Hot and dense matter: Hard probes from RHIC to LHC M. van Leeuwen, Utrecht University

Hard probes of QCD matter Use ‘quasi-free’ partons from hard scatterings Calculable with pQCD to probe ‘quasi-thermal’ QCD matter Interactions between parton and medium: Radiative energy loss Collisional energy loss Hadronisation: fragmentation and coalescence Quasi-thermal matter: dominated by soft (few 100 MeV) partons Use the strength of pQCD to explore QCD matter Sensitive to medium density, transport properties

Energy loss in QCD matter radiated gluon propagating parton QCD bremsstrahlung (+ LPM coherence effects) m2 Transport coefficient l Energy loss Energy loss probes: Density of scattering centers: Nature of scattering centers, e.g. mass: radiative vs elastic loss Or no scattering centers, but fields  synchrotron radiation?

Relativistic Heavy Ion Collider Au+Au sNN= 200 GeV PHENIX STAR RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+Cu Two large experiments: STAR and PHENIX Smaller experiments: PHOBOS, BRAHMS decomissioned Recent years: Large data samples, reach to high pT

Energy loss in QCD matter Yield per collision Nuclear modification factor : RAA = 1 C. Vale, K. Okada, Hard Probes 2008 0: RAA ≈ 0.2 D. d’Enterria : no interactions Hadrons: energy loss RAA = 1 RAA < 1 Hard partons lose energy in the hot matter

What can we learn from RAA? p0 spectra Nuclear modification factor PHENIX, PRD 76, 051106, arXiv:0801.4020 This is a cartoon! Hadronic, not partonic energy loss No quark-gluon difference Energy loss not probabilistic P(DE) Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 2 GeV for central collisions at RHIC Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this

Energy distribution from theory TECHQM ‘brick problem’ L = 2 fm, DE/E = 0.2 E = 10 GeV ‘Typical for RHIC’ ASW: Armesto, Salgado, Wiedemann WHDG: Wicks, Horowitz, Dordjevic, Gyulassy Not a narrow distribution: Significant probability for DE ~ E Conceptually/theoretically difficult T. Renk Significant probability to lose no energy For nuclear collisions, need to fold in geometry

Determining the medium density PHENIX, arXiv:0801.1665, J. Nagle WWND08 PQM (Loizides, Dainese, Paic), Multiple soft-scattering approx (Armesto, Salgado, Wiedemann) Realistic geometry For each model: Vary parameter and predict RAA Minimize 2 wrt data Models have different but ~equivalent parameters: Transport coeff. Gluon density dNg/dy Typical energy loss per L: e0 Coupling constant aS GLV (Gyulassy, Levai, Vitev), Opacity expansion (L/l), Average path length WHDG (Wicks, Horowitz, Djordjevic, Gyulassy) GLV + realistic geometry ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry AMY (Arnold, Moore, Yaffe) Finite temperature effective field theory (Hard Thermal Loops)

Medium density from RAA ^ +2.1 - 3.2 PQM <q> = 13.2 GeV2/fm +270 - 150 +0.2 - 0.5 GLV dNg/dy = 1400 ZOWW e0 = 1.9 GeV/fm +200 - 375 +0.016 - 0.012 WHDG dNg/dy = 1400 AMY as = 0.280 Quantitative extraction gives medium density to 10-20% Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated e.g. comparing different formalisms, varying geometry Different models approximately agree – except PQM, high density Density 30-50x cold nuclear matter

Heavy quark suppression Using non-photonic electrons Expected energy loss light M.Djordjevic PRL 94 Wicks, Horowitz et al, NPA 784, 426 PHENIX nucl-ex/0611018, STAR nucl-ex/0607012 Expect: heavy quarks lose less energy due to dead-cone effect Most pronounced for bottom Measured suppression of non-photonic electrons larger than expected Djordjevic, Phys. Lett. B632, 81 Armesto, Phys. Lett. B637, 362 Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al)

Di­hadron correlations 8 < pTtrig < 15 GeV Combinatorial background associated pTassoc > 3 GeV  trigger Near side Away side Use di-hadron correlations to probe the jet-structure in p+p, d+Au and Au+Au

Medium density from di-hadron measurement J. Nagle, WWND2008 8 < pT,trig < 15 GeV associated  trigger d-Au IAA constraint DAA constraint DAA + scale uncertainty Au-Au Medium density from away-side suppression and single hadron suppression agree Theory: ZOWW, PRL98, 212301 e0=1.9 GeV/fm single hadrons However: Theory curve does not match d+Au: need to evaluate systematics Experimental uncertainties will decrease in near future Data: STAR PRL 95, 152301 zT=pT,assoc/pT,trig

Lowering pT: gluon fragments/bulk response d+Au, 200 GeV  trigger Au+Au 0-10% STAR preliminary Jet-like peak associated 3 < pt,trigger < 4 GeV pt,assoc. > 2 GeV Long. flow J. Putschke, M. van Leeuwen, et al `Ridge’: associated yield at large  dN/d approx. independent of  Strong - asymmetry suggests effect of longitudinal flow or underlying event

Associated yields from coalescence Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination Baryon pT=3pT,parton Meson pT=2pT,parton Baryon pT=3pT,parton Meson pT=2pT,parton Hot matter Hot matter Hard parton (Hwa, Yang) No jet structure/associated yield Expect large baryon/meson ratio associated with high-pT trigger Expect reduced associated yield with baryon triggers 3 < pT < 4 GeV

Associated baryon/meson ratios pTtrig > 4.0 GeV/c 2.0 < pTAssoc < pTtrig C. Suarez et al, QM08 p+p / p++p- Inclusive spectra Associated yields Au+Au: Baryon enhancement Ridge (large Dh): Baryon enhancement p+p, d+Au: B/M  0.3 Jet (small Dh) B/M  0.3 Baryon/meson ratio in ridge close to Au+Au inclusive, in jet close to p+p Different production mechanisms for ridge and jet?

Parton energy from g-jet and jet reconstruction second-generation measurements at RHIC Qualitatively: known pQCDxPDF extract `known’ from e+e- Full deconvolution large uncertainties (+ not transparent) Fix/measure Ejet to take one factor out  Two approaches: g-jet Jet reconstruction

Direct-g recoil suppression  Expected recoil for various P(DE) T. Renk Measurement sensitive to energy loss distribution P(DE) Need precision to distinguish scenarios 8 < ET,g < 16 GeV 2 < pTassoc < 10 GeV J. Frantz, Hard Probes 2008 A. Hamed, Hard Probes 2008 STAR Preliminary ET,g DAA (zT) IAA(zT) = Dpp (zT) Large suppression for away-side: factor 3-5 Results agree with model predictions Uncertainties still sizable Some improvements expected for final results Future improvements with increased RHIC luminosity 17

Jet reconstruction in heavy ion events η pt per grid cell [GeV] STAR preliminary ~ 21 GeV Jets clearly visible in heavy ion events at RHIC Quantitative analysis requires: Good jet-finding algorithm Combinatorial background subtraction Use different algorithms to estimate systematic uncertainties: Cone-type algorithms simple cone, iterative cone, infrared safe SISCone Sequential recombination algorithms kT, Cambridge, inverse kT http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188

Jet spectrum – RAA of jets Energy resolution Pythia jet+Heavy ion background Effect of resolution on spectrum ET [GeV] Nbin scaled p+p Au+Au 0-10%  MB-Trig O HT-Trig R=0.4 pT cut =1 GeV Seed=4.6 GeV LOHSC Statistical Errors Only dNJet/dET (per event) dNJet/dET (per event) PyDet PyEmbed PyTrue LOHSC seed=4.6 GeV R=0.4 STAR Preliminary Counts pTcut =1.0 GeV Seed=4.6 GeV LOHSC pTcut =1 GeV R=0.4 ET=35±5 GeV S. Salur, Hard Probes 2008 STAR Preliminary ∆E E = EPyDet – EPyTrue E = EPyEmbed - EPyTrue E = EPyEmbed - EPyDet No significant suppression of jet production in AA  Jet reconstruction recovers unbiased jet sample

Jet fragmentation Fragmentation function Fragmentation function ratio A+A/p+p pt,jetrec. >30 GeV STAR preliminary stat. errors only J. Putschke, Hard Probes 2008 pthadron~10 GeV Au+Au HT Et>7.5 GeV p+p HT Et>5.4 GeV Jet-energy uncorrected – needs study No apparent modification in the fragmentation function with respect to p+p

Relating jets and single hadrons High-pT hadrons from jet fragmentation Qualitatively: Measured suppression of single hadrons: Suppression of jet yield (out-of-cone radiation) Modification of fragment distribution (in-cone radiation) First results from STAR show neither Work in progress: Better understanding of interplay between reconstruction biases (trigger bias, background subtraction) and jet-quenching needed Requires theory-experiment collaboration

New development: E-loss generators Event generators treat full in-medium QCD shower: Theory/model advantage over analytic approach: conserve energy/respect kinematic bounds Important experimental tool: can perform jet reconstruction (and other analysis) on model 2 models JEWEL K. Zapp, U. Wiedemann, arXiv:0804.3568 Focus on collisional energy loss + medium response Cluster hadronisation q-PYTHIA L. Cunqueiro, N. Armesto, C. Salgado, arXiv:0809.4433 Focus on radiative energy loss Modified shower evolution Important step for the field

Transport and medium properties Transport coefficient 2.8 ± 0.3 GeV2/fm (model dependent) e  23 ± 4 GeV/fm3 pQCD: T  400 MeV (Baier) (Majumder, Muller, Wang)  ~ 5 - 15 GeV/fm3 T ~ 250 - 350 MeV Viscosity t0 = 0.3-1fm/c Total ET From v2 (see previous talk: Steinberg) (Bjorken) Lattice QCD: h/s < 0.1 (Meyer) Broad agreement between different observables, and with theory A quantitative understanding of hot QCD matter is emerging

Outlook I: LHC Simulated result ALICE EMCal TDR For example: Can measure jet fragmentation with Ejet = 175 GeV Talks by T. Awes, M. Spousta, L. Sarycheva Use kinematic reach (DE >> E) to determine energy dependence of DE Large hard process yields: Jets to > 200 GeV Light, heavy hadrons to 100 GeV Test/validate understanding gained from RHIC

Projected performance for g-hadron measurement Outlook II: RHIC Accelerator upgrades Stochastic cooling Detector upgrades Vertex detectors PHENIX Projected performance for g-hadron measurement STAR Enables charm/bottom direct measurements Ongoing data analysis: Large sample Au+Au (run-7) and d+Au (run-8)

Conclusion Hard probes provide insight in nature of hot QCD matter RHIC results: ‘Standard’ measurements extent to high pT, start to constrain theory in detail Intermediate pT: interplay between hard and soft physics, e.g. ‘the ridge’ New large samples: experimental control over Eparton Gamma-jet analysis Jet reconstruction DE ~ 2 GeV at RHIC, DE/E not small Start systematic comparisons of different measurements with theory  Assessment of systematics in theory and experiment Progress towards ‘global analysis’ of heavy ion data Future: RHIC: larger luminosity, detector upgrades LHC: Large rates at high pT, exploit DE < E Theory: develop in-medium shower generators

Conclusion Hard probes provide insight in nature of hot QCD matter Recent developments: Large luminosity at RHIC Extend to higher pT Gamma-hadron analysis (fix parton energy) Jet reconstruction Near future at LHC Large rates at high pT, exploit DE < E Theory: develop in-medium shower generators DE ~ 2 GeV at RHIC, DE/E not small Start combining different measurements to improve understanding - Assessment of systematics in theory and experiment Progress towards ‘global analysis’ of heavy ion data

Extra Slides

Direct-g recoil yields  Run 4 p+p/Au+Au @ 200 GeV M. Nguyen, Quark Matter 2006 A. Hamed, Hard Probes 2008 Direct-g–jet measurements being pursued by STAR and PHENIX Requires large data samples Suppression of away-side yield visible Similar to di-hadrons, but now with selected parton energy

Away-side suppression  A. Hamed et al QM08 Away-side yield AuAu/pp Model predictions tuned to hadronic measurements First g-jet results from heavy ion collisions Measured suppression agrees with theory expectations Next step: measure pTassoc dependence to probe DE distribution

Intermediate pT: the ridge 3 < pt,trig< 4 GeV/c Jet-like peak 4 < pt,trig < 6 GeV/c pt,assoc. > 2 GeV/c Au+Au 0-10% STAR preliminary Au+Au 0-10% STAR preliminary J. Putschke et al, QM06  trigger `Ridge’: associated yield at large , small Df associated Weak dependence of ridge yield on pT,trig  Relative contribution reduces with pT,trig Strong - asymmetry suggests coupling to longitudinal flow

RAA at LHC GLV BDMPS T. Renk, QM2006 RHIC RHIC S. Wicks, W. Horowitz, QM2006 LHC: typical parton energy > typical E Expected rise of RAA with pT depends on energy loss formalism Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

Baryon/meson ratios Anti-Baryon/meson ratio p/p ratio P. Fachini et al, QM08 Theory: X.-N. Wang, PRC 70, 031901 Large baryon/meson ratio at intermediate pT  Hadronisation by coalescence of quarks? New data extend pT-range in p+p Expect p mainly from gluons and DEg>DEq, no stronger suppression seen for p New STAR results on baryon fragmentation in p+p, see M. Heinz

Heavy-light difference Expect: dead-cone effect M.Djordjevic PRL 94 (2004) Wicks, Horowitz et al, NPA 784, 426 Armesto et al, PRD71, 054027 light Armesto plot Below 10 GeV: charm loses 20-30% less energy than u,d Bottom loses ~80% less Expected suppression of D mesons ~0.5 times light hadrons

Heavy flavour: Discrepancy STAR/PHENIX spectra PHENIX: A/(exp(-a pT - b pT2) + pT/p0)n: A = 377 +/- 60 mb GeV^-2 c^3 a = 0.3565 +/- 0.014 c/GeV b = 0.0680 +/- 0.019 (c/GeV)^2 p0 = 0.70 +/- 0.02 (c/GeV) n = 8.25 +/- 0.04 Zhangbu Xu (STAR Collaboration) SQM08 35 35

Zhangbu Xu (STAR Collaboration) SQM08 e/p ratio STAR Preliminary STAR Preliminary e+e- STAR Preliminary Conversion in detector material x10 reduced Run8 is consistent with run3 NPE results Run8: 0.55%X0 (beampipe 0.29%, air: 0.1%, wrap ~0.14% …) run3: 5.5%X0 (+ SVT…) Detector Material is not the issue Jin Fu, Parallel session 12.I Zhangbu Xu (STAR Collaboration) SQM08 36 36

QCD and quark parton model At high energies, quarks and gluons are manifest At low energies, quarks are confined in hadrons S. Bethke, J Phys G 26, R27 Asymptotic freedom Running coupling: s grows with decreasing Q2 Running coupling: from confinement to asymptotic freedom QCD governs both extremes. Can we study/conceptualise the evolution? Study emergent behaviour at large coupling: confinement, bulk QCD matter

Radiative energy loss in QCD Energy loss process characterized by a single constant kT~m Transport coefficient l Energy loss Transport coefficient sets medium properties pQCD expectation (Baier et al) Non-perturbative: is a Wilson loop (Wiedemann) e.g. N=4 SUSY: From AdS/CFT (Liu, Rajagopal, Wiedemann) Transport coefficient is a fundamental parameter of QCD matter

Some open questions Urs Wiedemann, Hard Probes 2008

Di-hadron suppression in Au+Au d+Au Au+Au 20-40% Au+Au 0-5% pTassoc > 3 GeV pTassoc > 6 GeV High-pT hadron production in Au+Au dominated by (di-)jet fragmentation Suppression of away-side yield in Au+Au collisions Measures energy loss in di-jet events No detectable broadening or change of peak shape: fragmentation after energy loss