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## Osservabili ‘soft’ in ALICE

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**Osservabili ‘soft’ in ALICE**Quark Matter Italia Roma, 22-24 Aprile 2009 Luciano Ramello – Università del Piemonte Orientale & I.N.F.N. for the ALICE Collaboration**Indice**• L’esperimento ALICE a LHC • Osservabili ‘soft’ in ALICE: • Molteplicità di particelle cariche • Flusso direzionato (v1) ed ellittico (v2) • Spettri in pT, rapporti h/h, B/M, ... • Risonanze • Correlazioni HBT • Fluttuazioni L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**L’esperimento ALICE**Configurazione per le prime collisioni p-p e Pb-Pb a LHC L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**ALICE: dedicated to H.I.**• General purpose(as opposed to all SPS and some RHIC expt’s) • In contrast to LHC experiments mainly devoted to (hard) p-p physics • ALICE isfocussed on heavy-ion physics& therefore has: 1) Capability of coping with thehigh multiplicitygenerated in H.I. collisions (~2000 charged particles per unit rapidity, designed for 6000) • Not strictly necessaryfor selected hard probes (such as ), • but important to access the bulk of particle production, i.e. • to study soft observables • Implies, in particular: • High granularity • Large bandwidth for Data Acquisition system 2) Possibility ofpushing downas much as possible itspT reach • Implies anot too high B fieldfor momentum measurement • (note: Conflicting requirementwith accuracy forhard probes!) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Zero-degree calorimeters**Central barrel Muon arm L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Size: 16 x 26 meters**Weight: 10,000 tons TOF TRD HMPID ITS PMD Muon Arm PHOS Added since 1997: • V0/T0/ACORDE • TRD (’99) • EMCAL (’06) ALICE TPC L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**ALICE configuration at start-up**• Complete - fully installed & commissioned: • ITS, TPC, TOF, HMPID, MUONS, PMD, V0, T0, FMD, ZDC, ACORDE, TRIGGER, DAQ • Partially completed: • TRD (20%) to be completed by 2009 • PHOS (40%) to be completed by 2010 • EMCAL (0%) to be completed by 2010/11 • HLT (High Level Trigger) (~50%) • At start-up full hadron and muon capabilities, • Partial electron and photon capabilities L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**ALICE Detector Installation Goal 2009**2008/09 Shutdown used for additional installation and repairs Complete: ITS, TPC, TOF,HMPID, FMD, T0, V0, ZDC, Muon arm, Acorde PMD,Trigger,DAQ Partial installation: PHOS(3/5) 6-8/18 TRD 2-4/6 EMCAL ~ 50% HLT ALICE Status P. Kuijer QM 2009 P. Kuijer 8 Number of TRD & EMCAL modules depends on access conditions during LHC power test !**dNch/dy=6000**drop due to proton absorption TPC acceptance = 90% Momentum resolution ~ 5% @ 100 GeV Alice tracking performance • Central barrel tracking: ITS + TPC + TRD • Robust, redundant tracking from < 100 MeV/c to > 100 GeV/c • Very little dependence on dNch/dy up to dNch/dy ≈ 8000 • p/p < 5% at 100 GeV with careful control of systematics L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Alice particle identification**• ‘stable’ hadrons (π, K, p): 0.1<p<5 GeV/c; (π, p with ~ 80 % purity to ~ 60 GeV/c) • dE/dx in silicon (ITS) and gas (TPC) + time-of-flight (TOF) + Cherenkov (HMPID) • decay topologies (K0, K+, K-, Λ, cascades, D) • K and Λ decays beyond 10 GeV/c • leptons (e, μ ), photons, π0 • electrons TRD: p > 1 GeV/c, muons: p > 5 GeV/c, π0 in PHOS: 1<p<80 GeV/c L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Cosmic event with SPD trigger**Probably a muon interaction in the magnet’s iron about 350 tracks reconstructed in TPC L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Cosmic event with ACORDE trigger**L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**p beam in ALICE**LHC pilot beam at 450 GeV: p-Si collision in the SPD (Sept. 12, 2008) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Breve introduzione:cosa ci aspettiamo di imparare studiando**le osservabili ‘soft’ a LHC? Ovvero: dati i risultati ottenuti a RHIC c’è ancora qualcosa da scoprire a LHC?**Thermal freeze-out**Elastic interactions cease Particle dynamics (“momentum spectra”) fixed Tfo (RHIC) ~ 110-130 MeV Chemical freeze-out Inelastic interactions cease Particle abundances (“chemical composition”) are fixed (except maybe resonances) Tch (RHIC) ~ 170 MeV Thermalization time System reaches local equilibrium teq (RHIC) ~ 0.6 fm/c Reminder: space-time evolution L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Introduction/1**• Initial conditions: measuring dNch/d will be crucial for evaluating energy/entropy density & confirm (or reject) the saturation model (Lacey, QM 2009) • Thermo/hydrodynamics of QCD: at T ≥ 400 MeV (LHC) a new regime should emerge (Wiedemann, QM 2009) consequences for v2, HBT correlations, pT spectra • Chemical composition: will statistical models still work at LHC energy? Measuring hadron yields (& their ratios) will give the answer (limiting temperature of 160 MeV?) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Introduction/2**• Nature of phase transition: at LHC (high T, low B) a ‘crossover’ transition is expected… it should be verified by studying fluctuations of conserved quantities (Mohanty, QM 2009), while other experimental programmes (NA61/SHINE at CERN, the RHIC Critical Point Search, CBM at GSI) will explore other regions of the phase diagram • Lesson from RHIC: ‘know your reference’, in our case we will have collected p+p data before Pb-Pb ones, but it will be necessary later to collect p-A (d-A) data as well, to understand cold nuclear matter effects L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Molteplicità di particelle cariche**L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Charged multiplicity at the LHC**increasing s – decreasing x • Extrapolation of dNch/dmax vs s: • Fit to dN/dη ln s (limiting fragmentation)… • … or Saturation model (dN/dη s with =0.288)? • Clearly distinguishable with the first 10k LHC events Saturation model Armesto Salgado Wiedemann, PRL 94 (2005) 022002 Central collisions Models prior to RHIC Extrapolation of dN/dh ln s: 5500 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**dNch/d: PHOBOS extrapolation**W. Busza QM 2009 Au+Au Data from PHOBOS, Nucl. Phys. A757 (2005) 28 PHOBOS extrapolations to LHC energy, Wit Busza, J. Phys. G35, 044040 (2008): Total Nch (Pb+Pb √sNN = 5.5 TeV): 15 000 ± 1 000 Mid-rap. dNch/d @ Npart=386 (Pb+Pb √sNN = 5.5 TeV): 1 200 ± 100 Total Nch for inelastic p+p @ √s = 14 TeV (10 TeV): 60 ± 10 (56 ± 9) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Total particle production**PHOBOS: Nucl. Phys. A 757 28 (2005) E178: PRL34(1975)836 Wit Busza: Acta. Phys. Pol. B35(2004)2873 Similarity of total particle production in e+e-, pp, πA, KA, pA and AA collisions “What is the mechanism that makes the total particle production insensitive to the intermediate state?” W. Busza QM 2009 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Charged multiplicity in ALICE**Reconstructed dNch/d withtracklets in SPD (generated dNch/d = 3000) ALICE PRELIMINARY T. Virgili Wide rapidity coverage provided by ITS (SPD), TPC in the central region & FMD in the forward region L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**dNch/d in p+p: SPD**• dNch/d from tracklets (vertex + • 2 SPD layers): • larger and pT acceptance • less stringent calibration/alignment needs ALICE PRELIMINARY F. Prino M. Nicassio QM 2009 • Several careful corrections needed to go from Reconstructed to Corrected dNch/d: • background • algorithm + detector efficiency • geometrical acceptance • … and also: • vertexing efficiency • Min. Bias trigger efficiency ALICE PRELIMINARY L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**dNch/d in p+p: FMD**The FMD will complement the dNch/d measurement for -3.4<η<-1.7 and 1.7<η<5 Background correction is most crucial since >50% of particles crossing FMD are secondaries ALICE PRELIMINARY ALICE PRELIMINARY H.H. Dalsgaard L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Flusso direzionato (v1) ed ellittico (v2)**L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Anisotropic flow**Azimuthal asymmetry in coordinate space (transverse plane): Kolb + Heinz produces azimuthal asymmetry in momentum space: The amount of observed flow depends on centrality and on the spatial eccentricity: v1 = directed flow v2 = elliptic flow L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**y**z x Flow and reaction plane • The Fourier expansion is referred to the reaction plane angle (ΨRP) which must be located in the experiment’s reference frame • The reaction plane angle can be evaluated at any particular order of the expansion, using produced particles or even spectators (e.g. ‘bounce off’ of spectator neutrons, which can provide both the first order reaction plane and the amount of directed flow v1) Produced particles’ angular distribution in the transverse plane (xy) Reaction plane (xz) Figures courtesy F. Prino L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Directed flow in ALICE**• v1 can be measured in ALICE via spectator neutrons (>8.7), namely by their centroids as obtained by the two zero-degree ‘ZN’ calorimeters 7.04 cm • For a range of plausible v1 values (10% 20% 30%) at LHC, the first order event plane resolution obtained by combining both ZN’s is quite adequate • In addition, this measurement provides the sign of v2 2.76 TeV·A Pb-Pb minimum bias events HIJING simulation N. Demarco L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Elliptic flow: √s dependence**From the observed v2 dependence on √s … one expects v2(0) @ LHC/ALICE ~ 0.08 Large signal easy measurement, but.. beware of non-flow contributions (jets...)! v2 (elliptic flow) is supposed to scale as eccentricity (more on this later); from hydrodynamics calculations, it appears that the contribution to v2/ by the QGP phase (rather than from the cascade) is much larger at LHC with respect to lower energies T. Hirano, U. Heinz, D. Kharzeev, R. Lacey, Y. Nara, QM 2008 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Elliptic flow and eccentricity**• The ‘experimental’ definition of eccentricity is tricky: from geometry (Glauber) or taking into account event-by-event fluctuations in the actual number of participants? • the Participant Plane (PP) must be distinguished from the Reaction Plane (RP) A. Poskanzer QM 2009 momentum space coordinate space • v2 fluctuations (in events of the same centrality class) are closely related to fluctuations of part - which can be computed via Glauber MonteCarlo L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Elliptic flow: different methods**• Problem: measurement of v2 is affected by bothnon-flow and fluctuations • Different methods to extract v2 have been developed, based on event plane, 2-particle or many-particle correlations: they give different results… published corrected to PP STAR, J. Adams et al., PRC 72, 014904 (2005) A. Poskanzer QM 2009 … but they agree on mean v2 in Participant Plane! L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Elliptic flow: one result?**reaction plane corrected to RP • ‘A v2 for theorists’ (corrected to Reaction Plane): Glauber CGC Voloshin, Poskanzer, Tang, and Wang, Phys. Lett. B 659, 537 (2008) Still, some dependence on the assumption (Glauber vs. CGC) about fluctuations L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**What about viscosity?**• Relativistic viscous hydrodynamics is making nice progress (see e.g. D. Teaney, R. Snellings & P. Romatschke at QM 2009), 2nd order theory (with zero bulk viscosity) is under control • Shear viscosity () reduces v2: comparing hydrodynamics calculations with RHIC data, with high confidence one can conclude: /s < 0.5 ~6/4 M. Luzum and P. Romatschke, 2008 (erratum 2009) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**More on data vs. hydrodynamics**• Parametrization of v2/ deviation from ideal hydro: • v2/ = h / (1+B/(1/S· dN/dy)) = h / (1+Kn/K0) • h = ideal hydro limit for v2/ • 1/S· dN/dy inversely proportional to Knudsen number Kn = /L (mean free path/system size) • The B parameter scales with /s but is also sensitive to the EoS: • STAR data are well described using a CGC ε with soft EoS and η/s ~ 2/4π or Glauber ε with hard EoS and η/s ~ 4/4π • The shift of the peak of v2 as a function of pT may be an additional independent way of extracting /s R. Snellings, QM 2009 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**~**non-flow v2 (Hijing fit) Elliptic flow in ALICE ALICE Pb-Pb simulation: the v2 flow signal (for 3 different extrapolations) is clearly disentangled from the non-flow contribution over a wide range of centrality (charged multiplicity). v2/ε=0.33 hydro LDL v2/ε=0.22 Centrality class Relativistic hydrodynamics prediction: v2/ = constant [J. Y. Ollitrault – P.R. D 46 (1992)] Low Density Limit prediction: v2/ = const. (1/S)dNch/dy [Heiselberg, Levy – P.R. C 59 (1999); Poskanzer, Voloshin – P.L. B 474 (2000)] dNch/dh =2000 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Flow: event plane & other methods**event plane resolution (TPC) vs. v2 for 1000 charged tracks • Reaction Plane • Cumulants • Lee Yang Zeroes v2 vs pT for v2 =0.0625 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Flow with inner ITS layers**• SPD alone useful for low multiplicity events, and has higher acceptance • wrt TPC (low pT threshold ~15 MeV) • Simulations suggest • flow for negative and positively charged particles separately • coarse pT binning may be possible Layer 1 ׀η׀ < 2.0 Layer 2 ׀η׀< 1.4 In addition, FMD and PMD will measure flow at forward rapidities L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Spettri in pT e rapporti**Composizione chimica, meccanismi di adronizzazione**Statistical hadronization**• Statistical models assume that the hadronic system at freezeout can be described as a Hadron Resonance gas in chemical and thermal equilibrium, whose composition (ratios of particle species) follows statistical mechanics and depends on two parameters: chemical freezeout temperature Tch and baryonic chemical potential B • Other model parameters such as: fireball volume, I3, S are constrained from initial state • These models have been remarkably successful in describing hadron yields ratios up to RHIC - see e.g.: A. Andronic et al., Nucl. Phys. A772 (2006) 167 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Statistical hadronization at LHC**• Temperature Tchincreases rapidly at low √s, then reaches about 160 MeV at 7-8 GeV and stays constant; chemical potential B decreases continuously with increasing √s (A. Andronic et al., arXiv:0711.0974 [hep-ph]) TLHC = 161±4 MeV mBLHC=0.8(+1.2,-0.6) MeV L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Particle spectra in ALICE**• Transverse momentum ranges for particle identification in ALICE (central barrel): Expected charged hadrons yields for 107 Pb-Pb central collisions (TPC PID on statistical basis) L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Baryon asymmetry**AB = 2×(p–p)/(p+p) - - Antiparticle/particle ratios Phys. Rev. C 77, 061901(R) (2008) Phys. Rev. C 71, 021901(R) (2005) ALICE PRELIMINARY Systematic error on asymmetry <1.5% for a 10% material budget uncertainty W. Busza QM 2009 P. Christakoglou, M. Oldenburg L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Identified hadrons spectra (TPC)**ALICE–cosmics real data ALICE simulation A. Kalweit L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Strange particle spectra**: optimized, pT-dependent selection cuts L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**How to fit spectra?**Crucial to obtain the total yield (extrapolation down to pT=0: 7-15% of the yield is not measured directly) The proposed functional forms include: • Exponential in mT (Boltzmann) • Levy • Tsallis blast-wave (power law in mT) B. Hippolyte, H. Ricaud L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**62 GeV**Timmins (STAR) QM 2009 Staszel (BRAHMS) QM 2009 Baryon/meson ratios vs. pT/1 Sarah Blyth (STAR) QM 2006 High baryon to meson ratio (~1) at intermediate pT discovered at RHIC in Au+Au reactions, inconsistent with pQCD predictions • Baryon to meson ratio should be sensitive to: • hadronization mechanism (quark count) • radial flow of medium (hadron mass) • High B/M ratios may be explained by quark coalescence, but not uniquely… Au+Au 200 GeV L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Baryon/meson ratios vs. pT/2**− The ratio (+)/K0 at mid-rapidity in p+p collisions at RHIC energy is flat and below unity, while at higher energies (UA1, CDF) it rises above unity. − The maximum value of (+)/K0 at mid-rapidity in p+p collisions is not reproduced by PYTHIA. EPOS (with the mini-plasma option) predicts a strong increase of this ratio at LHC energy. B. Hippolyte, H. Ricaud L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Risonanze**L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Why resonances?**• Resonance Life-time [fm/c] • r(770) 1.3 • ++(1232) 1.7 • f0(980) 2.6 • K*(892) 4.0 • S*(1385) 5.7 • L*(1520) 13 • ω(783) 23 • (1020) 45 • Decay timecomparablewith (or even shorter than) QGP lifetime • Rescatteringandregenerationbetween chemical and kinetic freeze-out will affect the final yields • Comparing leptonic and hadronic final states provides further insight L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009**Resonances at RHIC**P. Fachini QM 2009 Mass shift ~45 MeV/c2 observed M. Naglis QM 2009 L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009