CHIMERA: Towards constraining properties of strongly interacting matter at RHIC and LHC Irakli Garishvili Group members: Ron Soltz, Betty Abelev, Andy Glenn 29 th Winter Workshop in Nuclear Dynamics

Motivation Achieved a very good qualitative understanding of the full picture of heavy ion collisions at RHIC and LHC Hybrid viscous hydro models coupled with microscopic transport simulations has been most successful describing soft observables One of the main challenges is constraining key properties of strongly interacting matter, η/s and T init Problem is compounded by the lack of the knowledge of the initial and pre- equilibrium states. Other challenges include EoS, freeze-out, etc.

/PhysRevC Where we stand? Hydro-based hybrid models provide a good qualitative agreement with the data More comprehensive and numerically rigorous approach clearly needed! Quote from paper:”Except for very peripheral collisions, the different lines in Fig. 1 overlap almost perfectly and thus are hard to distinguish optically.”

Our Solution/Suggestion: CHIMERA CHIMERA is Comprehensive Heavy Ion Model Evaluation Reporting Algorithm that consists of: 1.) Hybrid viscous hydro model of heavy ion collisions 2.) Database of measured key soft observables (Spectra, elliptic flow, HBT 3D-femtoscopic radii) 3.) Module for comprehensive statistical comparison between model and physics observables CHIMERA is a framework (designed to be a bridge between theory and experiment) that facilitates model evaluation by performing statistical comparison to the measured data global observables

5 CHIMERA part-I: Hybrid viscous hydro model Viscous Hydro: UVH2+1 Luzum, Romatschke PRC 78, (2008) Modified Freeze-out (with viscous corrections) Pre-equilibrium flow (“initial flow”): Vredevoogd, Pratt PRC 79, (2009) Equation of State of choice UrQMD v2.3/3.3.1 S. Bass et al. nucl-th/ MC Glaub er Opt Glaub er CGC b fm 7.0 fm 10 fm

6 CHIMERA part-II: Global data “ > dataSummary->Scan(“***”,”RHIC snapshot”) “ Compiled global data (Spectra, HBT, elliptic flow) from different experiments into single convenient ROOT format PHENIX: PRL 91, (2003), PRC (2004), PRL (2009) STAR : PRC (2005), EurPhys J. C 49, (2007), PRC (2008), PRC (2005) ALICE: PLB (2010), PLB (2010) + more recent CMS: recent flow measurements ATLAS: recent flow

Selecting a simulation and corresponding subset of experimental data Fit model predictions with n-th order Chebyshev polynomials ( For particle spectra multiplied by m T scaling function) Calculate “modified” χ 2 with respect to the fit by properly accounting for all types of systematic uncertainties associated with experimental measurements using PHENIX recipe from PRC 77, : p T (k T ) observable datapoints y i prediction μ(p T ) CHIMERA part-III: Statistical evaluation - with σ i point-to-point uncorrelated uncertainties (stat & sys) - with σ b fully point-to-point correlated uncertainties (sys) For each observable class and input initial state, χ2/ndf surface in full (T, η/s) phase space is constructed, which is subsequently fit by ellipsoid functions to find minima. Finally, resulting 2-σ contours are combined to find overlap.

First successful full RHIC analysis with pions resubmitted to PRC after approval from the referee pending mostly cosmetic changes to figures, etc

9 Setting up RHIC simulations & analyses Centra -lity Initial profile Initial flow τ 0 [fm/c] EoSObser- vables T init [MeV] η/s T sw [MeV] b = 4.4 [fm] N part : MC Glauber N coll : MC Glauber ON OFF 1.0 hisq Spectra V2 HBT 270 – 370 ΔT = 5 MeV 260 – 420, ΔT = 20 MeV 260 – 420, ΔT = 20 MeV , 0.16, 0.24, Varying parameters Start selecting data observables for a given centrality range. For initial RHIC analysis it was ~ 278 Each simulation is run by a given subset of specified parameters:

χ2/ndf comparison illustration with pion HBT (RHIC)

11 Illustration with pion V 2 (RHIC)

12 Evaluation example with pion spectra (RHIC)

Combining all three observables

14 Full RHIC results: 2σ contours First successful demonstration of numerical evaluation using global fit to RHIC data As expected Initial flow improves matching with HBT and V2. While contours overlap care needs to be taken when combining for full chi-squared More details in arXiv:

15 Setting up LHC simulations & analyses Centra -lity Initial profile Initial flow τ 0 [fm/c] EoSObser- vables T init [MeV] η/s T sw [MeV] 0-5%N part : MC Glauber N coll : MC Glauber ON OFF 0.6 hisq Spectra V2 HBT 420 – 480 ΔT = 5 MeV 400 – 480, ΔT = 10 MeV 360 – 460, ΔT = 20 MeV … , 0.16, 0.24, For initial analysis we selected ALICE, CMS and ATLAS data for 0-5% centrality in 2.76 TeV Pb+Pb collisions. Varying model parameters

16 χ2 evaluation example with h ± spectra (LHC) The χ2 is right What is the best χ2/ndf on the plot ? A: < 1 B: 1-10 C: D:

17 Snapshot with HBT & V 2 (LHC)

LHC picture, with pre-flow and N part scalling

Important caveat for LHC Reality for LHC: statistical (uncorrelated) uncertainties are order of magnitude smaller than for model! In this approximation initial statistical evaluation approach inadequate and needing adjustment N coll, pre-flow, T = 420 MeV Refined procedure: 1.) Fit simultaneously both data and model using ONLY statistical (~uncorrelated) errors 2.) Calculate modified χ2 by comparing the function from Step-1 to the “combined data and model array”. Within combined array data points allowed to move collectively following correlated errors.

Summary (so far) with 0-5% LHC data With refined approach χ2 now well behaved Need to reanalyze other observable and parameters After quick look, large discrepancy for V 2 for N part scalling, while N coll look very promising. First comprehensive numerical evaluation of hybrid hydro +URQMD model for LHC data, and also first prediction of HBT.

21 Summary/Outlook First comprehensive RHIC analysis (to appear in PRC very soon arXiv: ) indicating towards significant constraining power of CHIMERA approach. Even with a lot more room for improvement in modeling Initial (“modified”) LHC analysis clearly disfavors N part scaling with current parameters is insufficient to describe spatial anisotropy. N coll looking more promising. Separate study with different EoS’ underway For both RHIC and LHC data, further exploration of larger χ 2 phase-space by including all particle species and collision centralities planned. Testing more parameters (τ start, T freeze-out ) planned Further and “desirable” model Improvements: (1) Incorporating different initial state geometry and fluctuations: Lumpy initial conditions, higher harmonics (particularly V 3 ) (2) Incorporating 3D+1viscous hydro in CHIMERA

22 Acknowledgements Former LLNL Collaborators: M. Cheng, A. Linden-Levy, J. Newby Thanks to: S. Pratt, P. Romatschke, P. Huovinen, D. Molnar, S. Bass, D. Brown, colleagues from HotQCD Collaboration, PHENIX, STAR, ALICE, CMS and ALICE

Backup slides

Full LHC picture for 0-5% (NO pre-flow)

Initial Density and Pre-equilibrium Flow DNP 2011-OCT-28 MSUR.A. Soltz - LLNL-PRES MC GlauberOp GlauberCGC(MC-KLN) Implemented Advancing start (τ) approaches pre-equilibrium flow (  ) values For now : MC Glauber, avg. eccentricity, single impact parameter b=4.4 fm (0-20%)