Soft physics in PbPb at the LHC Hadron Collider Physics 2011 P. Kuijer ALICECMSATLAS Necessarily incomplete

Particle production in PbPb HCP11, soft physics at the LHC ArXiv: v1 dN ch /d  ~ 1600 ± 76 (syst) Particle production somewhat higher than expected AA and pp Good agreement between experiments Growth with energy faster in AA than in pp S 0.15 vs s 0.11 ArXiv: v1 JHEP 08 (2011)141 2

HBT radii scale roughly linearly with multiplicity 1/3 in pp and PbPb HBT radii in PbPb vs. trend from lower energy AA: – R long : perfectly agree – R side : reasonably agree – R out : clearly below the trend Behaviour of all 3 radii in qualitative agreement with hydro expectations – R out /R side decreases with √s due to higher initial temperature Volume and life time HCP11, soft physics at the LHC   r, t  p, E RHIC Volume at decoupling Few times nuclear volume (~5000 fm 3 ) R ~ 7 fm x 2 Multiplicity Lifetime: from collision to ‘freeze-out’ (hadron decoupling) RHIC + 40% Multiplicity 1/3 Observe (co-moving) volume using QM interferometry for bosons (ideas from 1950’s) Used by astronomers to measure star sizes with photons (Hanburry, Brown, Twiss ) Used in particle physics to measure source size with pions (Goldhaber) 3

Energy density HCP11, soft physics at the LHC Bjorken estimate for a thermalized system Even here bigger than at RHIC ~3*RHIC 4

Identified Spectra in heavy ions collisions Paris, 15 November 2011 Blast wave fits to extract yields and Hadron Collider Physics Symposium

6 Multi-strange baryons Text

Strangeness production: AA vs. pp 7 Production of multi-strange baryons in PbPb collisions at √s=2.76 TeV enhanced with respect to pp (depending on system size)

Comparison with results at lower energies 8 Enhancement decreases with √s (SPS>RHIC>LHC) Strange/pion ~ constant above SPS energies Increase with √s in pp

Particle Ratios in PbPb collisions Pb-Pb: - K/   from pp value towards thermal prediction -p/  ≈ like pp p/  off by factor > 1.5 from predictions ! similar to RHIC (where pbar/p = 0.8) ? RHIC Pb-Pb: K/  Range of Thermal model prediction (Prediction, no fit) Phys. Lett. B 673: ,2009  s = 1 HCP11, soft physics at the LHC9 PHENIX, Brahms, ALICE feed down corrected p/  9 STAR, not feed down corrected

Collective expansion HCP11, soft physics at the LHC10 x y x y In a thermalized system the radial expansion is driven by the pressure gradient from inside to outside. Resulting in boosted pt spectra If the system is asymmetric in spatial coordinates the expansion will lead to anisotropy in momentum space The final state anisotropy at low p t can be calculated using hydrodynamics with as input Initial conditions (eccentricity, volume, energy density,..) Properties of produced matter (viscosity,...) At RHIC strong anisotropies were measured.

Radial flow HCP11, soft physics at the LHC  p K PbPb pp Flow velocity depends on equation of state Momentum distributions for different mass particles show characteristic differences wrt pp P >> 0 P = 0 v0v0 v0v0 v0v0 Significant changes in slope compared to RHIC Especially for protons 11 Hydrodynamic Model Calculation <  ≈ 0.66 ArXiv:

Fit temperature and velocity HCP11, soft physics at the LHC12 STAR/200 GeV Centrality Common blast wave fit to π,K and p

Elliptic flow and viscosity HCP11, soft physics at the LHC Reaction plane X Z Y  Pressure  p x >  p y x y Spatial eccentricity  = (y 2 -x 2 )/(y 2 +x 2 ) source  anisotropic particle distributions due to pressure gradients:  Describe by Fourier analysis of the angular distribution dN/dφ = v 2 cos(2φ)  Measure coefficients using (two or more) particle correlations The second (elliptic flow) coefficient v 2 depends on the eccentricity and the equation of state. Viscosity reduces the elliptic flow  /s > 1/4  ≈  suggested by AdS/CFT RHIC v2 results indicate that the viscosity of the QGP is very small (less than 4 times the Ads/CFT limit) 13 Based on R. Lacey et al., Phys.Rev.Lett.98:092301,2007

+30% RHIC ALICE Flow coefficient STAR at RHIC Pions Protons Elliptic Flow Measurement at LHC v 2 versus p T – essentially no change from RHIC v 2 integrated over p T – 30% increase from RHIC – at the upper edge (but within) hydro-predictions particle mass dependence – as predicted by hydro How perfect is it ? Is  /s at the quantum limit 1/4  ? Hydro Interpretation passed the test ! CERN Press release, November 26, 2010: ‘confirms that the much hotter plasma produced at the LHC behaves as a very low viscosity liquid (a perfect fluid)..’ : still a (almost) perfect liquid HCP11, soft physics at the LHC14

Initial Conditions, fluctuations Current limit:  /s < (2-4) x 1/4  – initial conditions (pressure/energy distribution) not known precisely enough – 1) energy distribution (→ eccentricity  ) in overlap volume is model dependent different combinations of  and  /s describe data equally well – 2) event-by-event fluctuations → change  at fixed b & higher order deformation suggested in 2010 but controversial, higher v n where not directly 'seen' in the data Fourier series: dN/d  = v 1 cos(  ) + 2 v 2 cos(2  ) + 2 v 3 cos(3  ) +… Elliptic, v 2 Triangular, v 3 HCP11, soft physics at the LHC15

Are the structures really from Flow ? v 3 shows mass splitting expected from hydro flow ! Has the magnitude (and p T dependence) expected from geometry fluctuations (and has larger sensitivity to  /s than v 2 ! => reduces model dependence) v 3 for  /K/p  p K v 2 & v 3 versus p T v2v2 v5v5 Hydro calculation for v 3  /s = 0  /s = 1/4  v3v3 HCP11, soft physics at the LHC progress in precision measurements of  /s fluctuations discriminate & constrain models have large sensitivity to viscosity 16

Ridge HCP11, soft physics at the LHC ATLAS-CONF Two particle angular correlations should factorize if the correlations are ONLY due to the correlation with the event plane (collision geometry). near side jet correlations Remnants of away side jet (Mach cone)? Exclude the jet (η cut) and fit the underlying angular correlations Fourier order n Flow coefficient v n 17 ATLAS-CONF

Ridge HCP11, soft physics at the LHC However, putting in the coefficients from the flow analysis describes the curves for pt<4 GeV/c very well 18

HCP11, soft physics at the LHC19

Summary Bulk features (size, lifetime, energy density, radial flow,..) – LHC brings the hoped for (expected) quantitative improvements HBT radii, energy density, temperature,.. dN ch /d  : powerlaw difference pp - AA and centrality dependence. to be understood Particle ratios (p/  ) – surprising ! Flow Tomography (v n ) – Huge step forward: experimental handle on 'initial conditions' precision measurements of matter properties (  /s, c s, EoS) Unconventional/speculative ideas – to be explored HCP11, soft physics at the LHC20

HCP11, soft physics at the LHC More PbPb collisions on the way! 21

Backup slides HCP11, soft physics at the LHC22