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

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.