Low Mass Dimuon Production in Indium-Indium Collisions at the cern sps outline Physics Case



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Low Mass Dimuon Production in Indium-Indium Collisions at the CERN SPS


Outline

  • Physics Case

    • Study of meson spectral function in nuclear collisions
  • The NA60 Experiment

    • Detector strategy and apparatus
    • Event sample
  • Data Analysis

    • Event selection
    • Combinatorial background
    • Fake matches
  • Results

    • Understanding the peripheral data
    • Isolation of an excess in the more central data
    • Comparison of the excess to model predictions


Previous Observations

  • Low mass excess in Pb-Au Collisions well established by CERES (e+e-)

    • Modifications of  spectral function in nuclear collisions
    • Cannot distinguish different scenarios
      • Dropping mass
      • Broadening
  • Needed: clear discrimination between the various theoretical explanations

    • Good statistics AND mass resolution
    • Good signal / background


The NA60 Experiment



Event Sample: Indium-Indium

  • 5-week long run in Oct.–Nov. 2003

    • Indium beam of 158 GeV/nucleon
    • ~ 4 × 1012 ions delivered
    • ~ 230 million dimuon
    • triggers on tape
  • Several physics results from the

  • three dimuon mass regions

  • (different physics topics)

  • Present analysis: LMR, ~1/2 of total data

  • Data treatment

    • Select events with only one reconstructed vertex in target region (avoid re-interactions)
    • Match muon tracks from Muon Spectrometer with charged tracks from Vertex Tracker
    • Subtract Combinatorial Background
    • Subtract Fake Matches
  • Analysis in 4 centrality bins



Combinatorial Background



Fake Matches

  • “Fake Matches” are those tracks

  • where a muon track from the

  • Muon Spectrometer is matched

  • to the wrong track from

  • the Vertex Tracker

  • Fake matches of the signal pairs (<10% of CB) can be obtained in two different ways:

    • Overlay MC (used for this analysis)
    • 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 intermediate mass region analysis)
      • More rigorous, but more complicated.


Example of Overlay MC: the 



Subtraction of CB and Fake Matches



Signal and BG in 4 Multiplicity Windows



Phase Space Coverage in [m,pT] Plane



Analysis Strategy

  • Understanding the peripheral data

    • Fit peripheral mass spectrum with expected sources
    • Extract particle yields
    • Is acceptance under control?
  • Study of the excess

    • How to study more central data
    • Isolation of excess by subtraction of known sources
    • Comparison to theoretical models


Understanding the Peripheral Data

  •  2-body decays,

  • ’ 3-body decays;

  • open charm

  • 5 free parameters to be fitted

    • , DD, overall normalization
    • ( = 0.12, fixed)
    • Fit range: up to 1.4 GeV
  • 3 pT bins

  • Fit Results:

    •  and  nearly independent of pT;
    • 10% variation due to the 
    • Enhanced stronger at low pT
    • (due to  annihilation, see later)
  • Peripheral bin very well described in

  • terms of known sources



Analysis of Central Data



Isolating the Excess



Excess Spectra from Difference



Systematics

  • Illustration of sensitivity

    • to correct subtraction of combinatorial background and fake matches;
    • to variation of the  yield


Comparison to Model Predictions



Comparison of Data to RW(2+4+QGP)



Comparison of Data to RW(2+4+QGP)



Comparison to RR



Comparison to RR



Conclusions

  • NA60 results on Low Mass Dilepton production:

    • Lepton pair excess at SPS energies confirmed and studied in detail
      • High statistics
      • Centrality dependence
      • pT dependence
    • Models predicting strong mass shift of the intermediate  not confirmed
    • Models predicting strong broadening are in reasonable agreement with data


The NA60 Collaboration





Vector – Axial Vector Mixing







Associated track multiplicity distribution









Selection of primary vertex



Muon track matching



Combinatorial Background

  • CB (uncorrelated muon pairs coming from  and K decays) is estimated with an Event Mixing technique

    • Take muons from different events and calculate their invariant mass
    • takes account of
      • charge asymmetry
      • correlations between the two muons, induced by
        • magnetic field
        • sextant subdivision (detector geometry)
      • trigger conditions
  • Apparatus triggers both opposite sign (

  • and like sign () pairs.

  • Quality of CB is assessed comparing

  • LS spectra

    • Accuracy ~1%
    • over several
    • orders of magnitude!


Enhancement relative to cocktail 



Fake Matches

  • “Fake Matches” are those tracks

  • where a muon track from the

  • Muon Spectrometer is matched

  • to the wrong track from

  • the Vertex Tracker

  • They are present in both the CB and in the Signal.

  • 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 this analysis)
    • 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 intermediate mass region analysis)
      • More rigorous, but more complicated.




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