Based on work in progress in collaboration with: F. Scardina, S. Plumari and V. Greco

Transcription

1 Marco Ruggieri Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Catania (Italy) Based on work in progress in collaboration with: F. Scardina, S. Plumari and V. Greco Bari, 2012 December 20

2 Parton distribution functions Direction of increasing transverse partons area (1/Q 2 ) Phase Diagram Direction of increasing gluon number

3 Gluon saturation in deep inelastic scattering: forward scattering amplitude large x transverse probe transverse resolution proton area gluon number Unitarity constraint: small x: non linear effects

4 Soft gluon emission via QCD processes: Each gluon carries color charge, hence each soft gluon is able to emit a softer gluon, creating a gluon cascade which enhances the gluon occupation number. BFKL n BFKL JIMWLK GLR GLR JIMWLK Gluon saturation regime Gluon recombination y

5 n BFKL JIMWLK GLR Small x (i.e. slow) gluon dynamics: high density boson system, living in the background of large x (i.e. fast) color sources, the latter being randomly distributed in the proton. Gluon saturation regime y New state of matter: Color Glass Condensate Color Glass gluons carry color charge fast partons randomly distributed in the hadron, with frozen dynamics (by Lorentz time dilatation). Condensate small x gluons form a Bose-Einstein condensate, with typical correlation scale

6 Consider a heavy ion collision (HIC): HIC along the flying direction : Hadron in the transverse plane (CGC picture): cond cond Fast partons cond Strong chromo-electric and chromo-magnetic fields, correlated on the scale 1/Qs. After the collision, fields decay and produce the parton gas (QGP). Parton distribution in phase space will be affected by Qs.

7 A B perturbative Melting of the CGC leads to gluon production in the fireball: ugdf Unintegrated distribution functions (ugdfs) Saturation scale Qs depends on: 1.) position in transverse plane; 2.) gluon rapidity. It is computed selfconsistently: CGC (saturation built in) kt (GeV) Similar to the case of proton:

8 Au-Au collision, s=(200 GeV) 2, b=7.5 fm B A ra rb Density of Participants in the transverse plane Saturation scale in the transverse plane A B

9 Au-Au collision, s=(200 GeV) 2, b=7.5 fm Initial distribution of fireball gluons: configuration space Initial distribution of fireball gluons: transverse plane Collision direction Collision direction Impact parameter direction

10 In order to simulate the temporal evolution of the fireball we solve the Boltzmann equation for the parton distribution functions: f: distribution function of gluons C[f]: collision integral In C[f] we (preliminarily) consider only the elastic massless gluon scattering process: Distribution function is sampled at each time step by means of test particles which evolve according to the classical Hamilton equations of motion: The probability to have a collision is given by The simulation code accepts collisions among test particles with probability P 22.

11 Au-Au collision, s=(200 GeV) 2, b=7.5 fm t=0.6 fm/c t=1.6 fm/c t=4.6 fm/c t=8.6 fm/c Impact parameter direction Collision direction

12 Au-Au collision, s=(200 GeV) 2, b=7.5 fm Collision direction Impact parameter direction

13 Au-Au collision, s=(200 GeV) 2, b=7.5 fm CGC initial spectrum We work at fixed h/s; the cross section is computed in each phase space cell to give the wished value of h/s. Cross section at the center The system is in local equilibrium in less than 1 fm/c

14 Au-Au collision, s=(200 GeV) 2, b=7.5 fm PRELIMINARY v2 measures how efficiently the anisotropy of configuration in the initial state is transmitted to momenta in final states. Hirano and Nara, The large value of h/s needed to bring the CGC v2 onto exp. data is partly connected to the larger initial eccentricity of the CGC fireball, compared to that of the normal QGP phase. More investigation is needed. Impact parameter

15 CGC is an interesting state of high density QCD matter. Its effects on HICs are easily studied within a transport approach, due to its nonequilibrium nature. Outlook Inclusion of inelastic processes and of thermal masses. Detailed study of pre-equilibration times.