Global Event Features

1. Global Event Features

2. Charged multiplicity (central collisions) • Quantitative Difference from RHIC • dNch/dh ~ 1600 ± 76 (syst) • on high side of expectations • growth with √s faster in AA than pp : (√s -‘nuclear amplification’ • Energy density ≈ 3 x RHIC (fixed t) • lower limit, likely t0(LHC) < t0(RHIC) 15 GeV/fm3 …….. or more PRL105 (2010) 252301 PRL105 (2010) 252301

3. Bjorken’s formula A simple way to estimate the energy density from charged multiplicity or transverse energy measurements HP: particle are produced at formation time tf(tf = 1 fm/c or less) Consider a slice of longitudinal thickness Dz and section A This slice contains the particles with speed  And such a number can be expressed as: where we have used b ≈ y for b0 With some further calculations we get:

4. Saturation Models HIJING DPMJET Charged multiplicity: centrality dependence • dNch/dh as function of centrality (normalised to ‘overlap volume’ ~ Nparticipants) • DPMJET MC • fails to describe the data • HIJING MC • strong centr. dependentgluon shadowing • Others • saturation models: Color Glass Condensate,‘geometrical scaling’ fromHERA/ photonuclear react. Published on PRL Important constraint for models sensitive to details of saturation

5. Interferometry - I • Experimentally, the expansion rate and the spatial extent at decoupling are accessible via intensity interferometry, a technique which exploits the Bose–Einstein enhancement of identical bosons emitted close by in phasespace. This approach, known as Hanbury Brown–Twiss analysis p

6. Interferometry - II PLB 696 (2011) The three-dimensional correlation functions can be fitted with the following expression, accounting for the Bose-Einstein enhancement and for the Coulomb interaction between the two particles: l= correlation strenght, k(qinv)= squared Coulomb wawefuntion Time at decoupling: t ̴ Rout

7. System Size from pion interferometry • Spatial extent of the particle emitting source extracted from interferometryof identical bosons • Two-particle momentum correlations in 3 orth. directions -> HBT radii (Rlong, Rside, Rout) • Size: twice w.r.t. RHIC •  Lifetime: 40% higher w.r.t. RHIC

8. Kaon interferometry Kaon interferometry: complementary to pion due to different mT Results consisten with those with pions

9. Interferometry: pp vs Pb-Pb

10. Central collisions: radial flow (low) pT spectra : superposition of collective motion of particles on top of thermal motion Collective motion is due to high pressure arising from compression and heating . Low-pT particle production • “Blast-Wave” fit to pT spectra [1]: •  Radial flow velocity <b> ≈ 0.65 • (10 % larger than at RHIC) • Kinetic freezout temp. TK≈ 95 MeV • (same as RHIC within errors) [1] E. Schnedermann, et al.; Phys. Rev. C48, 2462 (1993) arXiv:1208.1974 [hep-ex]

11. Particle yields and ratios Assuming that the medium created in the collision reaches thermal equilibrium, one can compute particle yield and ratios with thermal models. Grand canonical ensamble: Where b =1/T and mi is the chemical potential Thermal models have been (and are being ) used to fit the measured particles yields (at different c.m. energies)  T and the baryochemical potential mb are free parameters to be obtained by the fit. N.B. T is the chemical freezout temperature……..

12. Yield and ratios at RHIC !

13. Particle yields and ratios at LHC - Extracted from pT- integrated identified particle spectra. - Comparison /Fit with Thermal/ Statistical models work well at RHIC  info on chemical freezout temperature and baryochemical potential • Predicted temperature T=164 MeV • A.Andronic, P.Braun-Munzinger, J.Stachel NP A772 167 • Thermal fit (w/o res.): T=152 MeV (c2/ndf = 40/9) • X and W significantly higher than statistical model p/p and L/p ratios at LHC lower than RHIC Hadronic re-interactions ? F.Becattiniet al. 1201.6349J.Steinheimer et al. 1203.5302

14. Anisotropic flow

15. Anisotropic flow: basic idea

19. Z Reaction plane Y X f Pz Py Px Nch yield Elliptic flow

20. v2: selected ALICE results Large elliptic flow oberved at RHIC  consistent with strongly coupled medium with low shear viscosity (ideal fluid) v2 for non-identified particles: v2 for identified particles: • Stronger mass dependence of the elliptic flow as compared to RHIC: • Due to the larger radial flow? • Some deviation from hydrodynamic predictions for (anti) protons in close-to-central collisions: rescattering?

22. V2 scaling at RHIC

27. More on anisotropic flow v2 and v3 over extended * h interval V3 sensitive to the fluctuations of the initial nucleon distribution *Results already published in the central h region: v2 Phys. Rev. Lett. 105, 252302 (2010), v3  Phys. Rev. Lett. 107, 032301 (2011)

28. High-pt and Jets

29. Particle spectra at high pT Pb-Pb at different centralities Reference: pp collisions Parton energy loss: A parton passing through the QCD medium undergoes energy loss which results in the suppression of high-pT hadron yields Related observable: nuclear modification factor RAA

30. RAA for identified particles • First measurement of (anti-)proton, K and p at high pT (>7 GeV/c) : • The RAA indicates strong suppression,confirming the indications from previous measurements for non-identified particles • The RAA for (anti-)protons, charged pions and K are compatible above ̴7 GeV/c  this suggests that the medium does not affect the fragmentation.

31. Charged jet: RAA and RCP Strong jet suppression observed for jets reconstructed with charged particles – RAA (jet) is smaller than inclusive hadron RAA(h±) at similar parton pT – data are reasonably well described by JEWEL model K.Zapp, I.Krauss, U.Wiedemann, arXiv:1111.6838

33. Isolation of near-side peak: Dh–D correlation with trigger Long-range (large Dh) correlation used as proxy for background sh s Evolution of near-side-peak sh and s with centrality: Strong sh increase for central collisions Interesting: AMPT describes the data very well Influence of flowing medium? Near-side (jet-like) structure N.Armesto et al., PRL 93, 242301