S. Damjanovic, QM2005, 4-9 August, Budapest1 First measurement of the  spectral function in high-energy nuclear collisions Sanja Damjanovic on behalf of the NA60 Collaboration Quark Matter 2005 August 4–9, Budapest, Hungary

S. Damjanovic, QM2005, 4-9 August, Budapest2 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, Budapest3 2.5 T dipole magnet hadron absorber Origin of muons can be accurately determined Improved dimuon mass resolution Matching in coordinate and momentum space targets beam tracker vertex tracker muon trigger and tracking magnetic field or ! Measuring dimuons in NA60: concept

S. Damjanovic, QM2005, 4-9 August, Budapest4 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 Event sample: Indium-Indium

S. Damjanovic, QM2005, 4-9 August, Budapest5 Data Analysis

S. Damjanovic, QM2005, 4-9 August, Budapest6 Selection of primary vertex Beam Tracker sensors windows The interaction vertex is identified with better than 20  m accuracy in the transverse plane and 200  m along the beam axis. (note the log scale) 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, Budapest7 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) 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.

S. Damjanovic, QM2005, 4-9 August, Budapest8 Determination of Combinatorial Background Basic method: Event mixing takes account of charge asymmetry correlations between the two muons, induced by magnetic field sextant subdivision trigger conditions talk by Ruben Shahoyan, 5b

S. Damjanovic, QM2005, 4-9 August, Budapest9 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, Budapest10 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 the signal 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, Budapest11 Example of overlay MC: the  Fake-match contribution localized in mass (and p T ) space   = 23 MeV  fake = 110 MeV

S. Damjanovic, QM2005, 4-9 August, Budapest12 Subtraction of combinatorial background and fake matches For the first time,  and  peaks clearly visible in dilepton channel Net data sample: events Mass resolution: 23 MeV at the  position  μμ channel also seen Fakes / CB < 10 % Real data !   

S. Damjanovic, QM2005, 4-9 August, Budapest13 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: Centrality binmultiplicity 〈 dN ch /dη 〉 3.8 Peripheral 4–28 17 Semi-Peripheral 28–92 63 Semi-Central 92– Central > Associated track multiplicity distribution

S. Damjanovic, QM2005, 4-9 August, Budapest14 Signal and background in 4 multiplicity windows S/B 2 1/3 1/8 1/11 Decrease of S/B with centrality, as expected

S. Damjanovic, QM2005, 4-9 August, Budapest15 Phase space coverage in mass-p T plane Final data after subtraction of combinatorial background and fake matches The acceptance of NA60 extends (in contrast to NA38/50) all the way down to small mass and small p T MC

S. Damjanovic, QM2005, 4-9 August, Budapest16 Results

S. Damjanovic, QM2005, 4-9 August, Budapest17 Understanding the Peripheral data Fit hadron decay cocktail and DD to the data 5 free parameters to be fit:   DD, overall normalization (  0.12  fixed) Fit range: up to 1.4 GeV

S. Damjanovic, QM2005, 4-9 August, Budapest18 Comparison of hadron decay cocktail to data all p T Very good fit quality log

S. Damjanovic, QM2005, 4-9 August, Budapest19 The  region (small M, small p T ) is remarkably well described Comparison of hadron decay cocktail to data → the (lower) acceptance of NA60 in this region is well under control p T < 0.5 GeV

S. Damjanovic, QM2005, 4-9 August, Budapest20 Particle ratios from the cocktail fits  and  nearly independent of p T ; 10% variation due to the  increase of  at low p T (due to ππ annihilation, see later) General conclusion:  peripheral bin very well described in terms of known sources  low M and low p T acceptance of NA60 under control

S. Damjanovic, QM2005, 4-9 August, Budapest21 Isolation of an excess in the more central data

S. Damjanovic, QM2005, 4-9 August, Budapest22 Understanding the cocktail for 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 p T 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, Budapest23 Conservative approach Use particle yields so as to set a lower limit to a possible excess

S. Damjanovic, QM2005, 4-9 August, Budapest24 ● data -- sum of cocktail sources including the  Cocktail definition: see next slide all p T Comparison of data to “conservative” cocktail Clear excess of data above cocktail, rising with centrality  fixed to 1.2 But: how to recognize the spectral shape of the excess?

S. Damjanovic, QM2005, 4-9 August, Budapest25 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  and  : fix yields such as to get, after subtraction, a smooth underlying continuum difference spectrum robust to mistakes even at the 10% level; consequences highly localized

S. Damjanovic, QM2005, 4-9 August, Budapest26 Excess spectra from difference: data - cocktail all p T Clear excess above the cocktail , centered at the nominal  pole and rising with centrality Similar behaviour in the other p T bins No cocktail  and no DD subtracted

S. Damjanovic, QM2005, 4-9 August, Budapest27 Systematics Level of underlying continuum more sensitive Illustration of sensitivity  to correct subtraction of combinatorial background and fake matches;  to variation of the  yield Structure in  region completely robust

S. Damjanovic, QM2005, 4-9 August, Budapest28 Enhancement relative to cocktail  use mass range 0.2–0.9 GeV to normalize to  Total data, no DD subtracted faster than linear rise with centrality, steeper for low p T Errors are systematic, statistical errors are negligible

S. Damjanovic, QM2005, 4-9 August, Budapest29 Comparison of excess to model predictions

S. Damjanovic, QM2005, 4-9 August, Budapest30 Predictions for In-In by Rapp et al (2003) for 〈 dN ch /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) Comparison of data to RW, BR and Vacuum 

S. Damjanovic, QM2005, 4-9 August, Budapest31 Comparison of data to RW, BR and Vacuum  p T dependence same conclusions

S. Damjanovic, QM2005, 4-9 August, Budapest32 Understanding the spectral shape Dilepton rate Example: thermal radiation based on white spectral function propagate this through NA60 acceptance: no structure ! recover white spectrum ! By pure chance, for all p T and the slope of the p T spectra of the direct radiation, the NA60 acceptance compensates for the phase space factors and “extracts” the integrate over space-time and momenta

S. Damjanovic, QM2005, 4-9 August, Budapest33 Comparison of data to RW, BR and Vacuum  Data and model predictions as shown (propagated through the NA60 detector) roughly represent the respective spectral functions, averaged over space- time and momenta.

S. Damjanovic, QM2005, 4-9 August, Budapest34 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