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
Low mass excess in Pb-Au Collisions well established by CERES (e+e-) - Modifications of spectral function in nuclear collisions
- Cannot distinguish different scenarios
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 where a muon track from the Muon Spectrometer is matched to the wrong track from 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
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; 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
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
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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