Osservabili ‘soft’ in ALICE

Osservabili soft in ALICE Quark Matter ItaliaRoma, 22-24 Aprile 2009Luciano Ramello Universit del Piemonte Orientale & I.N.F.N.for the ALICE Collaboration

L. Ramello Quark Matter Italia - Roma, 22-24 Aprile 2009

IndiceLesperimento ALICE a LHC Osservabili soft in ALICE:Molteplicit di particelle caricheFlusso direzionato (v1) ed ellittico (v2)Spettri in pT, rapporti h/h, B/M, ...RisonanzeCorrelazioni HBTFluttuazioni


Lesperimento ALICEConfigurazione per le prime collisioni p-p e Pb-Pb a LHC


ALICE: dedicated to H.I. In contrast to LHC experiments mainly devoted to (hard) p-p physics ALICE is focussed on heavy-ion physics & therefore has:1) Capability of coping with the high multiplicity generated in H.I. collisions (~2000 charged particles per unit rapidity, designed for 6000) Not strictly necessary for selected hard probes (such as ), but important to access the bulk of particle production, i.e. to study soft observables 2) Possibility of pushing down as much as possible its pT reach Implies, in particular: High granularity Large bandwidth for Data Acquisition system Implies a not too high B field for momentum measurement (note: Conflicting requirement with accuracy for hard probes!) General purpose (as opposed to all SPS and some RHIC expts)


ALICEHMPIDMuon ArmTRDPHOSPMDITSTOFTPCSize: 16 x 26 metersWeight: 10,000 tonsAdded since 1997:V0/T0/ACORDE TRD (99) EMCAL (06)


ALICE configuration at start-upComplete - fully installed & commissioned:ITS, TPC, TOF, HMPID, MUONS, PMD, V0, T0, FMD, ZDC, ACORDE, TRIGGER, DAQPartially completed:TRD (20%) to be completed by 2009PHOS (40%) to be completed by 2010EMCAL (0%) to be completed by 2010/11HLT (High Level Trigger) (~50%)

At start-up full hadron and muon capabilities,Partial electron and photon capabilities


ALICE StatusComplete: 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 Detector Installation Goal 2009Number of TRD & EMCAL modules depends on access conditions during LHC power test !2008/09 Shutdown used for additional installation and repairs*P. KuijerQM 2009


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 8000p/p < 5% at 100 GeV with careful control of systematics


Alice particle identificationstable hadrons (, K, p): 0.1 5 GeV/c, 0 in PHOS: 1

Cosmic event with SPD triggerProbably a muon interaction in the magnets iron

about 350 tracks reconstructed in TPC


Cosmic event with ACORDE trigger


p beam in ALICELHC pilot beam at 450 GeV: p-Si collision in the SPD (Sept. 12, 2008)


Breve introduzione:cosa ci aspettiamo di imparare studiando le osservabili soft a LHC?Ovvero: dati i risultati ottenuti a RHIC c ancora qualcosa da scoprire a LHC?


Reminder: space-time evolutionThermal freeze-outElastic interactions ceaseParticle dynamics (momentum spectra) fixedTfo (RHIC) ~ 110-130 MeV

Chemical freeze-outInelastic interactions ceaseParticle abundances (chemical composition) are fixed (except maybe resonances) Tch (RHIC) ~ 170 MeV

Thermalization timeSystem reaches local equilibriumteq (RHIC) ~ 0.6 fm/c


Introduction/1Initial 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 spectraChemical composition: will statistical models still work at LHC energy? Measuring hadron yields (& their ratios) will give the answer (limiting temperature of 160 MeV?)


Introduction/2Nature 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 diagramLesson 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


Molteplicit di particelle cariche


Charged multiplicity at the LHCExtrapolation of dNch/dmax vs s:Fit to dN/d ln s (limiting fragmentation) or Saturation model (dN/d s with =0.288)?Clearly distinguishable with the first 10k LHC eventsModels prior to RHICExtrapolation of dN/dh ln s:5500Saturation modelArmesto Salgado Wiedemann, PRL 94 (2005) 022002Central collisionsincreasing s decreasing x


dNch/d: PHOBOS extrapolationPHOBOS 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)W. BuszaQM 2009Au+Au Data from PHOBOS, Nucl. Phys. A757 (2005) 28


Total particle productionPHOBOS: Nucl. Phys. A 757 28 (2005)E178: PRL34(1975)836Wit Busza: Acta. Phys. Pol. B35(2004)2873Similarity of total particle production ine+e-, pp, A, KA, pA and AA collisionsWhat is the mechanism that makes the total particle production insensitive to the intermediate state?W. BuszaQM 2009


Charged multiplicity in ALICEReconstructed dNch/dwith tracklets in SPD(generated dNch/d = 3000)Wide rapidity coverage provided by ITS (SPD), TPC in the central region & FMD in the forward regionALICE PRELIMINARYT. Virgili


dNch/d in p+p: SPDdNch/d from tracklets (vertex + 2 SPD layers): larger and pT acceptance less stringent calibration/alignment needsF. PrinoM. NicassioQM 2009ALICE PRELIMINARYALICE PRELIMINARYSeveral 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


dNch/d in p+p: FMDALICE PRELIMINARYALICE PRELIMINARYH.H. DalsgaardThe FMD will complement the dNch/d measurementfor -3.4

Flusso direzionato (v1) ed ellittico (v2)


Anisotropic flowAzimuthal asymmetry in coordinate space (transverse plane):Kolb + Heinzproduces azimuthal asymmetry in momentum space:v1 = directed flow v2 = elliptic flowThe amount of observed flow depends on centrality and on the spatial eccentricity :


Flow and reaction planeThe Fourier expansion is referred to the reaction plane angle (RP) which must be located in the experiments reference frameThe 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)Reaction plane (xz)Figures courtesy F. PrinoProduced particlesangular distributionin the transverse plane (xy)


Directed flow in ALICEv1 can be measured in ALICE via spectator neutrons (>8.7), namely by their centroids as obtained by the two zero-degree ZN calorimeters2.76 TeVA Pb-Pb minimum bias eventsHIJING simulationFor a range of plausible v1 values (10% 20% 30%) at LHC, the first order event plane resolution obtained by combining both ZNs is quite adequateIn addition, this measurement provides the sign of v2N. Demarco7.04 cm


Elliptic flow: s dependenceFrom the observed v2 dependence on s one expects v2(0) @ LHC/ALICE ~ 0.08v2 (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 energiesLarge signal easy measurement, but..beware of non-flow contributions (jets...)!T. Hirano, U. Heinz, D. Kharzeev, R. Lacey, Y. Nara, QM 2008


Elliptic flow and eccentricityThe 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)coordinate spacemomentum spacev2 fluctuations (in events of the same centrality class) are closely related to fluctuations of part - which can be computed via Glauber MonteCarloA. PoskanzerQM 2009


Elliptic flow: different methodsProblem: measurement of v2 is affected by both non-flow and fluctuationsDifferent methods to extract v2 have been developed, based on event plane, 2-particle or many-particle correlations: they give different resultspublishedSTAR, J. Adams et al., PRC 72, 014904 (2005) but they agree on mean v2 in Participant Plane!corrected to PPA. Poskanzer QM 2009


Elliptic flow: one result?A v2 for theorists (corrected to Reaction Plane):corrected to RPVoloshin, Poskanzer, Tang, and Wang, Phys. Lett. B 659, 537 (2008)GlauberCGCreaction planeStill, some dependence on the assumption (Glauber vs. CGC) about fluctuations


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 controlShear viscosity () reduces v2: comparing hydrodynamics calculations with RHIC data, with high confidence one can conclude: /s < 0.5 ~6/4M. Luzum and P. Romatschke, 2008 (erratum 2009)


More on data vs. hydrodynamicsParametrization 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)R. Snellings, QM 2009The B parameter scales with /s but is also sensitive to the EoS:STAR data are well described using a CGC with soft EoS and /s ~ 2/4 or Glauber with hard EoS and /s ~ 4/4The shift of the peak of v2 as a function of pT may be an additional independent way of extracting /s


Elliptic flow in ALICEhydroLDLdNch/dh =2000v2/=0.33v2/=0.22Centrality classALICE 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).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)]


Flow: event plane & other methodsv2 vs pTfor v2 =0.0625event plane resolution (TPC)vs. v2 for 1000 charged tracksReaction PlaneCumulantsLee Yang Zeroes


Flow with inner ITS layersLayer 1 < 2.0Layer 2 < 1.4 SPD alone useful for low multiplicity events, and has higher acceptancewrt TPC (low pT threshold ~15 MeV) Simulations suggest flow for negative and positively charged particles separately coarse pT binning may be possible In addition, FMD and PMD will measure flow at forward rapidities


Spettri in pT e rapporti Composizione chimica, meccanismi di adronizzazione


Statistical hadronizationStatistical 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 BOther model parameters such as: fireball volume, I3, S are constrained from initial stateThese 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


Statistical hadronization at LHCTemperature Tch increases 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])mBLHC=0.8(+1.2,-0.6) MeVTLHC = 1614 MeV


Particle spectra in ALICETransverse momentum ranges for particle identification in ALICE (central barrel):Expected charged hadrons yields for 107 Pb-Pb central collisions (TPC PID on statistical basis)


Antiparticle/particle ratiosW. BuszaQM 2009Phys. Rev. C 77, 061901(R) (2008)

Phys. Rev. C 71, 021901(R) (2005)P. Christakoglou, M. OldenburgSystematic error on asymmetry

Identified hadrons spectra (TPC)A. KalweitALICEcosmicsreal dataALICEsimulation


Strange particle spectra: optimized, pT-dependent selection cuts


How to fit spectra?Exponential in mT (Boltzmann)LevyTsallis blast-wave (power law in mT)B. Hippolyte, H. RicaudCrucial to obtain the total yield (extrapolation down to pT=0: 7-15% of the yield is not measured directly)The proposed functional forms include:


Baryon/meson ratios vs. pT/1High baryon to meson ratio (~1) at intermediate pT discovered at RHIC in Au+Au reactions, inconsistent with pQCD predictionsBaryon 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 uniquelySarah Blyth (STAR) QM 2006Au+Au 200 GeV


Baryon/meson ratios vs. pT/2B. Hippolyte, H. RicaudThe 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.


Risonanze


Why resonances?Resonance Life-time [fm/c] r(770) 1.3++(1232) 1.7 f0(980) 2.6 K*(892) 4.0 S*(1385) 5.7L*(1520) 13 (783) 23(1020) 45 Decay time comparable with (or even shorter than) QGP lifetime

Rescattering and regeneration between chemical and kinetic freeze-out will affect the final yields

Comparing leptonic and hadronic final states provides further insight


Resonances at RHICP. Fachini QM 2009Mass shift ~45 MeV/c2 observedM. Naglis QM 2009


Resonances in ALICE Invariant mass reconstruction, background subtracted (like-sign method) mass resolutions ~ 1.5 - 3 MeV/c2 and pT stat. limits from 8 (r) to 15 GeV/c (f, K*)r0(770) p+p-106 central Pb-Pb Mass resolution ~ 2-3 MeV K*(892)0 K p 15000 central Pb-Pbf (1020) K+K-Mass resolution ~ 1.2 MeVGenerated & reconstructedf for 107 central Pb-PbInvariant mass (GeV/c2)ALICE PRELIMINARYA. Badal, A. Pulvirenti


Correlazioni HBT (Hanbury-Brown Twiss, a.k.a. Femtoscopy)


No more RHIC HBT puzzle?RHIC HBT PUZZLE: 1st order phase transition flow & spectra OK ideal hydro (no viscosity) HBT radii NOT OK 0=1.0 fm/cS. Pratt, QM 2009Solution:Early acceleration (t < 1 fm/c)Shear viscosityEoS (crossover)Initial energy profile


Another solution to the HBT puzzle1) semi-hard EoS2) Single freezeout modeled by THERMINATOR3) Initial conditions: 2-D gaussian in transverse plane, from Glauber MCSpectra, v2 and HBT radii well reproducedW. Florkowski WPCF 2008


Femtoscopy in ALICE/1Projected 3-D two-pion correlationfunction C2 for LHC Pb+Pb collisions,for b=8 fm centrality and pT bin 0-200 MeV/c


Femtoscopy in ALICE/2Anti-merging cut:Tracks that would bemerged in the TPC if they were in the sameevent have to be removed from the background(mixed event sample)Anti-splitting cut:here the effect of the cut on identical K correlations is shown


Fluttuazioni


QCD phase transition / Critical PointB. Mohanty QM 2009Lattice and other QCD based models :B = 0 Cross-over transitionTC ~ 170-195 MeVB > 160 MeV QCD critical pointExperiments:See distinct signatures that relevant d.o.f. are quark and gluonsTinitial(direct photons) > TC(Lattice) - PHENIX: direct photon arXiv:0804.4168No signature of QCD critical point established, possible hints at SPS (see C.Hoehnes talk)


Phase transition at B ~ 0Fluctuations at deconfinement transition visible through: Second moment of event-by-event distributions of multiplicity, mean pT, mean ET after removing non-dynamical fluctuations Fluctuations in particle ratios: sensitive to particle numbers at chemical FO (not kinetic FO), volume effects should cancel Fluctuations in conserved quantities: net charge, net baryon number, net strangenessB. Mohanty QM 2009See also: Spontaneous strongParity violation at RHICB. Voloshin QM 2009


Fluctuation results (QM 2009)

ObservableExperiment (Beam energy in GeV)ConclusionsK/ and p/NA49(6.3 - 17.3)arXiv:0808.1237M.I. Gorenstein et al, arXiv:0811.3089p/fluctuations : similar results from UrQMDK/ higher than UrQMD at lower energyHSD transport gives similar energy dependenceK/ and p/STAR(19.6 - 200 GeV)arXiv: 0901.1795G. Westfall - WWND09K/ : Statistical hadronisation model (q>1) agrees. HSD transport model similar results p/fluctuations similar to default UrQMD

ObservableExperiment (Beam energy in GeV)ConclusionsNet-chargeSTAR (19.6 - 200 GeV) p+p, Cu+Cu, Au+Au arXiv:0807.3269Lie between charge conservation effects and resonance gas model. Net-chargeNA49 (6 - 17 GeV) PRC 70,064903 (2004)Consistent with charge conservationNet-chargePHENIX (130 GeV)PRL 89, 082301 (2002)Similar to RQMD calculations.


EbyE in ALICE/1HIJING PbPb events @ sNN = 5.5 TeVParticle ratios protons from a singleHIJING central event:T = 31913 MeVgeneratedreconstructed300 central eventsTemperature fluctuations:C. Zampolli


EbyE in ALICE/2P. Christakoglou WPCF 2008 (Cracow)Balance functions


Future explorationsC. Hhne QM 2009complete scan of the QCD phase diagram with modern, 2nd generation experiments on the horizon! 30A GeVRHIC beam energy scan- evolution of medium properties- turn-off of established signatures- search for CP and PTNA61 at SPS (2007 acc. by SPSC)- search for CP and PT in energy-system size scanboth essentially limited to high yield observables- RHIC: energy dependentFAIR and NICA- new accelerator projects- FAIR: high intensities! rare probes!


Protone-protoneAlcuni esempi delle potenzialit di ALICE con i dati p-p del primo giorno / primo mese (Ecm = 10 TeV)


Charged multiplicityAfter unfoldingALICE PRELIMINARY


pT spectra: and anti-Tracking in ITS+TPCTopological PID (decay vertex)Gamma conversions partly removedNo correction for absorption


Resonances Example of first day analysis with TPC only, no PIDpp 10 TeV With 100K - a few 100K events: with PID, f and K* spectra feasible up to 3-4 GeVWith 1M a few M events: f reconstruction feasible without PID, being studied for K*


EbyE particle ratios M. FragkiadakisM. VassiliouSimulated pp data at 10 TeVITS, TPC and TOF PID + combined PID Effic. & Contamination

K/ and p/ Event-by-Event particle ratios extracted verified that (as expected) dynamical fluctuations in pp (PYTHIA) are zero ALICE PRELIMINARY


ConclusionsALICE detectors are installed, apart from full e.m. calorimetry (due for completion in 2011)All installed detectors are being commissioned with cosmic muons: calibration and alignment are well under wayAnalysis procedures have been developed and tested on the Grid, both for Pb+Pb and p+p first phyiscsALICE is well equipped, both detector-wise and analysis-wise, to produce a wealth of soft physics results with first p+p and Pb+Pb collisions


ALICE ReferencesALICE Physics Performance Report:Volume 1: F. Carminati et al., J. Phys. G. Nucl. Part. Phys. 30 (2004) 1517Volume 2: B. Alessando et al., J. Phys. G. Nucl. Part. Phys. 32 (2006) 1295ALICE Detector technical paper: K. Aamodt et al., The ALICE Experiment at the CERN LHC, 2008 JINST 3 S08002.http://aliceinfo.cern.ch/


Many thanks toALICE collaborators, in particular for their recent presentations/contributions:Bruno AlessandroPanos ChristakoglouAngela BadalBoris HippolyteChristian KuhnFrancesco PrinoAlberto PulvirentiEnrico Scomparin


RUN 60824, 20-SEP-2008SPD trigger: 7-8 events per minute, we can estimate 1-2 events per hour with high multiplicityRUN 61869, 06-OCT-2008 Event 9161ACORDE trigger rate: 82 HzD. Elia, M. Nicassio: PWG2 March 2009H.H. Dalsgaard, PWG2 Feb. 2009Radial flow is isotropic and has consequences for pT / mT spectra of particles having different mass.The angle Psi_RP in the transverse plane is determined by the impact parameter vector b.N. Demarco, PWG2 Dec. 2007; PHOBOS v1: nucl-ex/0510030; STAR v1: Phys. Rev. C 73, 034903 (2006).

See also: Voloshin et al., Phys. Lett. B 659, 537 (2008)

Assumptions needed to separate non-flow from fluctuations (see Poskanzer QM2009, slides 11-15)Matthew Luzum, Paul Romatschke arXiv:0804.4015Plus possibly a third parameter: strangeness suppression factor - gamma_s which is however not relevant at RHIC and LHC energies.W. Busza, QM 2009RHIC study: see arXiv:0812.1609v2[nucl-ex] c*Tau [fm] = (h/2pi)*c/Gamma = 197/Gamma[MeV]f0(980): Gamma = 40 to 100 MeV, assumed 75 MeVSee also: Gombeaud at QM 2009Jan Pluta, Erice 2005

Osservabili ‘soft’ in ALICE

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