Results from the NA50 experiment on J/? suppression and charged particle pseudorapidity distributions in Pb-Pb collisions

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Results from the NA50 experimenton J/?
suppression and charged particle pseudorapidity
distributions in Pb-Pb collisions

• NA50 Collaboration
• M.Monteno INFN, Torino (Italy)

XXXII International Symposium on Multiparticle
Dynamics September 7-13, 2002 Alushta, Crimea
(Ukraine)
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The NA50 Collaboration
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Outline of the presentation

• Introduction
• Goals and tools of the year 2000 Pb-Pb run
• Determination of normal nuclear absorption from
  new p-A data
• The standard analysis method s(J/?) /
  s(Drell-Yan)
• Published NA50 results on J/? suppression
• Results on J/? suppression from the year 2000
  Pb-Pb run
• PRELIMINARY s(J/?) / s(DY)2.9-4.5 GeV/c2 vs.
  transverse energy ET
• PRELIMINARY s(J/?) / s(DY)2.9-4.5 GeV/c2 vs.
  forward energy EZDC
• PRELIMINARY s(J/?) / s(DY)4.2-7.0 GeV/c2 vs.
  transverse energy ET
• Comparison with published results
• Results on dNch/d? distributions
• Measurements at two different Pb beam energies
  40 GeV/c and 158 GeV/c
• Analysis in bins of centrality
• Study of scaling behaviour versus Npart and
  versus energy
• Summary

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NA50 the experimental setup

• Pb beam
• 1998, 2000 158 GeV/nucleon
• 1999 40 GeV/nucleon

• Triggers
• DIMUON 2 muon tracks
• MIN.BIAS Non-zero energy deposit in the ZDC

• p beam
• 450 GeV/nucleon

• Centrality detectors
• E.M. Calorimeter (1.1lt?lt2.3)
• Zero Degree Calorimeter (?gt6.3)
• Multiplicity Detector (1.9lt?lt4.2)

• Muon spectrometer (2.7lt?lt3.9)
• MagnetMWPChodoscopes

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NA50 the Multiplicity Detector and the target
region
Silicon microstrip detector measuring the number
and the angular distribution of charged particles
produced in the collision

• Two planes (MD1, MD2)
• 36 azim.sectors (?F10?)
• 192 radial strips (??0.02)
• 6912 strips in each plane

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NA50 published results on J/? suppression

• Two different analysis methods
• Standard analysis 1) Drell-Yan used as a
  reference 2) fit of dimuon invariant mass
  spectra
• Minimum bias analysis Min.Bias events used as
  a reference
• Results
• Threshold effect at ET ? 40 GeV
• No saturation observed for most central
  collisions
• Limitations
• Analysis of peripheral Pb-Pb collisions
  possibly limited by Pb-air contamination (are
  they really compatible with collisions observed
  in lighter systems?)
• Comparison with ordinary nuclear absorption
  also limited by the low statistics of NA38 p-A
  and S-U data

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Goals and tools of the 2000 run

• GOALS
• Investigate peripheral interactions in improved
  experimental conditions
• Check behaviour of the anomalous J/?
  suppression against normal nuclear
  absorption, as determined by more accurate
  (high statistics) p-A data, collected by NA50
  with the same set-up
• TOOLS
• Target region put under vacuum, up to the
  pre-absorber
• Improved beam cleaning cuts
• New vertex recostruction method based on the
  Multiplicity Detector

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Rejection of pile-up and upstream interactions

• Beam cleaning cuts
• Rejected parasitic interactions of incident ion
  in Beam Hodoscope (33 m upstream from the target)
• Rejected double interactions by means of
  temporal analysis of signal in E.M. calorimeter

• Residual pile-up and upstream interactions
• Rejection based on a diagonal band cut in the
  ET-EZDC correlation plot

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Primary vertex reconstruction with the
Multiplicity Detector

• In the past, primary interaction vertex
  reconstruction was based on a system of quartz
  blades, located downstream of each sub-target.
  The efficiency of this method was low for
  peripheral collisions.

• A new method has been developed,
• based on the data recorded by the MD.
• Hits from MD1 and MD2 are combined, under
  different hypotheses on vertex position ?
  tracklets
• Tracklets are counted to calculate the
  likelihood of different vertex positions,
  measured by a statistical estimator.
• The largest estimator determines the best
  estimate for vertex position (if above a given
  threshold)

• The method works for ET gt 3 GeV.
• It has full efficiency for ET gt 15 GeV

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Effect of the target cuts (1)

• After the vertex reconstruction
• Selected candidate in-target events
• Event by event, a global cut rejects the muon
  tracks not pointing to
• the estimated primary vertex position

The effect of the above defined cuts are visible
in the following plots ?
For each selected dimuon, Zvertex is the Z of the
closest approach between the two
muons. Background tracks (produced far from the
target, in the absorber, and visible also in
dedicated empty target runs) are strongly
suppressed by this cuts.
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Effect of target cuts (2)
Here the effect of the same cuts is visible in
the plot of dimuon invariant-mass spectrum
In the low ET region, the contamination of
out-of-target tracks (as the ones produced in
dedicated empty target runs), for dimuons with
M lt MJ/? , is completely removed by these cuts.
For ETgt20 GeV, this kind of contamination is
already low.
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The standard J/? DY analysis
Fit to µµ- mass spectra with four contributions
(J/?, ?', Drell-Yan and Open Charm)
combinatorial background determined by fit of
µµ and µ-µ- mass spectra.
ni are free parameters in the fit
Extracted J/? and Drell-Yan yields, and
their ratio.

• Efficiencies cancel out in the ratio
• Absolute normalization (straightforward
• comparison to normal absorption)
• but (price to pay)
• Low statistics for high-mass Drell-Yan

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The Drell-Yan reference
The Drell-Yan yield is proportional to the number
of nucleon-nucleon collisions from p-p to Pb-Pb
It is a good normalization for the J/? yield
The centrality dependence of the cross section
ratio sJ/? / sDY (2.9-4.5) in Pb-Pb must be
compared with precise measurement of the same
ratio in lighter systems (p-A)
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The normal absorption of J/?
Fit to p-A and S-U data with an absorption model
à la Glauber are compatible ? simultaneous
fit with a common absorption cross section is
allowed
From new NA50 p-A data previous data
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The 2000 results analysis vs ET and EZDC

• Both analyses confirm the J/? suppression
  pattern
• Peripheral interaction agree with normal
  absorption
• There is a threshold followed by a steady
  decrease (no saturation) for the most
• central Pb-Pb collisions

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The 2000 results vs ET (with MRS43)
The analysis is affected by a systematic
uncertainty coming from the set of p.d.f. used
for calculation of DY cross section in the mass
range 2.9-4.5 GeV/c2
In order to estimate this effect, the analysis
of the 2000 data vs ET has been done also with
the set MRS43.
The same pattern vs. ET is observed, as in the
analysis with GRV LO, but slightly different
absolute values of the cross-section ratio.
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The 2000 results J/? / DY(4.2 7.0)

• sDY(2.9-4.5) depends on the extrapolation of
  dNDY/dM from a mass region
• where Drell-Yan is directly measured (without
  background)
• ? Different p.d.f. lead to different results
• If directly measured DY(4.2-7.0) used as a
  reference ? unique result !!

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dN/d? distributions vs centrality at 158 GeV/c
ET centrality selection
EZDC centrality selection

• Centrality intervals defined in terms of
  fraction of inelastic cross section, as
• determined from bands in dN/dET or dN/dEZDC
  (Min. Bias) distributions.
• Distributions fitted with Gaussians, to
  extract

• dNch/d? at the peak
• Gaussian width

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Scaling of dN/d? max and sGauss vs
centrality at 158 GeV/c
EZDC centrality selection
ET centrality selection
dN/d? at the peak scales linearly both with ET
and EZDC
Gaussian width decreases with centrality
(stopping power effect)
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dN/d? distributions vs centrality at 40 GeV/c
Nch density? 2 times smaller than at 158 GeV/c
Width smaller than at 158 GeV/c
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Charged particle scaling vs Npart
Fit with a power-law
ET 158 GeV/c EZDC 158 GeV/c
(with Npart estimated in the framework of the
Glauber model)
Results of the fits
(158 GeV/c)
(40 GeV/c)
Within errors, same Npart dependence observed at
158 and 40 GeV/c.
ET 40 GeV/c
Nearly linear scaling with Npart (as in WNM)
indicates dominance of soft processes in
particle production at the SPS energies
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Charged particle scaling vs energy
Pseudorapidity density of Nch at midrapidity, per
participant pair, as a function of c.m.s. energy
Two points from NA50 at

• Results
• The charged particle yield at
• 40 GeV/nucleon is compatible
• with the fit to data of inelastic
• interactions.
• The yield at 158 GeV/nucleon
• is more than 50 higher than any
• fit to data.
• A steep increase is observed that
• cannot be described by a simple
• energy scaling.

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Summary

• Preliminary results from the most recent NA50
  data
• confirm the J/? suppression pattern a threshold
  effect followed by
• a steady decrease for the most central Pb-Pb
  collisions
• confirm the departure from a normal nuclear
  absorption (newly
• determined from p-A and S-U data)
• indicate that the most peripheral Pb-Pb
  interactions (bgt8.5 fm)
• indeed follow the normal nuclear absorption
  pattern.

• Dedicated measurements of dN/d? distributions
  versus centrality
• showed that
• at the SPS energies, charged particles scale
  linearly with Npart,
• in agreement with the Wounded Nucleon Model
• the steep increase of charged particle
  production at midrapidity
• between 40 and 158 GeV/nucleon can not be
  described by the simple
• energy scaling observed in nucleon-nucleon
  collisions.