeV at the  position • Progress over CERES: statistics: factor >1000resolution: factor 2-3 ω   J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  8. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Comparison to theoretical models J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  9. Isolation of the excess • and  : fix yields such as to get, after subtraction, a smoothunderlying continuum •  : ()set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV/c2 (lower limit for excess). () use yield measured for pT > 1.4 GeV/c difference spectrum robust to mistakes on the 10% level, since the consequences of such mistakes are Highly localized. J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  10. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  11. Excess mass spectra for 12 centrality bins Cocktail from /=1.2 Excess always centered at nominal-pole, while width increases J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  12. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  13. Continuum/ρ Peak/ρ Ratio Shape modification vs. centrality Divide region around peak in 3 windows labeled L, C and U: “Peak” decreases with respect to cocktail ρ by ~2 from first to last point Change in trend for dNch/dy > 100 J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  14. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  15. Comparison to theory [talks by R. Rapp, V. Metag, G. Usai] • Predictions for In-In by Rapp et al. (2003) for <dNch/d> = 133, covering all scenarios • Data and predictions as shown, after acceptance filtering, roughly mirror the respective spectral functions, averaged over space-time and momenta. • Theoretical yields normalized to data in mass interval < 0.9 GeV/c2. Only broadening of  (RW) No mass shift (BR) observed J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  16. Outline • Event sample • Isolation of excess dimuons above hadron decays • Mass spectra • Shape analysis of mass spectra • Comparison to theory • Acceptance-corrected pT spectra • Conclusions J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  17. Strategy for acceptance correction • Reduce 3-dimensional acceptance correction in M-pT-y to a 2-dimensional correction in M-pT, using measured y distribution as an input. Use  for control. • Use slices of m = 0.1 GeV pT = 0.2 GeV • Check behaviour on 3 extended mass windows Subtract charm from the data (based on NA60 IMR results) before acceptance correction J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  18. arb. units Rapidity distribution of the excess • Agreement betweenthe three pT bins • Results close to rapidity distribution of ’s (from NA49 and NA60) for the same √s J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  19. Excess pT spectra: 3 centrality bins Hardly any centrality dependence BUT Significant mass dependence J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  20. Systematics of low-pT data: Combinatorial background • Enhanced yield at low-pT seen at all centralities, including the peripheral bin • Errors at low pT, due to subtraction of combinatorial background: Enhanced yield at low pT not due to incorrect subtraction of combinatorial background J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  21. Excess pT spectra: mass dependence • Since we do not observe any centrality dependence, we sum over centralities and keep dependence in mass • Differential fits to pT spectra: assume locally 1-par. mT scaling (~exp(-mT/Teff)) and use gliding windows of pT=0.8 GeV; get Teff for each bin J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  22. low Teff @ low pT Trend opposite to what expected from radial flow (see ) • -like region hardest • high-mass region softest Different sources in different mass regions Hadronic or partonic? Excess pT spectra: mass dependence Systematic errors: < 15 MeV J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  23. Conclusions (Mass spectra) • Pion annihilation seems to be a major contributor to the lepton pair excess at SPS energies • Strong broadening, but no significant mass shift of the intermediate  J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  24. Conclusions (pT spectra) • Strong mass dependence of pT spectra • Spectra behave oposite to expected from radial flow • pT spectra could serve as a handle to disentangle partonic from hadronic sources (breaking parton-hadron duality) J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  25. CERN Heidelberg Bern Palaiseau BNL Riken Yerevan Stony Brook Torino Lisbon Cagliari Clermont Lyon The NA60 experiment http://cern.ch/na60 ~ 60 people 13 institutes8 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. Damjanović, 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, P. Pillot,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  26. Backup J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  27. RMS of total excess • Consistency with shape analysis • Further rise starting atdNch/dy =100 • Bad fit (2=3) for linear rise abovedNch/dy=30) J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  28. J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai

  29. Systematics of low-pT due to the cocktail subtraction radical (unrealistic) increase of w+wDal subtraction  20% decrease of yield in mass window 0.6<M<0.9 enhanced yield at low pt cannot be due toincorrect w subtraction

  30. Systematics of low-pT data due to the cocktail subtraction Fit:0-0.4 GeV Teff=82 MeV Teff=97 MeV Teff=118MeV enhanced yield at low pt even after radical increase (by nearly a factor of 2) of the w+wDalitz yield

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