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

1. First measurement of the  spectral function in high-energy nuclear collisions Sanja Damjanovic on behalf of the NA60 Collaboration Quark Matter 2005August 4–9, Budapest, Hungary S. Damjanovic, QM2005, 4-9 August, Budapest

2. Outline • Event sample • Data analysis • event selection • combinatorial background • fake matches • Understanding the peripheral data • Isolation of an excess in the more central data • Comparison of the excess to model predictions S. Damjanovic, QM2005, 4-9 August, Budapest

3. muon trigger and tracking magnetic field hadron absorber or ! Measuring dimuons in NA60: concept 2.5 T dipole magnet beam tracker vertex tracker targets Matching in coordinate and momentum space • Origin of muons can be accurately determined • Improved dimuon mass resolution S. Damjanovic, QM2005, 4-9 August, Budapest

4. 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, QM2005, 4-9 August, Budapest

5. Data Analysis S. Damjanovic, QM2005, 4-9 August, Budapest

6. Selection of primary vertex The interaction vertex is identified with better than 20 mm accuracy in the transverse plane and 200 mm along the beam axis. (note the log scale) Beam Trackersensors windows Present analysis (very conservative): Select events with only one vertex in the target region, i.e. eliminate all events with secondary interactions S. Damjanovic, QM2005, 4-9 August, Budapest

7. Muon track matching Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Telescope (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. A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2< 3 are selected. Fake matches are subtracted by a mixed-events technique (CB) and an overlay MC method (only for signal pairs, see below) S. Damjanovic, QM2005, 4-9 August, Budapest

8. Determination of Combinatorial Background Basic method: Event mixing talk by Ruben Shahoyan, 5b • takes account of • charge asymmetry • correlations between the two muons, induced by magnetic field sextant subdivision trigger conditions S. Damjanovic, QM2005, 4-9 August, Budapest

9. Combinatorial Background from ,K→ decays Agreement of data and mixed CB over several orders of magnitude Accuracy of agreement ~1% S. Damjanovic, QM2005, 4-9 August, Budapest

10. Fake Matches Fake matches of the combinatorial background are automatically subtracted as part of the mixed-events technique for the combinatorial background Fake matches of thesignal pairs (<10% of CB) can be obtained in two different ways: • Overlay MC (used for LMR): 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 (used for IMR): More complicated, but vital for offset analysis S. Damjanovic, QM2005, 4-9 August, Budapest

11. Example of overlay MC: the  Fake-match contribution localized in mass (and pT) space  = 23 MeV fake = 110 MeV S. Damjanovic, QM2005, 4-9 August, Budapest

12. Subtraction of combinatorial background and fake matches Real data ! Net data sample: 360 000 events Fakes / CB < 10 % w f For the first time,  and  peaks clearly visible in dilepton channel h Mass resolution:23 MeV at the  position μμ channel also seen S. Damjanovic, QM2005, 4-9 August, Budapest

13. Associated track multiplicity distribution Track multiplicity from VT tracks for triggered dimuons, shown separately for opposite-sign pairs, combinatorial background and signal pairs after subtraction of total background (including fakes). Four multiplicity windows used in the further analysis: S. Damjanovic, QM2005, 4-9 August, Budapest

14. Signal and background in 4 multiplicity windows S/B Decrease of S/B with centrality, as expected S. Damjanovic, QM2005, 4-9 August, Budapest

15. 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, QM2005, 4-9 August, Budapest

16. Results S. Damjanovic, QM2005, 4-9 August, Budapest

17. 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) Fit range: up to 1.4 GeV S. Damjanovic, QM2005, 4-9 August, Budapest

18. all pT log Very good fit quality Comparison of hadron decay cocktail to data S. Damjanovic, QM2005, 4-9 August, Budapest

19. pT < 0.5 GeV Comparison of hadron decay cocktail to data The  region (small M, small pT) is remarkably well described → the (lower) acceptance of NA60 in this region is well under control S. Damjanovic, QM2005, 4-9 August, Budapest

20. Particle ratios from the cocktail fits • h/w and f/w nearly • independent of pT; • 10% variation due to • the w • increase of r/w • 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, QM2005, 4-9 August, Budapest

21. Isolation of an excess in the more central data S. Damjanovic, QM2005, 4-9 August, Budapest

22. 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, QM2005, 4-9 August, Budapest

23. Conservative approach Useparticle yields so as to set a lowerlimit to a possible excess S. Damjanovic, QM2005, 4-9 August, Budapest

24. 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, QM2005, 4-9 August, Budapest

25. 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 spectrumrobust tomistakes even at the 10% level;consequences highly localized S. Damjanovic, QM2005, 4-9 August, Budapest

26. 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, QM2005, 4-9 August, Budapest

27. Systematics Illustration of sensitivity to correct subtraction of combinatorial background and fake matches;to variation of the  yield Structure in  region completely robust Level of underlying continuum more sensitive S. Damjanovic, QM2005, 4-9 August, Budapest

28. Enhancement relative to cocktail  use mass range 0.2–0.9 GeV to normalize to  Total data,no DD subtracted Errors are systematic, statistical errors are negligible faster than linear rise with centrality, steeper for low pT S. Damjanovic, QM2005, 4-9 August, Budapest

29. Comparison of excess to model predictions S. Damjanovic, QM2005, 4-9 August, Budapest

30. 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, QM2005, 4-9 August, Budapest

31. Comparison of data to RW, BR and Vacuum  same conclusions pT dependence S. Damjanovic, QM2005, 4-9 August, Budapest

32. Understanding the spectral shape Dilepton rate Example: thermal radiation based on white spectral function propagate this through NA60 acceptance:no structure !recover white spectrum ! integrate over space-time and momenta By pure chance, for all pT and the slope of the pT spectra of the direct radiation, the NA60 acceptance compensates for the phase space factors and “extracts” the<spectral function> S. Damjanovic, QM2005, 4-9 August, Budapest

33. Comparison of data to RW, BR and Vacuum  Data and model predictions as shown (propagated through the NA60 detector) roughly representthe respective spectral functions, averaged over space-time and momenta. S. Damjanovic, QM2005, 4-9 August, Budapest

34. Conclusions • pion annihilation is a major contribution to the lepton pair excess in heavy-ion collisions • no mass shift of the intermediate  contrary to Brown / Rho scaling • broadening of the intermediate , consistent with Rapp / Wambach S. Damjanovic, QM2005, 4-9 August, Budapest