NA50 Experiment: J/ψ Suppression and Charged Particle Pseudorapidity Distributions in Pb-Pb Collisions

1. Results from the NA50 experimenton J/ψ 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)

2. The NA50 Collaboration

3. 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: σ(J/ψ) / σ(Drell-Yan) • Published NA50 results on J/ψ suppression • Results on J/ψ suppression from the year 2000 Pb-Pb run • PRELIMINARY σ(J/ψ) / σ(DY)[2.9-4.5 GeV/c2] vs. transverse energy ET • PRELIMINARY σ(J/ψ) / σ(DY)[2.9-4.5 GeV/c2] vs. forward energy EZDC • PRELIMINARY σ(J/ψ) / σ(DY)[4.2-7.0 GeV/c2] vs. transverse energy ET • Comparison with published results • Results on dNch/dη 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

4. 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 (1.1<η<2.3) • Zero Degree Calorimeter (η>6.3) • Multiplicity Detector (1.9<η<4.2) • Muon spectrometer (2.7<η<3.9) • Magnet+MWPC+hodoscopes

5. 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 (ΔΦ=10) • 192 radial strips (Δη=0.02) • 6912 strips in each plane

6. NA50 published results on J/ψ 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

7. Goals and tools of the 2000 run • GOALS: • Investigate peripheral interactions in improved experimental conditions • Check behaviour of the anomalous J/Ψ 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

8. 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

9. 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 > 3 GeV. • It has full efficiency forET > 15 GeV

10. 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.

11. 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 < MJ/ψ , is completely removed by these cuts. For ET>20 GeV, this kind of contamination is already low.

12. The “standard” J/ψ DY analysis Fit to μ+μ- mass spectra with four contributions (J/ψ, ψ', Drell-Yan and Open Charm) + combinatorial background determined by fit of μ+μ+ and μ-μ- mass spectra. ni are free parameters in the fit Extracted J/ψ 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

13. 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/ψ yield The centrality dependence of the cross section ratio σJ/ψ / σDY (2.9-4.5) in Pb-Pb must be compared with precise measurement of the same ratio in lighter systems (p-A)

14. The “normal” absorption of J/ψ 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:

15. The 2000 results: analysis vs ET and EZDC Both analysesconfirm the J/ψ 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

16. 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.

17. The 2000 results: J/ψ / DY(4.2 – 7.0) • σDY(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 !!

18. dN/dη 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/dη at the peak • Gaussian width

19. Scaling of dN/dη |max and σGauss vs centrality at 158 GeV/c EZDC centrality selection ET centrality selection dN/dη at the peak scales linearly both with ET and EZDC Gaussian width decreases with centrality (stopping power effect)

20. dN/dη distributions vs centrality at 40 GeV/c Nch density 2 times smaller than at158 GeV/c Width smaller than at 158 GeV/c

21. 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

22. 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.

23. Summary • Preliminary results from the most recent NA50 data: • confirm the J/ψ 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 (b>8.5 fm) indeed follow the normal nuclear absorption pattern. • Dedicated measurements of dN/dη 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.