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Charged particle pseudorapidity distributions in nucleus-nucleus collisions from SPS to LHC

Charged particle pseudorapidity distributions in nucleus-nucleus collisions from SPS to LHC. XLIV International Winter Meeting on Nuclear Physics Bormio, January 31st, 2006. Francesco Prino INFN – Sezione di Torino. OUTLINE: Physics motivation Experimental requirements and techniques

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Charged particle pseudorapidity distributions in nucleus-nucleus collisions from SPS to LHC

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  1. Charged particle pseudorapidity distributions in nucleus-nucleuscollisions from SPS to LHC XLIV International Winter Meeting on Nuclear Physics Bormio, January 31st, 2006 Francesco Prino INFN – Sezione di Torino OUTLINE: Physics motivation Experimental requirements and techniques Experimental results from SPS and RHIC Perspectives for ALICE at the LHC

  2. Introduction:Physics motivation

  3. Particle production in nuclear collisions • Particle multiplicity in nucleus-nucleus collisions (= number of particles produced in the collision) = global observable carrying important information about: • How initial energy available is redistributed for producing particles in the final state • Entropy of the system created in the collision • Initial energy density, parton density in the initial state • Centrality of the collision • Nucleus-nucleus collisions described in multicollision models as a superposition of elementary (nucleon-nucleon) collisions • Underlying dynamics of the particle production mechanism • Hard processes • Large momentum transfer • Small distance • Interactions at partonic level • Governed by perturbative QCD • Scale like the number of elementary collisions (Ncoll) • Soft processes • Small momentum transfer • Large distance • Described by phenomenological non-perturbative models • Scale like the number of participant nucleons (Npart)

  4. Evaluation of Npart and Ncoll r0 (Pb)= 0.16 fm-3 C (Pb)= 0.549 fm • Glauber model calculations: • Physical inputs: • Woods-Saxon density for colliding nuclei • Nucleon-nucleon inelastic cross-section inel • Numerical calculation of: • Interaction probability, Npart , Nspect, Ncoll ... vs. impact parameter b r0 (Pb)= 6.624 fm

  5. Key measurements • Scaling of particle multiplicity vs. energy • Change the energy available for particle production • Change the number of collision per participant • Handle for changing the balance of particle production between soft and hard processes • Scaling of particle multiplicity vs. centrality of the collision • Change the volume of particle production region ( Npart) • Handle for changing the system size • Change the number of collision per participant • Second handle for changing the balance between soft and hard processes • Scaling of particle multiplicity for different colliding nuclei • Second handle to change the system size

  6. Rapidity variable • Lorentz invariant • Pseudorapidity variable • h≈y for large momenta • h more easily accessed experimentally Particle momentum distributions Particle momenta decomposed Longitudinal momentum (pL) Transverse momentum (pT)

  7. pT = pL q = 45 (135) degrees h = ±0.88 pT>pL pL>>pT pL>>pT dN/dh – basics • Midrapidity peak / plateau • Sensitive to hadroproduction details • Related to energy density • Bjorken formula (requires a “central-plateau structure” in the y distribution of produced particles) • Boost-invariant central plateau? • Width of the distribution • Information on longitudinal expansion and stopping power (stopping vs. transparency) • Fragmentation regions • Investigate effects connected with target and projectile fragmentation

  8. Experimental requirementsand techniques

  9. Experimental issues • Acceptance: • Large h coverage to measure particles at mid-rapidity and in fragmentation regions • Low pT cut-off if a magnetic field is present • Analysis techniques • Count fired channels (hits) on detectors NA50, PHOBOS • In general 1 hit NOT EQUAL to 1 particle because of: • PHYSICAL PROCESSES in the detector volume (multiple occupancy, charge sharing…) • INSTRUMENTAL PROBLEMS (electronic noise, cross-talk …) • What is done is to count CLUSTERS (i.e. groups of contiguous strips firing together) and apply a correction to go from clusters to crossing particles • Measure energy deposition in detector channels NA57, BRAHMS, PHOBOS • Correction for Landau distribution of energy deposition required • Match hits between 2 detectors (TRACKLETS) PHOBOS • More precise alignment and knowledge of primary vertex required • Correction for tracking efficiency to be applied • Full tracking NA49, STAR

  10. Example: NA50 analysis Silicon microstrip detectormeasuring the number and the angular distribution of charged particles produced in the collision • 2 Planes (MD1, MD2) • each plane made of 2 layers (up/down) • 36 azimuthal sectors (=10o) • 192 radial strips (=0.02) • 6912 strips in each plane • Only 128 innermost strips used

  11. High multiplicity bin NA50 analysis method • Extract number of particles from number of clusters in bins of h (=0.15) and centrality • Cluster size distribution not reproduced by a VENUS+GEANT simulation (only physical clusters) • Dedicated MC, aimed at reproducing cluster size distribution observed in data • Calculate primary dNch/d. • Subtract the delta electron contribution (GEANT) • Max. 5% of the occupancy in the most peripheral bin • Divide by secondary/primary ratio (extracted from VENUS+GEANT simulation) • VENUS+GEANT data reconstructed with the same method as experimental data. • 1.2 –1.8 correction factor. • Does not depend on centrality. • Depends on target thickness, target position, particular MD plane. • Large corrections due to thick (3 mm) target

  12. Cross checks (I) Compare results from different detector planes • Average between detector planes • Wide  coverage • Compare results from runs with two different target thicknesses and positions • Average between different thicknesses • Wider  coverage

  13. Cross checks (II) Compare results from 2 independent centrality estimators • dN/dh values obtained with ET and EZDC centrality selections agree within 1.5% • Both centrality estimators independent of MD ET centrality selection EZDC centrality selection  Abreu et al. (NA50 collaboration) Phys. Lett. B530 (2002) 43

  14. Experimental results:width of the distribution

  15. Width of dN/dh distribution (I) • Information on longitudinal expansion and degree of stopping • How would an isotropic source emitting at rest look like? FWHM = 1.8

  16. √s= 200 GeV √s= 130 GeV Width vs. centrality at SPS and RHIC NA50 at 158 GeV/c (√s=17.2 GeV) • Gaussian width (FWHM) decreases with increasing centrality • Observed also by NA35, WA80, Helios/Emulsion, E802 • Stopping power effect • Decreasing contribution of protons from target and projectile fragmentation

  17. Width vs. energy NA50 most central Pb-Pb • Available phase space in rapidity increases with √s • Fit with the simple scaling law sh = a + b · ln s • At SPS energies dN /dh (dN/dy) are twice as large as the one expected from a thermal fireball (Senger and Strobele, nucl-ex/9810007) E877 central Au-Au

  18. Experimental results:particle density at midrapidity

  19. Midrapidity peak / plateau • The maximum of pseuorapidity distribution (dNch/dh | max ) at hcm=0: • Most frequently used variable to characterize the multiplicity of the interaction • Independent of phase space acceptance  allows comparison between different experiments • Increases with collision energy (√s) and centrality central central peripheral peripheral

  20. Scaling with centrality at the SPS (I) • Agreement within 10% among experiments at 158 GeV/c • Fit with the power law • Values of exponent a between 1.00 (NA50) and 1.08 (WA98) • Depends on the model to calculate Npart (NA50 finds a=1.00 with a Glauber estimation of Npart and 1.08 with a VENUS estimation) • Two-component fit: • Values of B compatible with 0

  21. Scaling with centrality at the SPS (II) NA50 • Npart describes the centrality dependence of particle production at midrapidity at SPS energies • Soft processes dominate particle production at such energies • No important contribution from hard processes (as expected) • Introduce yield per participant pair: • A flat behaviour reflects the linear dependence of dN/dhmax on Npart

  22. PHOBOS PRC 2004, nucl-ex/0405027 Scaling with centrality at RHIC • Yield per participant pair increases by ≈ 25% from peripheral to central Au-Au collisions • Contribution of the hard component of particle production ? • BUT: • The ratio 200 / 19.6 is independent of centrality • A two-compoment fit with dN/dh  [ (1-x) Npart /2 + x Ncoll ] gives compatible values of x (≈ 0.13) at the two energies.

  23. s = 130 GeV NA50 at 158 A GeV/c Warning • Npart is not a direct experimental observable and affects the scale of both axes of plots of yield per participant vs. Npart • Different methods of evaluating Npart give significantly different results!

  24. dN/dhmax in central heavy ion collisions increases as ln(s) from AGS to top RHIC energies Different √s dependence in pp and heavy ion collisions Density at midrapidity vs. energy • WARNING when comparing dN/dhmax between collider and fixed target experiments:pseudo-rapidity h is not boost invariant • Conversion from dN/dh|lab to dN/dy (Lorentz invariant) and then to dN/dh|cm

  25. Experimental results:total charged multiplicity

  26. √s= 200 GeV Multiplicity vs. density at midrapidity central • The shape of pseudorapidity distributions is not independent of centrality (Npart) • Height increases more than linearly with Npart • Width decreases with increasing centrality • BUT Height  Width ≈ constant peripheral

  27. Total multiplicity vs. Npart • Total multiplicity obtained integrating dN/dh distributions • Small extrapolation thanks to the wide h coverage of PHOBOS • Total charged-particle multiplicity proportional to Npart • Total yield per participant is the same as in e+e- collisions at the same energy

  28. 62.4 GeV 200 GeV Cu+Cu Preliminary 3-6%, Npart = 100 PHOBOS PHOBOS Cu+Cu Preliminary 3-6%, Npart = 96 Au+Au 35-40%, Npart = 99 Au+Au Preliminary 35-40%,Npart = 98 Gold vs. copper • Unscaled dN/dh very similar for Au-Au and Cu-Cu collisions with the same Npart • Compare central Cu-Cu with semi-peripheral Au-Au • For the same system size (Npart) Au-Au and Cu-Cu are very similar

  29. Multiplicity in AA collisions • Below pp and e+e- at AGS energies • Cross through pp at SPS energies • Joins e+e- data above top SPS energy • No leading particle effect AA collisions at RHIC energies • Due to multiple collisions per participant ? Integrated yield vs. energy • Multiplicity in pp collisions lower than in e+e- • Understood as due to leading particle effect • The outgoing proton takes away a substantial amount of energy 1

  30. Experimental results:fragmentation regions

  31. Limiting fragmentation • Study particle production in the rest frame of one of the two nuclei • Introduce the variable y’ = y - ybeam (or h’ = h – ybeam ) • Limiting fragmentation • Benecke et al., Phys. Rev. 188 (1969) 2159. • At high enough collision energy both d2N/dpTdy and the particle mix reach a limiting value in a region around y’ = 0 • Also dN/dh’ reach a limiting value and become energy independent around h’=0 • Observed for p-p and p-A collisions • In nucleus-nucleus collisions • Particle production in the fragmentation region independent of energy, but NOT necessarily independent of centrality

  32. Limiting fragmentation (I) PHOBOS Phys. Rev. Lett. 91, 052303 (2003) • Particle production independent of energy in fragmentation regions • Extended limiting fragmentation (4 units of h at 200 GeV) • No evidence for boost invariant central plateau Spectator emission ?

  33. Limiting fragmentation (II) • Different limiting curves for central and peripheral data • Particle production in the fragmentation region changes significantly with centrality • The hypothesis of limiting fragmentation does not imply that the limiting curve is independent of centrality • BUT both (central and peripheral) energy independent

  34. What have we learned so far ? • Charged particle multiplicities follow simple scaling behaviours • Total yield at RHIC energies ≈ Npart multiplicity in e+e- at the same energy • Extended (up to 4 h units) fragmentation regions where particle production is independent of energy (BUT not of centrality) • No evidence for a boost invariant central plateau also at top RHIC energy • From STAR White paper: • “Most bulk properties measured appear to fall on quite SMOOTH CURVES with similar results from lower energy collisions…Similarly the centrality dependences observed at RHIC are generally smooth… These experimental results contrast with theoretical speculations and predictions… which often suggested strong energy dependences accompanying the hadron-to-QGP phase transition” • Energy density from Bjorken formula and measured dN/dy (dN/dh) at top RHIC energy gives values of ~ 5 GeV/fm3 “well above the critical density (1 GeV/fm3) predicted by Lattice QCD for a transition to the QGP

  35. Perspectives for ALICE at the LHC

  36. Energy dependence and the LHC Detectors planned for dN/dh > 5000 Saturation model Armesto, Salgado, Wiedemann hep-ph/0407018 dN/dη ~ 1800 dN/dη ~ 1100 Models prior to RHIC Log extrapolation

  37. dN/dη ~ 1800 Limiting fragmentation dN/dη ~ 1100 W. Busza, Zakopane ’04 Limiting fragmentation and the LHC

  38. ALICE at the LHC Forward Multiplicity Detector (FMD) Inner Tracking System (ITS) Time Projection Chamber (TPC)

  39. ALICE pseudorapidity coverage • Different measurement techniques • CLUSTERS on innermost ITS layers (Silicon Pixels) • TRACKLETS with 2 innemost layers of ITS (Silicon Pixels) • FULL TRACKING (ITS+TPC) • ENERGY DEPOSITION in the pads of Forward Multiplicity Detector (FMD) p-p collisions at LHC: s = 14 TeV ybeam = 9.6

  40. Silicon Pixel Detectors (2D) Silicon Drift Detectors (2D) Silicon Strip Detectors (1D) R= 43.6 cm L= 97.6 cm dN/dh measurement with ITS • Multiplicity from: • 2 innermost layers of Silicon Pixel Detectors: • Wider h coverage • No energy loss information • Analysis techniques: • Count “clusters” on the 2 layers • Count “Tracklets” (associations between 2 layers)  ALICE collab. - Pysics Performance Report - Vol II

  41. dN/dh at mid-rapidity with ITS • dN/dh in |h|<0.5 for: • 100 HIJING events • Standard noise level • No magnetic field • zVERTEX = 0 • Hits = number of primary particles crossing a layer • Number of clusters • Lower than generated multiplicity in layer 1 • due cluster merging at high multiplicity • Enhanced in layer 2 • due to secondary particles produced in the inner layer • Tracklets • Association efficiency decreases with increasing multiplicity

  42. (looser cut) (tighter cut) Generated mult. Standard noise level Systematic effects Magnetic field effect • Clusters in layer 1 insensitive to the field • low pT tracks do not reach layer 2 • Field = 0 best condition to measure multiplicities Noise level effect • Tracklet method more stable against noise level • Noise effect almost completely removed at the price of a decrease of efficiency (larger MonteCarlo correction needed)

  43. dN/dh reconstruction in ITS (I) • dN/dh distribution for: • 1 central HIJING event (dN/dh = 6000) • Standard noise level • No magnetic field • zVERTEX = 0 • With zVERTEX smearing an acceptance correction has to be included

  44. zVERTEX spread allows to increase the h coverage dN/dh reconstruction in ITS (II) • dN/dh distribution for: • 300 semi-central HIJING events (dNch/dh ≈ 3000) • Standard noise level • No magnetic field • zVERTEX spread = ± 5 cm • + acceptance correction

  45. Thanks to… Tiziano Virgili (NA57), Gunther Roland (PHOBOS) for giving me plots and material …and to… Marek Idzik, Marco Monteno, Marzia Nardi, Luciano Ramello for discussions and clarifications …and to… NA50 and ALICE collaborations

  46. Backup slides

  47. Multiplicity and collision centrality • The impact parameter (b) determines the “centrality” of the event • SMALL IMPACT PARAMETER (Central events) • Many participant nucleons (large Npart ) and few spectators • Many nucleon-nucleon collisions ( large Ncoll ) • Big system • Many produced particles (~ 5000 at top RHIC energy ) • LARGE IMPACT PARAMETER (Peripheral events) • Few participant nucleons (small Npart ) and many spectators • Few nucleon-nucleon collisions (small Ncoll ) • Small system • Few produced particles

  48. “Glauber” calculations • Optical approximation  Czyz and Maximon, Annals Phys. 52 (1969) 59. Nucleus thickness functions Nucleus-nucleus thickness function Nucleon-nucleon collision probability

  49. PHOBOS Apparatus

  50. Centrality determination in NA50 • Two detectors independent from MD to measure event-by-event centrality related observables • Electromagnetic calorimeter transverse energy of neutral particles (ET) • Zero Degree Calorimeter energy of spectator nucleons (EZDC) • Centrality intervals for dN/dh analysis defined in terms of fraction of total inelastic cross section

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