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

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 2 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

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 p T spectra  Conclusions

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

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 5 Event sample  5-week long run in Oct.–Nov  Indium beam of 158 GeV/nucleon ~ 4 × 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: 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 >1000 resolution: 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 p T 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 smooth underlying continuum   : (  ) set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV/c 2 (lower limit for excess). (  ) use yield measured for p T > 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 p T 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 p T spectra  Conclusions

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

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 17 Strategy for acceptance correction  Reduce 3-dimensional acceptance correction in M-p T -y to a 2-dimensional correction in M-p T, using measured y distribution as an input. Use  for control.  Use slices of m = 0.1 GeV p T = 0.2 GeV  Check behaviour on 3 extended mass windows 0.4<M<0.6 GeV 0.6<M<0.9 GeV 1.0<M<1.4 GeV Subtract charm from the data (based on NA60 IMR results) before acceptance correction

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 18 Rapidity distribution of the excess  Agreement between the three p T bins  Results close to rapidity distribution of  ’s (from NA49 and NA60) for the same √s arb. units

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

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 20 Systematics of low-p T data: Combinatorial background  Enhanced yield at low- p T seen at all centralities, including the peripheral bin  Errors at low p T, due to subtraction of combinatorial background: Enhanced yield at low p T not due to incorrect subtraction of combinatorial background peripheral1% semiperipheral10% semicentral20% central25%

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

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

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 (p T spectra)  Strong mass dependence of p T spectra  Spectra behave oposite to expected from radial flow  p T 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 Lisbon CERN Bern Torino Yerevan Cagliari Lyon Clermont Riken Stony Brook Palaiseau Heidelberg BNL ~ 60 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. 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 The NA60 experiment

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 27 Backup

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 28 RMS of total excess  Consistency with shape analysis  Further rise starting at dN ch /dy =100  Bad fit (  2 =3) for linear rise above dN ch /dy=30)

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 29 Different model descriptions Renk/Ruppert

J. Seixas (NA60 Coll.) Quark Matter 2006, Shanghai 30

Systematics of low-p T due to the cocktail subtraction radical (unrealistic) increase of  Dal subtraction  20% decrease of yield in mass window 0.6<M<0.9 enhanced yield at low pt cannot be due to incorrect  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  Dalitz yield Systematics of low-p T data due to the cocktail subtraction