Elliptic fl ow from initial state s of fast nucle i.

1. Elliptic flow from initial statesof fast nuclei. A.B.Kaidalov ITEP, Moscow (based on papers with K.Boreskov and O.Kancheli)

2. Contents: • Introduction.Models for elliptic flow. • Reggeon diagrams for inclusive cross sections and anisotropic flows. • Impact parameter picture. • Inclusive vertex. • Role of multi-pomeron interactions. • Conclusions.

3. Introduction. • Heavy ions collisions at high energies - a way to study hadronic matter at extreme conditions Т > Tc in the deconfinement phase: quark-gluon plasma (QGP) .

4. Collisions of heavy ions andQGP. • In recent years ideas about properties of QGP have changed (mainly due toRHICdata) . Previously QGP was usually considered as a gas of quarks and gluons , while now – as a(nearly ideal) liquid:QGPs.Strong interaction between quarks and gluons is natural at Т ~ 200 MeV~ΛQCD. Important role of nonperturbative effects. Chromomagnetic part of the correlator of gluonic fieldsdoes not decrease forT > Tc .

5. Models for elliptic flow. Anisotropic flows give an important information on role of collective effects in heavy ions collisions. Anisotropy of the overlap region leads to anisotropic angular distributions. w(φ)=v0 + ∑ 2vncos(nφ) v2 – elliptic flow. v2(b,pT,y,s) Determined from event by event analysis

6. Hydrodynamical model. • Successful description of RHIC data on V2 in hydrodynamical model (HM). • Follows from the assumption of local equilibrium. Small mean free path. • For an ideal fluid (without viscosity) Energy-momentum and baryon number conservation. It is necessary to fix equation of state and initial conditions.

7. Elliptic flow and hydro. HM describes dependence of v2 onрТ (up to 2 GeV) and on types of particles.

8. Problems of HM. а) Does not describe data forрТ >2 GeV. Viscosity?

9. Problems of HM. b) Too fast increase withb. в) Dependence ony. г) Value of v4. v4/ (v2)²= ½; exp: 1.17±0.01 Small number of rescatterings and an absence of the local equilibrium. Blaizot et al.

10. Recombination model. Scaling forv2/n , pT/n , n – number of constituent quarks.

11. Final state interaction models of elliptic flow. In the model with final state interaction, whichdescribes suppression of large pT particlesv2is also reproduced. F.Capella, E.G.Ferreiro

12. Final state interaction model for elliptic flow. The model reproduces dependence ofv2on рТ (for all рТ). Too weak dependence onb.

13. Anisotropy from initial state. Most of theoretical models are based on classical concepts. What is the role of quantum effects? Field-theoretical approach shows that, the anisotropy can appear also due to distributionsof partons in the initial wave functions ofnucleons and nuclei. К.Boreskov,А.К.,О.Kancheli Thus not only final, but also initial state interactions contribute tov2!

14. Reggeon diagrams for inclusive cross sections and anisotropic flows. Consider single particle inclusive cross sections in the central rapidity region at high energies. For nonenhanced diagrams they are expressed in terms of the KM-diagrams

15. Reggeon diagrams for inclusive cross sections and anisotropic flows. • To find impact parameter representation of the cross section we need the amplitude for nonzero

16. Reggeon diagrams for inclusive cross sections and anisotropic flows. where is the nuclear profile function. Inclusive vertex Odd powers of are absent due to symmetry properties.

17. Elliptic flow. • Elliptic flow can be written as

18. Estimate of v2. To estimate the effect use the gaussian parameterization of nuclear form-factors

19. Impact parameter picture. Interaction of nuclei in the impact parameter plane

20. Impact parameter picture.

21. Impact parameter picture. In b-space it is easier to understand an origin of the elliptic flow. The inclusive vertex has a corelation between vectors and . Convolution of Tint with is sensitive to gradients of nuclear densities.

22. Inclusive vertex. Feynman diagrams for inclusive vertex For scalar particles

23. Inclusive vertex. For production of particles with spins (e.g. vector mesons) asymmetry is larger and has different sign. Behavior of the vertex at large pt also depends on spins of exchanged particles. In scalar theory In QCD for

24. Two-particle azimuthal correlations. Reggeon diagrams for two-particle inclusive cross sections

25. Two-particle azimuthal correlations. Contribution of Fig. a) has a factorized form Diagram of Fig. c) corresponds to non-flow component.

26. Role of multi-pomeron interactions. • In the supercritical pomeron theory with role of interactions between pomerons increases with energy and A.

27. Multi-pomeron interactions. • The diagrams of the type b) correspond to interactions between particles from chains produced in different NN-interactions. In this case number of pomerons is the same below and above emission point. These contributions are positive and can lead to a substantial increase of v2.

28. Multi-pomeron interactions. • Indeed  and R²A  R²A/n . Thus v2 ~<n>² . For central collisions at RHIC energies <n> = 2÷3 and it decreases to <n> =1 for peripheral collisions.

29. Estimate of v2 . • V2 as a function of pt for two centralities (b=5.6 fm and 7.9 fm)

30. Conclusions. • Investigation of anisotropic flows gives an important information on dynamics of collisions and collective effects. • Elliptic flow is generated not only in the final state, but also in the initial state interactions. Distributions of partons in colliding nuclei are asymmetric.