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First measurement of the  spectral function in high-energy nuclear collisions

First measurement of the  spectral function in high-energy nuclear collisions. Sanja Damjanovic NA60 Collaboration. Villasimius, Sardinia, 18 May 2006. Outline. Experimental set-up Data analysis Understanding the peripheral data Isolation of an excess in the more central data

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First measurement of the  spectral function in high-energy nuclear collisions

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  1. First measurement of the  spectral function in high-energy nuclear collisions Sanja Damjanovic NA60 Collaboration Villasimius, Sardinia, 18 May 2006 S. Damjanovic, Hot Quarks 2006

  2. Outline • Experimental set-up • Data analysis • Understanding the peripheral data • Isolation of an excess in the more central data • Comparison of the excess to model predictions • Conclusions S. Damjanovic, Hot Quarks 2006

  3. Event sample: Indium-Indium 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 S. Damjanovic, Hot Quarks 2006

  4. Subtraction of combinatorial background and fakes Net data sample: 360 000 events Fakes / CB < 10 % For the first time,  and  peaks clearly visible in dilepton channel ; even μμ seen w f h Mass resolution:23 MeV at the  position Progress over CERES: statistics: factor >1000resolution: factor 2-3 S. Damjanovic, Hot Quarks 2006

  5. Phase space coverage in mass-pT plane Final data after subtraction of combinatorial background and fake matches MC The acceptance of NA60 extends (in contrast to NA38/50) all the way down to small mass and small pT S. Damjanovic, Hot Quarks 2006

  6. Associated track multiplicity distribution Track multiplicity from VT tracks for triggered dimuons for opposite-sign pairscombinatorial backgroundsignal pairs 4 multiplicity windows: S. Damjanovic, Hot Quarks 2006

  7. Results S. Damjanovic, Hot Quarks 2006

  8. Understanding the Peripheral data Fit hadron decay cocktail and DD to the data 5 free parameters to be fit: h/w, r/w, f/w, DD, overall normalization (h/h = 0.12, fixed) do the fits for all pT and three bins in pT Extrapolatefit parameters to full phase space (using particle generator “Genesis”) S. Damjanovic, Hot Quarks 2006

  9. all pT log Very good fit quality Comparison of hadron decay cocktail to data S. Damjanovic, Hot Quarks 2006

  10. Comparison of hadron decay cocktail to data pT < 0.5 GeV The  region (small M, small pT) is remarkably well described → the (lower) acceptance of NA60 in this region is well under control S. Damjanovic, Hot Quarks 2006

  11. Particle ratios from the cocktail fits • h/w and f/w nearly • independent of pT; • 10% variation due to • the w • enhanced r/w, mostly • at low pT (due to ππ • annihilation, see later) • General conclusion: • peripheral bin very well described in terms of known sources • low M and low pT acceptance of NA60 under control S. Damjanovic, Hot Quarks 2006

  12. Isolation of an excess in the more central data S. Damjanovic, Hot Quarks 2006

  13. Understanding the cocktailfor the more central data Need to fix the contributions from the hadron decay cocktail Cocktail parameters from peripheral data? How to fit in the presence of an unknown source?  Nearly understood from high pT data, but not yet used Goal of the present analysis: Find excess above cocktail (if it exists) without fits S. Damjanovic, Hot Quarks 2006

  14. Conservative approach Useparticle yields so as to set a lowerlimit to a possible excess S. Damjanovic, Hot Quarks 2006

  15. Comparison of data to “conservative” cocktail all pT Cocktail definition: see next slide / fixed to 1.2 ● data -- sum of cocktail sources including the  Clear excess of data above cocktail, rising with centrality But: how to recognize the spectral shape of the excess? S. Damjanovic, Hot Quarks 2006

  16. Isolate possible excess by subtracting cocktail (without ) from the data  :set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV  leads to a lower limit for the excess at very low mass  andf : fix yields such as to get, after subtraction, a smooth underlying continuum difference spectrum robust to mistakes even on the 10% level, since the consequences of such mistakes are highly localized. S. Damjanovic, Hot Quarks 2006

  17. Excess spectra from difference: data - cocktail all pT No cocktail and no DD subtracted Clear excess above the cocktail , centered at the nominal  pole andrising with centrality Similar behaviour in the other pT bins S. Damjanovic, Hot Quarks 2006

  18. Enhancement relative to cocktail  use mass range 0.2<m<0.9 GeV to normalize to  Total data, no DD subtracted Non-linear rise with centrality, steeper for low pT S. Damjanovic, Hot Quarks 2006

  19. Systematics Illustration of sensitivity to correct subtraction of combinatorial background and fake matches;to variation of the  yield Systematic errors of continuum 0.4<M<0.6 and 0.8<M<1GeV 25% Structure in  region completely robust S. Damjanovic, Hot Quarks 2006

  20. Comparison of excess to model predictions S. Damjanovic, Hot Quarks 2006

  21. Acceptance filtering of theoretical prediction all pT Input (example): thermal radiation based on RW spectral function Output:spectral shape much distorted relative to input, but somehow reminiscent of the spectral function underlying the input; by chance? S. Damjanovic, Hot Quarks 2006

  22. Understanding the spectral shape at the output all pT Input: thermal radiation based on white spectral function Output: white spectrum ! By pure chance, for all pT and the slope of the pT spectra of the direct radiation, the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the <spectral function> S. Damjanovic, Hot Quarks 2006

  23. Comparison of data to RW, BR and Vacuum  Predictions for In-In by Rapp et al (2003) for 〈dNch/d〉 = 140, covering all scenarios Theoretical yields, folded with acceptance of NA60 and normalized to data in mass interval < 0.9 GeV Only broadening of (RW) observed, no mass shift (BR) S. Damjanovic, Hot Quarks 2006

  24. Comparison of data to RW, BR and Vacuum  same conclusions pT dependence S. Damjanovic, Hot Quarks 2006

  25. New theoretical developments since QM05 Brown and Rho, comments on BR scaling, nucl-th/0509001Brown and Rho, formal aspects of BR scaling, nucl-th/0509002 Rapp and van Hees, parameter variations for 2p, unpublished Rapp and van Hees, 4p, 6p… processes , hep-ph/0603084 Rapp and van Hees, 4p, 6p… processes , hep-ph/0604269 Renk and Ruppert, finite T broadening, Phys. Rev. C71 (2005) Renk and Ruppert, finite T broadening and NA60, hep-ph/0603110 Renk, Ruppert, Müller, BR scaling and QCD Sum Rules, hep-ph/0509134 Renk, Ruppert, Müller, theoretical thoughts on NA60, unpublished Skokov and Toneev, BR scaling and NA60, Phys. Rev. C73 (2006) Dusling and Zahed, Chiral virial approach and NA60, nucl-th/0604071 Bratkovskaya and Cassing, HSD and NA60, in progress S. Damjanovic, Hot Quarks 2006

  26. Brown/Rho scaling ? S. Damjanovic, Hot Quarks 2006

  27. Dropping Mass (DM) vs Rapp/Wambach Modification of DM by • change of the fireball parameters Results of Rapp (2/2006):(now in absolute terms and propagated through the NA60 acceptance filter) even switching out all temperature effects does not lead to agreement between DM and the data S. Damjanovic, Hot Quarks 2006

  28. Dropping Mass (DM) vs Rapp/Wambach Still same conclusions in all pT windows (χ2 at low pT !) loss of sensitivity at low pT because of acceptance cut S. Damjanovic, Hot Quarks 2006

  29. Chiral Virial Approach Dusling/Zahed First attempt to describe the centrality dependence of the excess data. Reasonable description, but increasing overestimate of central  peak S. Damjanovic, Hot Quarks 2006

  30. Is there still more in the data to help understanding the origin of the broadening? Shape analysis of excess mass spectra S. Damjanovic, Hot Quarks 2006

  31. Excess mass spectra in 12 centrality windows S. Damjanovic, Hot Quarks 2006

  32. Shape vs. centrality nontrivial changes of all three variables at dNch/dy>100? r 3/2(L+U) “continuum” R=C-1/2(L+U) “peak” RR peak/continuum S. Damjanovic, Hot Quarks 2006

  33. RMS of total excess r Consistency with shape analysis Further rise starting at dNch/dy =100 significant! (bad fit (c2=3) for linear rise above dNch/dy=30) S. Damjanovic, Hot Quarks 2006

  34. Conclusions (I) : data • pion annihilation seems to be a major contribution to the lepton pair excess in heavy-ion collisions at SPS energies • no significant mass shift of the intermediate  • only broadening of the intermediate  S. Damjanovic, Hot Quarks 2006

  35. Conclusions (II) : interpretation • all models predicting strong mass shifts of the intermediate r,including Brown/Rho scaling, are not confirmed by the data • models predicting strong broadening roughly verified; not completely clear whether broadening due to T or baryon density • theoretical investigation on an explicit connection between broadeningand the chiral condensate clearly required S. Damjanovic, Hot Quarks 2006

  36. 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 S. Damjanovic, Hot Quarks 2006

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