Melting the QCD Vacuum with Relativistic Heavy-Ion Collisions

Transcription

1 Melting the QCD Vacuum with Relativistic Heavy-Ion Collisions Steffen A. Bass QCD Theory Group Introduction: the Quark-Gluon-Plasma How can one create a QGP? Basic tools for a Theorist: Transport Theory Examples of High-Impact Research Near-Ideal Fluids & Elliptic Flow Jet Energy-Loss Parton Recombination Heavy Quarks work supported through grants by

2 11 Science Questions for the New Century formulated by the National Research Council: 1. What is dark matter? 2. What is dark energy? 3. How were the heavy elements from Iron to Uranium made? 4. Do neutrinos have a mass? 5. Where do ultra-high energy particles come from? 6. Is a new theory of light and matter needed to explain what happens at very high energies and temperatures? 7. Are there new states of matter at ultra-high temperatures and densities? 8. Are protons unstable? 9. What is gravity? 10. Are there additional dimensions? 11. How did the Universe begin?

3 Introduction: Phases QCD Matter

4 Phases of Matter Ordinary Matter: phases determined by (electromagnetic) interaction apply heat & pressure to study phase-diagram Phases of QCD matter: heat & compress QCD matter: collide heavy atomic nuclei numerical simulations: solve partition function (Lattice)

5 QCD on the Lattice Goal: explore the thermodynamics of QCD evaluate QCD partition function: path integral with N steps in imaginary time can be numerically calculated on a 4D Lattice Equation of State for an ideal QGP: (ultra-relativistic gas of massless bosons) F. Karsch LGT predicts a phase-transition to a state of deconfined nearly massless quarks and gluons QCD becomes simple at high temperature and/or density

6 QGP and the Early Universe a few microseconds after the Big Bang the entire Universe was in a QGP state compressing & heating nuclear matter allows to investigate the history of the Universe the only means of recreating temperatures and densities of the early Universe is by colliding beams of ultrarelativistic heavy-ions

7 Telescopes for the Early Universe: Heavy-Ion Collider Facilities

8 Heating & Compressing QCD Matter The only way to heat & compress QCD matter under controlled laboratory conditions is by colliding two heavy atomic nuclei!

9 Heating & Compressing QCD Matter ALICE CERN: typical Pb+Pb LHC: typical Pb+Pb LHC: scientists from 105+ institutions dimensions: 26m long, 16m high, 16m wide weight: 10,000 tons 1000s of tracks task: reconstruction of final state to characterize matter created in collision two more experiments w/ Heavy-Ions: CMS, ATLAS

10 The ALICE Experiment tracking of low momentum hadrons with p T >100 MeV large solid angle good particle-id for low and intermediate momenta

11 RHIC: A dedicated QGP Machine Brookhaven National Laboratory: Relativistic Heavy-Ion Collider 2 large experiments (STAR, PHENIX) 2 small experiments (PHOBOS, BRAHMS) scientists from 80+ institutions typical RHIC: 1000s of tracks task: reconstruction of final state to characterize matter created in collision

12 Tools of the Trade Transport Theory

13 Probing QCD in Heavy-Ion Collisions initial state QGP and hydrodynamic expansion hadronic phase and freeze-out Challenges: pre-equilibrium hadronization time-scale of the collision process: seconds! [too short to resolve] characteristic length scale: meters! [too small to resolve] confinement: quarks & gluons form bound hadronization, experiments don t observe them directly Experiments: Lattice QCD: Transport-Models: observe only the final state rely on QGP signatures predicted by Theory rigorous calculation of QCD properties in equilibrium full description of collision dynamics connect intermediate state to measurements & lattice

14 Transport Models for RHIC microscopic transport models based on the Boltzmann Equation: transport of a system of microscopic particles all interactions are based on binary scattering t + p E r f 1(p, r, t) = processes diffusive transport models based on the Langevin Equation: C(p, r, t) transport of a system of microscopic particles in a thermal medium interactions contain a drag term related to the properties of the medium and a noise term representing random collisions p(t + t) =p(t) 2T v t+ (t) t (viscous) relativistic fluid dynamics: transport of macroscopic degrees of freedom based on conservation laws: µt µ =0 T ik = u i u k + P ( ik + u i u k ) iu k + k u i 2 3 ik u + ik u (plus an additional 9 eqns. for dissipative flows) hybrid transport models: combine microscopic & macroscopic degrees of freedom current state of the art for RHIC modeling Each transport model relies on roughly a dozen physics parameters to describe the time-evolution of the collision and its final state. These physics parameters act as a representation of the information we wish to extract from RHIC.

15 Visualization of a Model Calculation 3+1D Hydro + Boltzmann Hybrid

16 initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium hadronization Near-Ideal Fluids & Elliptic Flow S.A. Bass & A. Dumitru, Phys. Rev C61 (2000) D. Teaney et al, nucl-th/ T. Hirano et al. Phys. Lett. B636 (2006) 299 C. Nonaka & S.A. Bass, Phys. Rev. C75 (2006)

17 RHIC in the press: Perfect Liquid on April 18 th, 2005, BNL announced in a press release that RHIC had created a new state of hot and dense matter which behaves like a nearly perfect liquid. how does one measure/ calculate the properties of a near ideal liquid? are there any other near ideal liquid systems found in nature? Steffen A. Bass

18 Relativistic Fluid Dynamics (RFD) transport of macroscopic degrees of freedom based on conservation laws: µ T µν =0 µ j µ =0 for ideal fluid: T µν = (ε+p) u µ u ν - p g µν and j i µ = ρ i u µ Equation of State needed to close system of PDE s: p=p(t,ρ i ) connection to Lattice QCD calculation of EoS initial conditions (i.e. thermalized QGP) required for calculation assumes local thermal equilibrium, vanishing viscosity applicability of hydro is a strong signature for a thermalized system Viscosity: shear and bulk viscosity are defined as the coefficients in the expansion of the stress tensor in terms of the velocity fields: 2 T ik = u i u k + P ( ik + u i u k ) i u k + k u i 3 ik u + ik u viscous RFD requires solving an additional 9 eqns. for the dissipative flows Note: for quasi-particulate matter, viscosity decreases with increasing cross section for viscous RFD, the microscopic origin of viscosity is not relevant!

19 Collision Geometry: Elliptic Flow two nuclei collide rarely head-on, but mostly with an offset: Reaction plane z y only matter in the overlap area gets compressed and heated up x elliptic flow (v 2 ): gradients of almond-shape surface will lead to preferential emission in the reaction plane asymmetry out- vs. in-plane emission is quantified by 2 nd Fourier coefficient of angular distribution: v 2 vrfd: good agreement with data for very small /s

20 Elliptic flow: early creation initial energy density distribution: spatial eccentricity momentum anisotropy Most model calculations suggest that flow anisotropies are generated at the earliest stages of the expansion, on a timescale of ~ 5 fm/c if a QGP EoS is assumed. P. Kolb, J. Sollfrank and U.Heinz, PRC 62 (2000)

21 Elliptic flow: ultra-cold Fermi-Gas Li atoms at release from an optical trap: initial almond shape, similar to interaction area in heavy-ion collision Li-atoms released from an optical trap exhibit elliptic flow analogous to what is observed in ultrarelativistic heavy-ion collisions Elliptic flow is a general feature of strongly interacting systems! K. M. O Hara, S. L. Hemmer, M. E. Gehm, S. R. Granade, J. E. Thomas: Science 298 (2002) 2179

22 initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium hadronization Jet Energy-Loss

23 Jet-Quenching: Basic Idea hadrons q q leading particle suppressed suppression can be experimentally quantified in terms of R AA ratio: R AA d 2 N AA /dydp d 2 N pp /dydp Ncoll AA hadrons leading particle suppressed partons lose energy and/or fragment differently than in the vacuum: radiative energy loss q L ˆq = ρ dσ q 2 dq 2 2 ρσ k dq 2 T q g = µ 2 transport coefficient q is sensitive to density of (colored) charges λ f CNM pion gas QGP data q-hat RHIC data shows values for q-hat far larger than expected even for a QGP!

24 LHC: Additional Model Constraints LHC: leading particle energy loss range extended up to p T =300 GeV additional constraints on energy-loss models: high p T behavior of R AA indicative of pqcd driven processes challenge to AdS/CFT and other strong coupling calculations (tend to over-predict amount of energy-loss) CMS Collaboration: Eur. Phys. J. C72 (2012) 1945

25 Jet Suppression at LHC ATLAS & CMS observe unbalanced jets direct evidence for QGP jet-suppression

26 initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium hadronization Hadronization: Parton Recombination featured in Thompson ESI: Fast Moving Fronts March JNS publication prize for Young Nuclear Theorists awarded to C. Nonaka citations since January 2003 R.J. Fries, C. Nonaka, B. Mueller & S.A. Bass, PRL 90 (2003) R.J. Fries, C. Nonaka, B. Mueller & S.A. Bass, PRC 68 (2003) C. Nonaka, R.J. Fries & S.A. Bass, Phys. Lett. B 583 (2004) 73 R. J. Fries, S.A. Bass & B. Mueller, PRL 94 (2005)

27 The baryon RHIC species dependence of v 2 saturation: RFD distributions scale by mass, not Meson/Baryon Number: why does RFD fail at intermediate momenta? why do baryons overtake mesons? v 2 why do protons not exhibit the same jet- suppression as pions? fragmentation starts with a single fast parton: energy loss affects pions and protons in the same way! what drives the physics that makes intermediate p t physics dependent on Meson/Baryon Number?

28 Recombination+Fragmentation Model basic assumptions: at low p t, the quark- and antiquark spectrum is thermal and they recombine into hadrons locally at an instant : dn M d 3 P = C M at high p t, the parton spectrum is given by a pqcd power law, partons suffer jet energy loss and hadrons are formed via fragmentation of quarks and gluons: V d 3 (2π ) q w( 1 3 (2π ) P q ) w( 1 P + q ) ˆϕ M (q) 2 E dn h d 3 P = P u dz dσ w (2π ) 3 α (R, 1 P) D z α h (z) Reco: baryons shifted to higher p t than mesons, for same quark distribution shape of spectrum determines if reco or fragmentation is more effective: for thermal distribution recombination yield dominates fragmentation yield vice versa for pqcd power law distribution 1 0 z 2 α understand behavior of baryons, since jet-quenching is strictly high-p t!

29 Quasiparticles at T C : Parton Recombination The parton recombination model predicts that the elliptic flow of a hadron as a function of p t scales with the number of its constituent quarks: v M 2 (p t ) = v B 2 (p t ) = 3 vp 2 2 v p v p 2 p t 2 p t 2 2 p t 3 +3 v p v p 2 p t 3 2 p t 3 3 Lessons from Constituent-Quark-Scaling: as the system evolves towards T C, the DoF of the QGP become quasi-particles at hadronization, constituent quarks are most likely the dominant DoF most direct evidence for the creation of a QGP to date!

30 initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium hadronization Heavy Quarks Quarkonium Suppression Heavy Quark Energy Loss

31 Quarkonium Suppression Basic Idea: color screening in deconfined matter weakens potential (force) between heavy quark and antiquark. strong screening seen in Lattice calculations screening sets in at shorter and shorter distances as T increases Free energy of a static Q-Qbar in N f =2+1 ϒ(1S), ϒ(2S) & J/ψ: three Quarkonium states relatively close in mass, but of significantly different radius expect sequential dissociation measured suppression pattern indicative of sequential melting! A. Mocsy & P. Petreczky: PRL 99 (2007)

32 Heavy Quark Energy Loss: Experiment Expectation: Heavy Quarks should show significantly less energy-loss, due to their large mass vs. light quarks & gluons heavy quarks exhibit nearly as much energy-loss as light quarks! interplay between collisional and radiative energy loss?

33 The QGP Discovery at RHIC initial state QGP and hydrodynamic expansion hadronic phase and freeze-out pre-equilibrium Jet Energy-Loss medium exhibits large opacity for a hard parton traversing it suppression of high-pt hadrons hadronization Elliptic Flow early time phenomenon conversion of spatial to momentum anisotropy due to hydrodynamic flow Parton Recombination partons form hadrons via recombination manifest in quark number scaling laws v M p p 2 ( p t ) 2v t 2 2 v 2 3 B p p ( p t ) 3v t 2

34 The Future of Hot & Dense QCD Heavy-Ion collisions at RHIC & LHC have produced a state of matter which can be called the Quark-Gluon-Plasma: challenging fundamental open questions (initial state, thermalization) the properties of the QGP can be characterized by its transport coefficients, such as η/s and q-hat large opacity: values of q-hat beyond the pqcd expectation near ideal fluidity: the smallest value of η/s observed in nature Transition from Discovery Phase to Exploratory Phase and onwards to Precision Spectroscopy of the QGP: establish the physics driving the small value of η/s (e.g. particles vs. fields) and its dependence on temperature and fugacities determine proper structure of the QGP to use in jet energy-loss & HQ calculations for a precision measurement of q-hat & D explore medium response to jets and heavy quarks and develop a unified picture on the nature of the QGP

35 The End