Dynamical description of heavy-ion collisions Elena Bratkovskaya - PowerPoint PPT Presentation

Dynamical description of heavy-ion collisions Elena Bratkovskaya (GSI, Darmstadt & Uni. Frankfurt) for the PHSD group COST Workshop on Interplay of hard and soft QCD probes for collectivity in heavy-ion collisions 25 February - 1 March 2019, Lund university, Sweden 1

The ‚holy grail‘ of heavy-ion physics: The phase diagram of QCD • Study of the phase transition from hadronic to partonic matter – Quark-Gluon-Plasma • Search for the critical point • Search for signatures of chiral symmetry restoration • Study of the in-medium properties of hadrons at high baryon density and temperature 2

Theory: Information from lattice QCD + II. chiral symmetry restoration I. deconfinement phase transition with increasing temperature with increasing temperature m q =0 lQCD BMW collaboration: q q Δ ~ T l, s q q 0  Crossover: hadron gas  QGP  Scalar quark condensate is viewed as an order parameter for the restoration of chiral symmetry:  both transitions occur at about the same temperature T C for low chemical potentials

Degrees-of-freedom of QGP  lQCD gives QGP EoS at finite m B ! need to be interpreted in terms of degrees-of-freedom Non-perturbative QCD  pQCD Thermal QCD pQCD: = QCD at high parton densities:  weakly interacting system  strongly interacting system  massless quarks and gluons  massive quarks and gluons  quasiparticles = effective degrees-of-freedom  How to learn about degrees-of- freedom of QGP? Theory  HIC experiments 4

Basic models for heavy-ion collisions • Statistical models: basic assumption: system is described by a (grand) canonical ensemble of non-interacting fermions and bosons in thermal and chemical equilibrium = thermal hadron gas at freeze-out with common T and m B [ - : no dynamical information] • Hydrodynamical models: basic assumption: conservation laws + equation of state (EoS); assumption of local thermal and chemical equilibrium - Interactions are ‚hidden‘ in properties of the fluid described by transport coefficients (shear and bulk viscosity h, z, ..), which is ‘input’ for the hydro models [ - : simplified dynamics] • Microscopic transport models: based on transport theory of relativistic quantum many-body systems - Explicitly account for the interactions of all degrees of freedom (hadrons and partons) in terms of cross sections and potentials - Provide a unique dynamical description of strongly interaction matter in- and out-off equilibrium: - In-equilibrium: transport coefficients are calculated in a box – controled by lQCD - Nonequilibrium dynamics – controled by HIC Actual solutions: Monte Carlo simulations [+ : full dynamics | - : very complicated] 7

History: Semi-classical BUU equation Boltzmann-Uehling-Uhlenbeck equation (non-relativistic formulation) - propagation of particles in the self-generated Hartree-Fock mean-field potential U(r,t) with an on-shell collision term: Ludwig Boltzmann                p f         f ( r , p , t ) f ( r , p , t ) U ( r , t ) f ( r , p , t ) collision term:    r r p   t m t   elastic and coll inelastic reactions   f ( r , p , t ) is the single particle phase-space distribution function - probability to find the particle at position r with momentum p at time t  self-generated Hartree-Fock mean-field potential:  1          3 3   U ( r , t ) d r d p V ( r r , t ) f ( r , p , t ) ( Fock term )  3 ( 2  )  occ  Collision term for 1+2  3+4 (let‘s consider fermions) :      4 d               3 3 3 I d p d p d | | ( p p p p ) ( 1 2 3 4 ) P coll 2 3 12 1 2 3 4  3  ( 2 ) d Probability including Pauli blocking of fermions: 3  t 1       P f f ( 1 f )( 1 f ) f f ( 1 f )( 1 f ) 3 4 1 2 1 2 3 4 Gain term: 3+4  1+2 Loss term: 1+2  3+4  4 2 12 6

Elementary hadronic interactions Consider all possible interactions – eleastic and inelastic collisions - for the sytem of (N,R,m), where N -nucleons, R- resonances, m -mesons, and resonance decays Low energy collisions: High energy collisions:  binary 2  2 and (above s 1/2 ~2.5 GeV)  + p 2  3(4) reactions  1  2 : formation and Inclusive particle decay of baryonic and production: mesonic resonances BB  X , mB  X, mm  X BB  B´B´ X =many particles BB  B´B´m described by mB  m´B´ string formation and decay mB  B´ (string = excited color pp mm  m´m´ singlet states q-qq , q-qbar ) mm  m´ . . . using LUND string model Baryons: B = p, n, (1232) , N(1440), N(1535), ... Mesons: M =  , h , r, w, f, ... 8

From weakly to strongly interacting systems In-medium effects (on hadronic or partonic levels!) = changes of particle properties in the hot and dense medium Example: hadronic medium - vector mesons, strange mesons QGP – dressing of partons Many-body theory: Strong interaction  large width = short life-time  broad spectral function  quantum object  How to describe the dynamics of L (1783)N -1 broad strongly interacting quantum and S (1830)N -1 states in transport theory? exitations  semi-classical BUU first order gradient Barcelona / expansion of quantum Valencia group Kadanoff-Baym equations  generalized transport equations based on Kadanoff-Baym dynamics 8

Dynamical description of strongly interacting systems  Semi-classical on-shell BUU: applies for small collisional width, i.e. for a weakly interacting systems of particles How to describe strongly interacting systems?!  Quantum field theory  Kadanoff-Baym dynamics for resummed single-particle Green functions S < (1962) Green functions S < / self-energies S : Integration over the intermediate spacetime   ret  c    a  S S S S S retarded ˆ       m iS η {Φ ( y ) Φ ( x ) }      1 x 2 S ( M ) xy xy xy xy xy xy m 0 x x 0        adv c a S S S S S advanced      iS {Φ ( y ) Φ ( x ) } xy xy xy xy xy xy h   1 ( bosons / fermions )      c c iS T {Φ ( x ) Φ ( y ) } causal xy a c    T ( T ) ( anti ) time ordering operator  a   a   iS T {Φ ( x ) Φ ( y ) } anticausal xy Leo Kadanoff Gordon Baym 9

From Kadanoff-Baym equations to generalized transport equations After the first order gradient expansion of the Wigner transformed Kadanoff-Baym equations and separation into the real and imaginary parts one gets: Generalized transport equations (GTE): drift term Vlasov term backflow term collision term = ‚gain‘ - ‚loss‘ term Backflow term incorporates the off-shell behavior in the particle propagation ! vanishes in the quasiparticle limit A XP   (p 2 -M 2 )  GTE: Propagation of the Green‘s function i S <XP =A XP N XP , which carries information not only on the number of particles ( N XP ), but also on their properties, interactions and correlations (via A XP )  Spectral function:    S   ret Im 2 p – ‚width‘ of spectral function 4-dimentional generalizaton of the Poisson-bracket: XP XP 0 = reaction rate of particle (at space-time position X)  c    Life time  W. Cassing , S. Juchem, NPA 665 (2000) 377; 672 (2000) 417; 677 (2000) 445 10 10

General testparticle off-shell equations of motion W. Cassing , S. Juchem, NPA 665 (2000) 377; 672 (2000) 417; 677 (2000) 445  Employ testparticle Ansatz for the real valued quantity i S < XP insert in generalized transport equations and determine equations of motion !  General testparticle Cassing‘s off-shell equations of motion for the time-like particles: with 11 11

Collision term in off-shell transport models Collision term for reaction 1+2->3+4: ‚gain‘ term ‚loss‘ term with The trace over particles 2,3,4 reads explicitly for bosons for fermions additional integration The transport approach and the particle spectral functions are fully determined once the in-medium transition amplitudes G are known in their off-shell dependence! 12

Goal: microscopic transport description of the partonic and hadronic phase  How to model a QGP phase in line with lQCD data? Problems:  How to solve the hadronization problem? Ways to go: ‚Hybrid‘ models: pQCD based models:  QGP phase: hydro with QGP EoS  QGP phase: pQCD cascade  hadronic freeze-out: after burner -  hadronization: quark coalescence hadron-string transport model  AMPT, HIJING, BAMPS  Hybrid-UrQMD  microscopic transport description of the partonic and hadronic phase in terms of strongly interacting dynamical quasi-particles and off-shell hadrons  PHSD 16