Saturation physics and baryon stopping in the SPS, RHIC, and LHC energy regions

1. Saturation physics and baryon stopping in the SPS, RHIC,and LHC energy regions 冯 笙 琴 20100726 13届全国核结构会议 赤峰

2. Topics • CGC and saturation physics • The rapidity distributions of net baryon in the SPS, RHIC, and LHC energy regions • The results and discussions

3. Color Glass Condensate Hadronic interactions at very high energies are controlled by a new form of matter, a dense condensate of gluon. Color:gluon are colored Glass:the fields evolve very slowly with respect to the natural time scale and are disordered. Condensate:There is a very high density of massless gluons. These gluons can be packed until their phase space density is so high that interactions prevent more gluon occupation.

4. HERA Data pQCD ok ! 1.Small x problem McLerran, hep-ph/0311028

5. HERA Data 2. Geometric scaling One expects structure functions from DIS depend on (in general) x and Q2/Λ2, but the function depend only on at Bjorken x < 0.01 (HERA), i.e. independent of x.  Some kind of scaling at small x.

6. 5. Individual hadrons freeze out 4. Hadron gas cooling with expansion 3. Quark Gluon Plasma thermalization, expansion 2. Pre-equilibrium state collision 1. Nuclei (initial condition) The picture Relativistic Heavy Ion Collisions in High Energy Limit  CGC Transverse momentum spectra and rapidity distribution of net baryon or final hadrons in Relativistic Heavy Ion Collisions.

7. 纵向光锥变量 LC time LC longit. coordinate invariant 4-product LC energy LC longit. momentum rapidity

8. Notation Feynman, Bjorken x:

9. Color Glass Condensate theory Higher energies (smaller x  0) Valence partons gluon cascade dense gluon state = CGC Color Glass Condensate Saturation scaleQs >> LQCD typical transverse size ~ 1/Qs  weak coupling as(Qs) << 1 at high energy Strong gauge fieldA ~ Qs /g, E, B ~ Qs2/g CGC is a weakly-coupled many body system with high non-linearity ! Valence partons as static random color source Small x gluons as radiation field created by r(x). Stochastic Yang-Mills equation Hadrons at Very High Energies

10. pT spectra PRC_80_054905 hep-ph:0307037 So …… Experiment Theory Saturation physics HERA Small x problem Geometric scaling “Baryon stopping as a test of geometric scaling”

11. Particle production in hadronic collisions Leading particles (projectile or target ) have rapidity close to original rapidity. Produced particle populate the region around zero-rapidity. Feynman scaling of rapidity distribution of produced particles.

12. Deep inelastic scattering Hadron = collections of partons with momentum distribution dN/dx Rapidity: 强子内部胶子分布图

13. 2. The distribution function of net baryons The net-baryon number is essentially transported by valence quarks that probe the saturation regime in the target by multiple scatterings. The fast valence quarks in one nucleus scatter in the other nucleus by exchanging soft gluons, leading to their Redistribution in rapidity space. To access the gluon distribution at small x, we use the valence quark distributions at large x

14. 2. The distribution function of net baryons The distribution of net baryon is proportional to the valence quark rapidity distribution, by integrating over PT: this is indeed a good approximation at high energy heavy-ion collisions

15. One important prediction of the color glass condensate theory is geometric scaling: the gluon distributions depends on and only through the scaling variable , where

16. with the changing of variables Thus we rewrite the formula as where is the scaling variable.

17. In the NF-model (No Fragmentation) — the unintegrated gluon distribution is The valence quarks distribution is

18. So the net-baryon distribution originates from the projectile is The effective quark mass are considered by substituting the transverse momentum for i.e. m: the mass of effective quark

19. The results Rapidity distributions of net baryonin different centrality collisions at SPS energiesof . Rapidity distributions of net baryons in different centralities collisions at RHIC energies of .

20. Rapidity distributions of net baryon in central Au+Au collisions at RHIC energies of .

21. The results The mean rapidity loss is plotted as a function of the beam rapidity.

22. 3. The results The effective quark mass is plotted as a function of the beam rapidity, are among 0.25-0.28GeV.

23. r The ratios of number of baryons which locate in the central region contributing to the whole ones.

24. The results The rapidity distribution of net baryons in central Pb+Pb collisions at LHC energies of with .

25. The discussions Coming to ultrarelativistic heavy ion collisions, as experimentally realized at RHIC and, in perspective, at LHC, we note that the CGC should be the appropriate description of the initial conditions. Indeed, most of the multiparticle production at central rapidity is from the small-x. Theearly stages of a nuclear collision, can thus be described as the melting of the Colour Glass Condensates in the two nuclei.

26. The discussions The experimental data at SPS、RHIC have been analyzed from the perspective of the CGC. The nuclear stopping have been discussed by saturation model. LHC results has been predicted

27. 谢谢