Partonic transport simulations of jet quenching Z. Xu, C. Greiner Institut für Theoretische Physik J. W. Goethe-Universität, Frankfurt
Outline motivation introduction to the model simulations of Au+Au at 200 AGeV some more details summary, discussion, outlook
Motivation high energy particles considered to be promising probes of the medium created in AA-collisions nuclear modification factor relative to pp (binary collision scaling) K. Reygers, hep-ex/0512015 experiments show approx. factor 5 of suppression in hadron yields investigation of jet quenching within our stochastic parton cascade model (Z. Xu, C. Greiner, Phys. Rev. C71)
The Model (1) LO pqcd cross sections calculate transition probabilities for particles within spatial cells no geometric interpretation of cross sections simulate gluon plasma including the processes gg gg gg ggg use test particles to ensure sufficient statistics employ this framework to simulate static media (central) heavy-ion collisions matrix element integrated over momentum space
The Model (2) gg gg cross section employ small-t (small angle) approximation gg ggg Gunion-Bertsch Debye screening thermal case:
Inclusion of LPM-effect obtain total cross section for gg ggg via integration of the matrix element energy conservation LPM cutoff LPM-effect (Landau-Pomeranchuk-Migdal) incoherent treatment of gg ggg processes parent gluon must not scatter during formation time of emitted gluon with for gluons energy conservation LPM cutoff
Iterative calculations large MFP small cutoff large cross section small MFP self-consistent, iterative calculations but: very time-consuming, only used for high-energy particles total cross section in a static, thermal gluon plasma (T = 400 MeV) example of iterative calculation of MFP
Au+Au Setup central (b=0 fm) Au-Au collision at 200 AGeV sampling of initial gluon plasma: initial momentum distribution (mini-jets) according to Glück-Reya-Vogt parameterization for structure functions; K = 2 lower cut-off: p 0 = 1.4 GeV number of produced particles via standard nuclear geometry (Wood-Saxon density profile, Glauber-Model) populate η-region from -5 to 5 additionally each parton is given a formation time keep only gluons for further simulation (~3500 out of ~4800) 35 testparticles
Au+Au Setup (2) simulate evolution of fireball up to ~5 fm/c when energy density in a cell drops below ε =1GeV free streaming (in the respective cell) iterative mean free path calculations for p T > 8 GeV calculate nuclear modification factor R AA from comparison of final to initial p T -spectrum
Mean free path time evolution of the gluonic mean free path in simulations of central Au+Au
Thermalization (1) time evolution of p T spectra (central region, x T < 1.5 fm, η < 0.5)
Thermalization (2) kinetic equilibration on a timescale of 1-2 fm/c! time evolution of momentum anisotropy, thermal value: 0.65 (central region, x T < 1.5 fm, η < 0.5)
Rapidity density final dn/dy ~ 800 (initial dn/dy ~ 700) Only gluons! Remember, #gluons~3500 out of #total~4800 (~70%)
Elliptic flow time evolution of v 2 at midrapidity for different impact parameters
Experimental v 2 vs. centrality STAR White Paper, nucl-ex/0501009
Au-Au Reconstruction partons with high-pt too rare simulate large number of initial conditions select events according to highest pt-(test)particle simulate only selected events and weight results full: 200000 events; reconstruction: 40 events per pt-bin, ~1000 total
Au-Au Nuclear modification factor (1)
Au-Au Nuclear modification factor R AA ~ 0.04 0.05 suppression too strong, by factor of ~4-5 ~ 300 days of CPU-time, -> CSC
P 0 contribution to R AA R AA from gluons having escaped without any interaction (triangles) about 40% of the observed high-p T gluons have not interacted
Energy loss in a static medium
Energy loss in a static medium gluon jet in a static, thermal medium T = 400 MeV dominant process is 2->3
Static medium Relevant processes Collision rates of the projectile as a function of its energy E. T=400 MeV, N f =0 Mean relative loss of energy, (E i E f )/E f, as a function of incoming energy E i 2 -> 3 is the most relevant process for the energy loss
What we have seen consistent incorporation of inelastic processes into a partonic transport model rapid thermalization (on the order of 1-2 fm/c) considerable amount of collectivity in simulated Au+Au events possibility to examine v 2 and R AA (and various other observables) within one framework non-perturbative transport simulations of jet events up to transverse momenta of 35 GeV simulated R AA about a factor 4-5 below Pion-R AA data but remember: it s only gluons (cp. nucl-th/0512076) very high de/dx in static medium simulations
Some issues no quarks included so far quarks will suffer a smaller energy loss energy loss depends crucially on the effective modeling of the LPM effect How to incorporate interference effects into a classical transport simulation? LPM for thermal particles? Abelian vs. non-abelian LPM effect
The to-do list current implementation of LPM effect needs careful checking frame dependence distribution of energies in inelastic collisions, scattering angles etc. improve implementation of LPM effect? non-abelian LPM effect? effects on thermalization and flow-observables? include quark degrees of freedom and examine quark jets think about the initial conditions investigate angular correlations and other observables linked with partonic energy loss simulate LHC-energies incorporate hadronization scheme
Backup slides
Temperature time evolution of the temperature in the central region
Gluon-Quark-Ratio
Radiation spectrum radiated gluon is taken to be the outgoing particle that is assigned k T (in Gunion-Bertsch matrix element)
Comparison to BDMPS