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Transport description of viscous effects

Transport description of viscous effects. Che-Ming Ko Texas A&M University. Introduction A multi-phase transport (AMPT) model Anisotropic flow - Elliptic flow - Higher-order anisotropic flow - Rapidity dependence - System size dependence

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Transport description of viscous effects

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  1. Transport description of viscous effects Che-Ming Ko Texas A&M University • Introduction • A multi-phase transport (AMPT) model • Anisotropic flow - Elliptic flow - Higher-order anisotropic flow - Rapidity dependence - System size dependence - Flavor dependence • Summary Collaborators: Lie-wen Chen (SJTU), Zie-wei Lin (Univ. Alabama), Bin Zhang (Arkansas State Univ.)

  2. Elliptic flow from parton cascade Zhang, Gyulassy & Ko, PLB 455, 45 (1999) Based on Zhang’s parton cascade (ZPC) (CPC 109, 193 (1998)) using minijet partons from HIJING for Au+Au @ 200 AGeV and b=7.5fm v2 of partons is sensitive to their scattering cross section

  3. Zhang’s parton cascade (ZPC) Bin Zhang, Comp. Phys. Comm. 109, 193 (1998) • Using αs=0.5 and screening mass μ=gT≈0.6 GeV at T≈0.25 GeV, • then <s>1/2≈4.2T≈1 GeV, and pQCD gives σ≈2.5 mb and a • transport cross section • σ=6 mb → μ≈0.44 GeV, σt≈2.7 mb • σ=10 mb → μ≈0.35 GeV, σt≈3.6 mb

  4. A multiphase transport (AMPT) model Default: Lin, Pal, Zhang, Li &Ko, PRC 61, 067901 (00); 64, 041901 (01); 72, 064901 (05); http://www-cunuke.phys.columbia.edu/OSCAR • Initial conditions: HIJING (soft strings and hard minijets) • Parton evolution: ZPC • Hadronization: Lund string model for default AMPT • Hadronic scattering: ART String melting: PRC 65, 034904 (02); PRL 89, 152301 (02) • Convert hadrons from string fragmentation into quarks and antiquarks • Evolve quarks and antiquarks in ZPC • When partons stop interacting, combine nearest quark and antiquark to meson, and nearest three quarks to baryon (coordinate-space coalescence) • Hadron flavors are determined by quarks’ invariant mass

  5. Elliptic flow from AMPT Lin & Ko, PRC 65, 034904 (2002) σp= 6 mb • Based on coordinate-space coalescence • Need string melting and large parton scattering cross section • Mass ordering of v2 at low pT as in ideal hydrodynamic model

  6. PRL 90, 202102 (2003); PRC 68, 034904 (2003) Coalescence model Number of hadrons with n quarks and/or antiquarks Spin-color statistical factor e.g. Quark distribution function Coalescence probability function For baryons, Jacobi coordinates for three-body system are used.

  7. Momentum-space coalescence model Kolb, Chen, Greco, & Ko, PRC 69 (2004) 051901 Including 4th order quark flow Meson flow Baryon flow

  8. Higher-order anisotropic flow Chen, Ko, & Lin, PRC 69, 031901 (04) Data can be described by a multiphase transport (AMPT) model Parton cascade Data Momentum-space coalescence

  9. Strange quarks from AMPT Chen & Ko, PRC 73, 044903 (2006)

  10. Phi & Omega flows Chen & Ko, PRC 73, 044903 (2006) same for phi & Omega but different from quark instead of 1.1 and 0.7 from dynamical coalescence model

  11. Flows of phi & Omega with large sizes satisfied for large phi and Omega sizes of 4.5 fm

  12. Pseudorapidity dependence of v1 and v2 Chen, Greco, Ko & Koch, PLB 605, 95 (2005) • String melting describes data near mid-rapidity (||<1.5) • At large rapidity (||>3), hadronic picture works better

  13. System size dependence of elliptic flow Chen & Ko, nucl-th/0505044 v2 is proportional to size of colliding system

  14. Elliptic flow in collisions of asymmetric systems Compared to symmetric collisions • Directed flow v1 is stronger • Elliptic flow v2 is more sensitive to parton cross section • Both directed and elliptic flows are asymmetric in forward and backward rapidities

  15. Quark elliptic flow from AMPT • PT dependence of charm quark v2 is different from that of light quarks. • At high pT, charm quark has similar v2 as light quarks. • Charm elliptic flow is also sensitive to parton cross sections

  16. Charm elliptic flow from AMPT Zhang, Chen & Ko, PRC 72, 024906 (05) 15 10 5 0 -5 • Current light quark masses are used in AMPT. Charmed meson elliptic flow will be larger if constituent quark masses are used. • To increase v2 at low pt requires going beyond parton cascade.

  17. Summary • Elliptic flow is sensitive to parton scattering cross section • The AMPT model with string melting can reproduce observed large elliptic flow and mass ordering at low pT with large parton • cross section (quasi bound states in QGP and/or multiparton dynamics • gg↔ggg?) • Observed hadron v4≈1.2v22 is reproduced with parton v4≈v22 • v1 and v2 at midrapidity requires formation of partonic matter, while those at large rapidity is consistent with a hadronic matter • AMPT predicts: - v2 scales with the size of colliding system - Stronger v1 and more sensitivity of v2 to parton cross section in asymmetric than symmetric collisions, and both show forward-backward asymmetry in rapidity • Observed large charmed meson flow requires large charm quark scattering cross section (resonance effect?)

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