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Soft Physics in ALICE

Soft Physics in ALICE. Global event properties. Bulk properties: soft hadrons + interplay hard–soft Identified particle spectra (wide p T range). Chemical composition Hadronisation mechanisms. Expansion dynamics Space-time structure

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Soft Physics in ALICE

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  1. Soft Physics in ALICE Global event properties Bulk properties: soft hadrons + interplay hard–soft Identified particle spectra (wide pT range) Chemical composition Hadronisation mechanisms Expansion dynamics Space-time structure Radial, anisotropic flow Momentum correlations Event by event physics Fluctuations Christian KUHN (IPHC - Strasbourg) for the ALICE collaboration 1 SQM 2007 @ Levoca

  2. From SPS & RHIC to LHC 2 Pb-Pb at √sNN= 5.5 TeV ALICE LHC SPS Au-Au √sNN = 200 GeV Many open questions ! Need excitation functions on the largest possible energy range -> transition between hard and soft processes At LHC: “old” (different / better conditions) + new observables ALICE at LHC: one dedicated heavy ion experiment which will offer many novel opportunities to study the physics of strongly interacting matter and the QGP Dense, color opaque medium

  3. New conditions & new probes at LHC 3 Better overall conditions to study the QGP (x 30 wrt. RHIC) Much higher energy density (x 3-10) Initial temperature T (x2) > 3Tc Larger QGP volume, Longer QGP life time t (x3-5) Hard processes Multiplicity Bulk properties strongly influenced by hard processes dNch/dη = 2600 Very hard probes copiously produced 1200 Event by event physics

  4. ~ 1000 Members ~ 30 Countries ~ 90 Institutes ALICE TOF TRD HMPID L3 MAGNET PMD FMD ITS MUON TRIGGER CHAMBERS TPC PHOS ABSORBER MUON TRACKING CHAMBERS 4 EMCAL Designed to cover essentially all observables of interest in the soft and hard regimes (hadron, lepton and photon sectors) -> Tracking & particle identification in a large acceptance and pT domain Size: 16 x 26 meters Weight: 10,000 tons

  5. Tracking and particle identification central Pb – Pb TPC 5 Physical efficiency For dNch/dy = 4000 in Pb-Pb Reconstructed / generated ( |η|<0.9) ~ 90% for pT > 1 GeV (limited by dead zones) Protons : large absorption Kaons : in-flight decays Momentum resolution: ~1% at PT = 1 GeV/c, ~4% at PT = 100 GeV/c Precise vertexing (better than 100 mm) p,K,p: dE/dx (in TPC & ITS) + TOF and RICH 100 MeV < p < a few tens GeV electrons: TRD p >1 GeV muons: p > 4 GeV photons: PHOS 1 < p < 80 GeV

  6. Global event properties in Pb-Pb and pp Multiplicity distribution (dNch/dh) in Pb-Pb 60000 Pythia pp events Energy density Silicon Pixel Detector (SPD) -1.6 < h < +1.6 Forward Multiplicity Detector (FMD) h -5, +3.5 generated dN/dh measurement in pp @ 14 TeV with the TPC LHC commissioning with optimal luminosity (<1030 cm-2s-1) Benchmark for Pb-Pb + genuine pp physics J.F. Grosse Oetringhaus C. Jorgensen Study of underlying event structure Advantage of ALICE: pT cut-off ~100 MeV 1 central Hijing event Explore minimum bias pp (adequate triggers) 6

  7. A -0.9 y=0 0.9 Baryon yields in pp collisions 7 Asymmetry of the proton to anti-proton number A = 2 (Np – Nanti-p) / (Np + Nanti-p) The experimental challenge • Distinguish between two mechanisms • of Baryon Number (BN) transport: • Quark – diquark string breaking where BN is carried • by the valence quarks • 2)Baryon stopping = stopping of the string junction • Effect of a few percents but the asymmetry is • predicted to be dependent on the multiplicity BN ASSOCIATED TO GLUONIC FIELD BN ASSOCIATED TO VALENCE QUARKS Asymmetry (uncorrected) Extract PID efficiency map & absorption map

  8. Chemical equilibrium vs non-equilibrium at LHC Hot (T~170 MeV) & under- saturated (gq < 1) super-cooled & over-saturated Statistical hadron resonance gas model at equilibrium : -> SPS -> RHIC -> LHC ( T~170 MeV, mB ~1) ? Hadronization from a super-cooled ( T~140 MeV) over-saturated system with higher entropy – > out of equilibrium strangeness abundance LHC ? 8

  9. Equilibrium versus non equilibrium at LHC 9 B. Hippolyte et al. EPJ C49 (2007) Non equilibrium Oversaturation of s (gs = 3 - 5) Equilibrium gs = 1 And many other questions: strangeness enhancement % energy Correlation volume (Npart GC, Nbin hard processes) ? Evolution in pp ?

  10. 10 Hard / soft interplay at intermediate pT Rcp: central over peripheral yields/<Nbin> Baryon/meson ratio Elliptic flow Parton recombination / coalescence + pert. QCD: parton fragmentation ? LHC ? RHIC Strange particles give access to a wide pT range R. Fries et al. LHC But baryon & meson production must be first understood in pp !

  11. 11 Strange baryon / meson ratio @ UA1 and CDF: pp @ 630 GeV & 1.8 TeV Ratio at mid-pT already surprisingly high in pp data at high energies Extracting mixed ratio from 1996 UA1 strange particle data B. Hippolyte et al. EPJ C49 (2007) This feature is not observed in pp PYTHIA simulations and ratio at 14 TeV stays well below unity. Better but still missing a factor > 2 wrt RHIC -> UA1 -> CDF extrapolations Need to investigate NLO contribution and baryon creation mechanisms (diquark to popcorn scenario or gluonic baryon junctions). Important evolutions: PYTHIA v6.2 -> v6.3: - multiple parton interactions New PDF

  12. Recombination of shower and thermal quarks R.Hwa et al, Phys.Rev.C70 (024904) 2004 • triggered azimuthal correlations to measure fragmentation (shower quark contribution). If W comes from TTTat intermediate pT, no W correlations will be seen L(STAR) W(STAR) End of reco ? Enhanced production from recombination of thermal partons in the ridge ? 2-dim. correlations: Df-Dh 12

  13. Topological identification of strange hadrons Statistical limit for 1 year: ~ 107 central Pb-Pb, 109 min. bias pp pT ~ 13 - 15 GeV for K+, K-, K0s, L pT ~ 9- 12 GeV for X, W Pb-Pb central L 300 Hijing events 13 reconstructed L/event 700K pp collisions (14 TeV) K K0s Secondary vertex and cascade finding pp collisions Reconstruction rates: X: 0.1/event W: 0.01/event H. Ricaud Identification of K+, K- via their kink topology K mn 13

  14. Resonances (r, f, K*, …) 14 Time difference between chemical and kinetic freeze-out In medium modifications of mass, width, comparison between hadronic and leptonic channels partial chiral symmetry restoration r0(770)p+p- 106central Pb-Pb See talks: A. Badala, P. Ganoti, D.Tapia Takaki Invariant mass reconstruction, background subtracted (like-sign method or event mixing) Mass resolutions ~ 1.5 - 3 MeV pT stat. limits from 8 (r) to 15 GeV (f,K*) Mass resolution ~ 2-3 MeV 200K Pythia pp min. bias events (0.9TeV) Invariant mass (GeV/c2) ~4000 K* reconstructed K*(892)0 K p 15000 central Pb-Pb f (1020) K+K- Mass resolution ~ 1.2 MeV

  15. Anisotropic Flow 15 Hydro limit (full local thermalization) at RHIC ? More likely at LHC ? Continuous increase with Kn-1 (Kn = mean freepath/system size, Kn-1 ~ s (1/S) dN/dy -> no saturation seen in data Initial conditions CGC + hydro (until T ~ 170 MeV) i.e., contribution of the QGP + hadronic cascade At LHC, contribution from QGP much larger than at RHIC Data: v4/v22~1.2 suggest Kn~1: No thermalisation at RHIC! Hydro limit : Kn << 1 v4=0.5 (v2)2 at large pT T. Hirano N. Borghini, J.Y. Ollitrault ? In ideal hydrodynamics v2 driven by the (space time averaged) velocity of sound, v2/e = constant In the low density limit v2 is driven by e and dN/dy Qualitative predictions for LHC • Closer to ideal hydro. • Significant increase of v2 • v4/(v2)2 smaller than at RHIC LHC 60

  16. Flow and non-flow effects at LHC ~ non-flow v2 (Hijing fit) hydro LDL 16 E. Simili Estimate of non flow effects: Hijing simulations with v2=0 & no jet quenching Signal clearly disentangled in a wide range of multiplicity Event plane resolution ~ 10o Track multiplicity = 1000 V2 = 0.06 ALICE TPC fREC - fMC The difference oberved at RHIC between v2{2} and v2{4} could be dominated by event by event fluctuations The relevant eccentricity (i.e. epart) indeed varies event-by-event Measure flow with different methods and detectors covering different h to disentangle flow, nonflow and fluctuations BUT

  17. Particle correlations Rlong p1 x1 p2 qside Rside x2 qout qlong Rout 17 The HBT puzzle at RHIC • Rout/Rside does not increase as expected • from hydro which predicts a long system • lifetime … • pT dependence of Rout/Rside also not • reproduced by hydro … • And many open questions : • surprising scaling of the radii with pp • non-femtoscopic q-anisotropic • behaviour: EMCIC ? Rout/Rside √sNN (GeV) Could the long awaited QGP signal of extended lifetime-scales appear only at LHC ?

  18. Particle correlations 18 Two pion momentum correlation analysis Study of event mixing, two track resolutions, track splitting/merging, pair purity, Coulomb interactions, momentum resolution corrections, PID corrections Radii can be recontructed up to 15-20 fm Correlation functions Rrec(fm) C(q out) Rsimul. (fm) C(q side) Rsim = 8fm 1 event : 5000 p C(q inv) Other potential analyses Two kaon & two proton correlations Single event HBT Direct photon HBT, … C(q long) q (GeV/c) q inv (GeV/c)

  19. Event by event fluctuations 19 Lattice computations at small chemical Potential (S. Ejiri, F. Karsch, K. Redlich) Fluctuations of temperature, entropy, energy density and of quark number susceptibilities (net charge, isospin, strangeness content) associated with phase transition 4th moment of the net charge T/Tc High multiplicities at LHC => ALICE suited for the measurement of event/event fluctuations of <pT>, T multiplicity, particle ratio, strangeness, azimuthal anisotropy, intermediate / high pT phenomena, long range correlations, balance function, … C. Zampolli Mini-jets and jets expected to increase strongly the level of fluctuations Fluctuations of flow viscosity Fluct. of particle ratio constraints on statistical models p Resolution T/T: 0.5 % for 

  20. Conclusions & Outlook With one month of Pb-Pb collisions (107 central + 107 MB events) many questions left open from the RHIC era should be answered We also hope many many surprises !!!! Installation and commissioning of all ALICE subsystems are essentially on schedule for the first pp collisions Major effort ongoing to finalize the DAQ and offline projects in order to be able to perform the physics analysis from day one Busy months ahead, but working detector well on track for first collisions 20

  21. The End

  22. Global event characterization in Pb-Pb Centrality determination brec(fm) sb ~ 1fm Event by event determination of the centrality Zero degree hadronic calorimeters (ZDC) + electromagnetic calorimeters (ZEM) EZDC , EZEM Nspec Npart impact parameter (b) bgen (fm) Correlations between ZDC and ZEM EZDC (TeV) Events reconstructed generated sNpart ~15 Npart EZEM (GeV) Npart

  23. dNch/dh measurement with the TPC h vs. z J. F. Grosse-OetringhausC. E. Jørgensen Corrections • Track-to-particle (track level) • Geometrical acceptance, reconstruction efficiency, decay, feed-down • Function of h, z vtx, pT • Vertex reconstruction efficiency (event level) • Function of z vtx, multiplicity • Trigger efficiency (event level) • Function of z vtx, multiplicity pT projection

  24. Multiplicity distribution and dNch/dh with the SPD (tracklets) zvertex SPD efficiency correction chip by chip inserted in the geometrical acceptance Map (here arbitrary dead channels) Ongoing: - Finalize method to determine detector efficiency maps - Evaluate dN/dη for 900 GeV sample with the detailed SPD efficiency map - Include corrections and systematics from beam-gas background and trigger acceptance Tracklets Integration over φ T. Virgili, D. Elia Generated pp 900 GeV pp 14 TeV

  25. Identified particle spectra at intermediate pT solid: STAR open: PHENIX PRL91(03) Interplay between hard and soft processes V2: constituent quark scaling -> Coalescence ? Recombination % Fragmentation (RF) Soft (hydro-> flow) + quenching, … baryons / mesons Hadron production in pp at LHC (Pythia)

  26. Baryons and mesons at intermediate pT Coalescence of thermal partons qualitatively explains the baryon excess at mid-pT in AuAu at RHIC L/K0s P. Levai MICOR model : quark-coalescence (0 < pT < 4-5 GeV ) + pert. QCD : jet-fragment. (2 < pT < 10-20 GeV) reproduces many RHIC features at mid-pT: baryon/ meson , Rcp, v2, … RHIC LHC Soft-hard overlap at LHC: pT=4±1 for p, pT=6±1 for p

  27. y x • Find a trigger particle W (pT>2 GeV/c) • Find an associated particle • (1.5 GeV/c < pTas< pTtrg) in the same event • 3. Compute Df at primary vertex Statistical jet measurements High pT track v2 B 0 same-side away-side W Df Possibility: enhanced production from recombination of thermal partons in the ridge (elongation in Dh under the jet peak) If no SSS contribution to W spectrum W-h correlation will be flat after v2 subtraction

  28. Another interesting idea … C. Salgado …is that coalescence models could open up the exciting possibility to use the unconfined state to probe the constituent quark composition of exotic hadronic states (f0, a0), thus discriminating between conflicting models. f0 /a0 candidates for a new class of mesons having a 4-quark structure Joffe & Wilczeck; also Maiani, Piccinini, Polosa, Riquer Or f0 = ss ? [s u ] [ s u ] ± [ s d ] [ s d ] / √2 ? f0 = Based on reco. + frag. model (R. Fries et al) RHIC LHC

  29. Identified particle spectra Particle reconstruction and identification capabilities: unique to ALICE Global tracking (ITS-TPC-TRD) + dE/dx (low pT + relativ. rise), TOF, HMPID, PHOS, … Invariant mass, topological reconstruction Acceptance / efficiency / reconstruction rate (e) / contamination pT range (PID or stat. limits) for 107 central Pb-Pb and 109 min. bias pp For ~ 20 particle species for -1 < y < +1 and -4 < y < +2.5 p, K, p: 0.1- 0.15 50 GeV Weak or strong decaying particles: until 10-15 GeV Mid-rapidity p PID in the relativistic rise K p Pb-Pb Pb-Pb pT (GeV/c)

  30. K0S K0s and L reconstruction in pp H. Ricaud L PDC06: 700 K pp(14 TeV) with PROOF K0s L reconstruction rate

  31. Resonances pp @ 900 GeV F. Riggi, R. Vernet, D. Tapia Takaki, M. Spyropoulou Uncertainties on LHC running scenario: 1-2 days ? maximum expected number of events < 106 ? Not very difficult to get above Tevatron statistics -> 900 GeV data could become best data at this √s. With 2 x 105 events and PID: Extraction of yields at least for K*(892), Φ(1020), Λ*(1520) Pt-distribution for K*(892) up to 1.5-2 GeV/c Particle ratios K*/K-, Φ/K*, Λ*/K*, Φ/Λ* measurable With larger statistics, possibility to explore: Extended momentum range (1.5-2.5 GeV/c) for K* Mass shifts analysis vs pt and charged multiplicity Pt-distribution also for other resonances

  32. Resonance reconstruction in p-p @ 0.9 TeV 200 000 Pythia pp min. bias events (PDC06) with PID F. Riggi, R.Vernet K* K*(892) PT reach for 2 x105 events Evt mix. & like sign methods give similar results ~ 180 reconstructed Φ(1020)

  33. Anisotropic flow (J.Y. Ollitrault et al.) Kn = mean free path/system size Ideal hydro : universal prediction v4=0.5 (v2)2 at large pt Kn-1~σ/S (dN/dy) v4/(v2)2 Data ~1.2 suggest Kn~1: No thermalisation at RHIC! The hydro limit is Kn«1. If not satisfied, one expects smaller v2 than in hydro. pT

  34. Non flow and fluctuations PHOBOS v2result band: 90% CL PHOBOS epartprediction Reaction Plane Estimate in ALICE from spectator neutrons N. De Marco Event Plane resolution: <cos(phiZDC – phiRP)> 2 arms: event plane azimuth phiZDC from the mean of the centroids of spectator neutrons spot on ZN1 and ZN2 front face. The distance between the 2 centroids can be a tool to select events with a better e.p. resolution

  35. EbyE P. Christakoglou PDC06 • Balance function extended into a 2-dim. differential analysis in pT-η . • Decrease of the width with increasing <PT>. The method is sensitive to the transverse expansion of the system. • Net charge fluctuations, particle ratio fluctuations, long range correlations <PT> fluctuations 700K p+p events @ √s = 14TeV 200K p+p events @ √s = 900GeV <PT> rises with increasing multiplicity

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