Exploring the Quark-Gluon Plasma with ALICE at the LHC

Transcription

1 Exploring the Quark-Gluon Plasma with ALICE at the LHC Yvonne Pachmayer, University of Heidelberg Introductory remarks Selected results Pb-Pb collisions Global observables Hard probes Summary and outlook Pb-Pb snn = 2.76 TeV (2011)

2 Quantum Chromodynamics Theory of strong interaction Part of the standard model Quarks as constituents, gluons as interaction carriers Confinement Quarks and gluons are not observed as free particles, they are confined in hadrons Baryon Meson Chiral symmetry breaking Hadrons are much heavier than their constituents 2

3 Asymptotic Freedom Coupling α s between color charges gets weaker for high momentum transfers, i.e. for small distances r Vanishing QCD coupling constant at short distances r implies that the interactions of quarks and gluons are negligible at very high temperatures creation of practically non-interacting quark-gluon plasma at extreme temperatures S. Bethke Eur.Phys.J. C64 (2009) 3

4 Tracing back the Big Bang Electroweak phase transition QCD phase transition x Tcore sun Non perturbative! 4

5 Predictions from First Principles: Lattice QCD 4 ε/t ~ # degrees of freedom F. Karsch and E. Laermann, arxiv: hep lat/ Hadron HadronGas Gasto toqgp QGPphase phase transition transition many d.o.f. deconfined few d.o.f. confined 5

6 Expected QCD Phase Diagram 6

7 LHC Heavy Ion Program CERN-SPS Fixed target program since 1986 snn 20 GeV press release February 2000 Discovery of new State of Matter BNL-RHIC Heavy ion collider program since 2000 snn 200 GeV press release April 2005 Appears to be more like a liquid What is different at LHC? Large energy: snn = 2.76 TeV Large abundance of jets and heavy quarks, very large volumes, temperatures, densities, decoupling times Characterize the Quark-Gluon Plasma 7

8 Time Evolution of a Heavy Ion Collision adapted from A. Mocsy Quark-Gluon Plasma Phase of strongly interacting matter Hydrodynamic expansion Chemical and Thermal Freeze-out 8

9 Geometry Plays a Key Role in Ultra Relativistic Heavy Ion Physics Number of participants: number of nucleons in the overlap region Number of binary collisions: number of inelastic nucleon nucleon collisions Small impact parameter b corresponds to large particle multiplicity 9

10 Central Pb-Pb Collision Pb-Pb snn = 2.76 TeV (2011) : pp at s = 0.9, 2.76, 7 and 8 TeV Autumn 2010: Pb-Pb at snn = 2.76 TeV Autumn 2011: Pb-Pb at snn = 2.76 TeV Autumn 2012: p-pb at snn = 4.4 TeV 10

11 The Large Hadron Collider and ALICE LHC 8.6 km World s largest and most powerful particle accelerator Four large experiments ALICE dedicated to heavy-ion physics 11 11

12 A Large Ion Collider Experiment The Collaboration 1300 Members 35 Countries 120 Institutes ALICE Covers Forward Muon Arm: -2.4 η -4.0 Central Barrel: -0.9 η 0.9 STRIP ACORDE PIXEL ITS EMCAL FMD T0 & V0 TRD HMPID PMD ZDC V0 T0 FMD TRACKING TRACKING CHAMBERS CHAMBERS MUON FILTER ~116m from IP V0 T0 TRIGGER CHAMBERS TPC ZDC ALICE measures Hadrons, muons, electrons, photons ~116m from IP TOF Transverse impact parameter resolution PHOS < 75(20) μm for pt > 1(20) GeV/c ABSORBER DIPOLE MAGNET Momentum resolution <2% for p < 20 GeV/c Particle identification with various systems Drift Size: 16 x 26 meters Weight: 10,000 tons Subsystems: 18 Tracking: 7 PID: 6 Calo.: 5 Trigger, Nch:

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14 Particle Identification in ALICE ITS Silicon Drift/Strip de/dx TPC de/dx TRD TOF Excellent particle identification from low to high momenta 14

15 Anti-Particle Identification Antimatter Helium-4 nucleus Heaviest observed antinucleus TPC de/dx 15

16 Global Observables Charged Particle Multiplicity in Central Pb-Pb Collisions Charge particle multiplicity increases with Bjoerken estimate: beam energy significantly steeper than in pp Multiplicity factor 2.1 larger than at top RHIC energy (200 GeV) 2 ετ ~ 15 GeV/fm c dn /dη = 1600 in central Pb-Pb collisions ch (~factor 3 larger than RHIC) (~3000 in +/- 45 ) ε α T4 30% temperature increase Predictions cover wide range of QGP compared to RHIC many excluded by very first data 16

17 Global Observables Hanbury Brown-Twiss Interferometry and Space-Time Extent of Fireball Technique of intensity interferometry developed by Hanbury-Brown and Twiss in astrophysics as a means to determine size of distant objects γ (π) r, t γ (π) J. Stachel p, E Freeze-out volume huge growth at LHC From Rlong: expansion at LHC 10 fm/c 17

18 Global Observables Identified Particle Ratios 0-5% centrality Very similar particle/anti-particle production Net baryon density compatible with 0 18

19 Global Observables Particle Ratios Comparison with Thermal Model Thermal model: A. Andronic et al., Phys.Lett.B673: ,2009 Same results from THERMUS S. Wheaton, J. Cleymans and M. Hauer, Comput. Phys. Commun. 180 (2009) Tch= 164 MeV μb = 1 MeV All yields (except protons) described by thermal model for grand-canonical ensemble with Tch= 164 MeV and μb= 1 MeV Proton/pion ratio below expectation Tension already present at RHIC Strange particles constrain fit 19

20 Global Observables Collective Effects Fourier decomposition of momentum distribution relative to reaction plane z Reaction plane dn 1 2v n p T cos n φ Ψ RP dφ n=1 2v2 y x y py φ x px Coordinate space: initial asymmetry Collective interaction pressure Momentum space: final asymmetry 20

21 Integrated Elliptic Flow Collision Energy Dependence Viewpoint: A 'Little Bang arrives at the LHC E. Shuryak Physics 3 (2010) 105 Some researchers expected that the QGP at LHC would be more weakly interacting due to higher temperature leading to - larger mean free path of particles - larger viscosity/entropy density - to smaller flow components (v2) QGP remains a nearly ideal liquid Hydro behavior continues at LHC v2 (pt int.) LHC ~1.3x (pt int.) RHIC The overall increase is consistent with the increased radial expansion leading to a higher mean pt 21

22 Identified particle elliptic flow Pronounced mass dependence due to radial flow Hydrodynamics predicts mass ordering: v2 1 ( pt v m T ) T <ν> = average flow velocity Hydrodynamic model predictions are able to describe the data for π, K v 2 = cos ( 2 ( φ Ψ RP ) ) Small viscosity/entropy density 22

23 Triangular flow Initial spatial anisotropy and its fluctuations lead to event by event fluctuating symmetry planes Odd harmonics are not zero Triangular flow (v3) only weak centrality dependence When calculated w.r.t. participant plane v3 vanishes More sensitive to shear viscosity η/s and initial conditions Small viscosity/entropy density Perfect liquid 23

24 Higher Harmonics 0-2% most central Pb-Pb ALICE Collaboration arxiv: v1 24

25 Higher Harmonics 0-2% most central Pb-Pb cosmic microwave background radiation (WMAP data) power spectrum - typical angular separation of the fluctuations ALICE Collaboration arxiv: v1 25

26 Higher Harmonics 0-2% most central Pb-Pb ALICE Collaboration arxiv: v1 B. Schenke et al. e-by-e hydro Viscosity suppresses higher harmonics, vn provide additional sensitivity to η/s 26

27 Hard Probes Hard probes are highly penetrating observables (particles, radiation) used to explore properties of matter that cannot be viewed directly High pt hadrons, jets Open heavy flavors (charm and beauty) Quarkonia (J/Ψ, Ψ(2S), Ψ(1S), Ψ(2S), Ψ(3S)) Hard probes originate from hard processes, characterized by large momentum transfer Q2, where pqcd can be applied Well understood in vacuum (pp collisions) 27

28 Hadron Production High p T hadrons/open heavy flavours are produced in hard scattering, in the earliest collision time, by fragmentation of high p T partons Hadron production cross section in pp can be calculated in pqcd Modification in the nucleus? Modified in Pb-Pb collisions! Depends on the properties of the medium (temperature, density, color charge, Transport coefficient,...) p-pb measurement autumn 2012 Quenching 28

29 Nuclear Modification Factor RAA R AA p T = 1 2 d N AA /dp T dy N coll d 2 N pp / dpt dy Needs pp reference at same s! At high pt: RAA = 1 if no nuclear modification! 29

30 Charged particle RAA pp Reference pp: power law at high pt Consistent with pqcd 30

31 Charged particle RAA Pb-Pb Measurement Strong discrepancy from scaled pqcd result Strong suppression of particle production at high pt in central Pb-Pb collisions 31

32 Charged particle RAA R AA p T = 1 2 d N AA /dp T dy N coll d 2 N pp / dpt dy Max. suppression at pt ~ 7 GeV/c Rise at pt ~ GeV/c Shape of pt distribution changes with collision centrality different suppression pattern depending on collision centrality Hint of leveling off above pt =30 GeV/c - maybe pqcd limit is never reached? 32

33 Charged particle RAA R AA p T = 1 2 d N AA /dp T dy N coll d 2 N pp / dpt dy All models tuned with RHIC data Rise qualitatively described Strength of suppression model dependent LHC data very sensitive to details of energy loss mechanisms Program for the next years 33

34 Energy Loss in the Medium Heavy Quarks Heavy quarks (charm, beauty) expected to be produced in primary partonic scatterings and to coexist with the surrounding medium Energy loss depends on Properties of the medium (gluon densities, size) Properties of the probe (color charge, mass) Dead cone effect Gluon radiation is suppressed for angles θ < MQ/EQ Heavy flavour energy loss should be smaller than the one of light hadrons: ΔEg > ΔEup,down > ΔEcharm > ΔEbeauty RAA (light hadrons) < RAA (D) < RAA (B) path length L Color Charge CR=3 for gluons, CR=4/3 for quarks Dokshitzer and Kharzeev, PLB 519 (2001) 199. Armesto, Salgado, Wiedemann, PRD 69 (2004) Djordjevic, Gyulassy, Horowitz, Wicks, NPA 783 (2007) 493. BDMPS approach Possible other mechanisms for the interaction with the medium Transport coefficient related to (collisional energy loss, in-medium dissociation, resonance scattering) medium characteristics and gluon density 34

35 D Meson RAA pp reference at 2.76 TeV: measured 7 TeV spectrum scaled with FONLL Cross-checked against measured result at 2.76 TeV (large errors and limited pt-coverage due to short data taking period) arxiv: R. Averbeck et al, arxiv: Suppression in central collisions: factor 3-4 for pt > 5 GeV/c Suppression for 40-80% CC: factor 1.5 for pt > 5 GeV/c Reduce errors with Pb-Pb data from 2011 run: ~factor 6-7 more statistics in 0-7.5% CC 35

36 D Meson RAA RAA (π) < RAA (D) < RAA (B)? arxiv: arxiv: (CMS) Initial State Effects Comparison to NLO + shadowing AA coincides with charged hadron RAA for pt > 5 GeV/c (EPS09 parametrization) Strong suppression likely to be a final state effect p-pb measurement end of 2012 will give decisive answer Preliminary charged π R Hint of RAA (π) < RAA (D) RAA (D) < RAA (B)? not conclusive different pt range Reduce errors (2011 Pb-Pb run) 36

37 D0 and D+ Meson v2 in 30-50% Collision Centrality arxiv: (BAMPS), arxiv: (Aichelin et al.) Indication of non zero v2 3σ effect in 2-6 GeV/c for D0 D meson v2 comparable with v2 of charged hadrons measured with ALICE in the same rapidity region Models based on charm transport in the medium predict a non-zero v2 of up to

38 D0 and D+ Meson v2 in 30-50% Collision Centrality arxiv: arxiv: (BAMPS), arxiv: (Aichelin et al.) Indication of non zero v2 3σ effect in 2-6 GeV/c for D0 D meson v2 comparable with v2 of charged hadrons measured with ALICE in the same rapidity region Models based on charm transport in the medium predict a non-zero v2 of up to , but do not describe well the RAA 38

39 Charmonia (I) State J/ψ χc ψ' Mass (GeV/c2) Radius (fm) Bound state of heavy quarks (cc) Original idea (1986) Implant charmonia into the QGP and observe their T/TC modification (Debye screening of QCD) Sequential melting J/ψ Estimate initial temperature c χc c ψ C o lo r S c r e e n in g L. Kluberg and H. Satz, arxiv:

40 Charmonia (II) New insight (2000) QGP screens all charmonia, but charmonium production takes place at the phase boundary Enhanced production at high energy Signal for deconfinement Start of Collision Development of QGP Hadronisation P. Braun-Munzinger and J. Stachel, arxiv:

41 J/ψ Meson Reconstructed via μ+μ- decay Challenging measurement in Pb-Pb collisions Resonance well visible despite large combinatorial background Down to pt = 0 GeV/c, rapidity range: 2.5 < y < Pb-Pb collisions Proton-proton collisions: provide important reference R AA ( pt )= 1 N coll d N 2 AA 2 pp /dp T dy d N / dpt dy 41

42 J/ψ Nuclear Modification Factor RAA Larger RAA (at forward rapidity) at LHC compared to RHIC Hint of J/ψ regeneration 42

43 J/ψ Nuclear Modification Factor RAA Larger RAA (at forward rapidity) at LHC compared to RHIC Hint of J/ψ regeneration Precisely measure charm cross section in nucleus collisions Need p-pb collisions: disentangle initial and final state effects 43

44 Plans and Outlook Have created the hottest matter ever on Earth T > 2 x 1012 K, > 100,000 times hotter than the core of Sun It has characteristics of a soup of quarks and gluons It flows like a liquid, better than any we know or have made The LHC is a hard probes machine There has been a burst of new data and observables. Much more to come, leading to full characterization of this new (and old) state of matter Upcoming p+pb run in fall 2012 will reduce some of the uncertainties due to initial state effects Upgrade for

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46 A Large Ion Collider Experiment ACORDE STRIP Drift PIXEL ITS EMCAL FMD T0 & V0 TRD HMPID ZDC TRACKING CHAMBERS PMD ~116m from IP V0 T0 TPC MUON FILTER V0 T0 FMD TRIGGER CHAMBERS ZDC ~116m from IP TOF PHOS DIPOLE ABSORBER MAGNET B-Field: 0.5 T Forward Muon Arm: -2.4 η -4.0 Central Barrel: -0.9 η 0.9 Transverse impact parameter resolution < 75(20) μm for pt > 1(20) GeV/c Momentum resolution <2% for p < 20 GeV/c Particle identification with various systems 46

47 Higher Harmonics 0-2% most central Pb-Pb Sensitivity to in-medium speed-of-sound and to expansion velocity ALICE Collaboration arxiv: v1 P. Staig, E. Shuryak arxiv: v2 (initial perturbation 0.7fm; Tf = 120 MeV) 47

48 Signal Extraction for D Meson v2 Event plane determination from Q-vector n w i cos ( 2 φi ) Q2= i 1 n w i cos ( 2 φi ) i 1 Using tracks in TPC at central rapidity φi-weights to improve EP flatness Signal extraction in-plane and out-of-plane Calculate v 2 π N inp N outp v 2= 4 N inp + N outp Correct for EP resolution 48