Jet modifications at RHIC Marco van Leeuwen, Utrecht University

2 QCD and quark parton model S. Bethke, J Phys G 26, R27 Running coupling:  s grows with decreasing Q 2 Asymptotic freedom At low energies, quarks are confined in hadrons At high energies, quarks and gluons are manifest Running coupling: from confinement to asymptotic freedom QCD governs both extremes. Can we study/conceptualise the evolution? This is the basic theory, but what is the phenomenology? QCD Lagrangian

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

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

5 Relativistic Heavy Ion Collider PHENIX STAR Au+Au  s NN = 200 GeV RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+Cu Two large experiments: STAR and PHENIX Smaller experiments: PHOBOS, BRAHMS decomissioned Dedicated to study QCD: proton spin and Quark Gluon Plasma

6 High-p T hadron suppression Size of medium Compare Au+Au spectra to properly scaled p+p spectra: ‘nuclear modification factor’  : no interactions Hadrons: energy loss R AA = 1 R AA < 1 Direct photons confirm volume scaling Hadrons suppressed: energy loss

7 Energy loss in QCD matter  : R AA = 1  0, h ± : R AA ≈ 0.2 Au+Au 200 GeV, 0-5% central Compare Au+Au spectra to properly scaled p+p spectra: ‘nuclear modification factor’ D. d’Enterria Hard partons lose energy in the hot matter Hadron suppression ~ independent of pT for pT>~4 GeV  : no interactions Hadrons: energy loss R AA = 1 R AA < 1

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

p T assoc > 3 GeV p T assoc > 6 GeV d+Au Au+Au 20-40% Au+Au 0-5% 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 High-p T hadron production in Au+Au dominated by (di-)jet fragmentation Highest p T : focus on fragmentation

10 Di­hadron yield suppression No suppression Suppression by factor 4-5 in central Au+Au Away-side: Suppressed by factor 4-5  large energy loss Near side Away side STAR PRL 95, < p T,trig < 15 GeV Yield of additional particles in the jet Yield in balancing jet, after energy loss Near side: No modification  Fragmentation outside medium?

11 Theory vs. data I PHENIX, arXiv: , J. Nagle WWND08 PQM (Loizides, Dainese, Paic), Multiple soft-scatttering approx (Armesto, Salgado, Wiedemann) Realistic geometry GLV (Gyulassy, Levai, Vitev), opacity expansion (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) For each model: 1.Vary parameter and predict R AA 2.Minimize  2 wrt data Models have different but ~equivalent parameters: transport coeff. gluon density dN g /dy energy density  0 coupling constant  S

12 Medium density from R AA PQM = 13.2 GeV 2 /fm ^ GLV dN g /dy = WHDG dN g /dy = ZOWW  0 = 1.9 GeV/fm AMY  s = Quantitative extraction gives medium density to 10-20% Method takes into account only exprimental uncertainties Theory uncertainties need to be further evaluted by comparing different formalisms and other model parameters Different models approximately agree – except PQM, high density Density 30-50x cold nuclear matter

13 d-Au Au-Au Medium density from di-hadron measurement I AA constraint D AA constraint D AA + scale uncertainty J. Nagle, WWND2008 associated  trigger  0 =1.9 GeV/fm single hadrons Medium density from away-side suppression and single hadron suppression agree Some open questions: disagreement in d+Au?

14 Fundamental quantity P(  E) ~15 GeV Renk, Eskola, hep-ph/ Salgado and Wiedemann, Phys. Rev. D68, Radiation spectrumRadiation in realistic medium In realistic systems, energy loss is a broad distribution P(  E) Single-hadron and di-hadron observables fold production spectra with P(  E) Can we access P(  E) experimentally? Need to fix parton energy:  -jet events  E jet = E 

15  -jet in Au+Au Use shower shape in EMCal to form  0 sample and  -rich sample Combinatorial subtraction to obtain direct-  sample A. Hamed, STAR, QM08

16 Away-side suppression with direct-  triggers A. Hamed et al QM08 First  -jet results from heavy ion collisions Measured suppression agrees with theory expectations Next step: measure p T assoc dependence to probe  E distribution Model predictions tuned to hadronic measurements

17 Lowering p T : gluon fragments/bulk response 3 < p t,trigger < 4 GeV p t,assoc. > 2 GeV Au+Au 0-10% STAR preliminary associated  trigger d+Au % Jet-like peak `Ridge’: associated yield at large  dN/d  approx. independent of  Strong  -  asymmetry suggests coupling to longitudinal flow Long. flow J. Putschke, M. van Leeuwen, et al

18 More medium effects: away-side 3.0 < p T trig < 4.0 GeV/c 1.3 < p T assoc < 1.8 GeV/c A. Polosa, C. Salgado Vitev, PLB630 Mach Cone/Shock wave T. Renk, J. Ruppert Stöcker, Casseldery-Solana et al Gluon radiation +Sudakov Au+Au 0-10% d+Au Near side: Enhanced yield in Au+Au consistent with ridge-effect Away-side: Strong broadening in central Au+Au ‘Dip’ at  =  Medium response (shock wave) or gluon radiation with kinematic constraints? (other proposals exist as well: k T -type effect or Cherenkov radiation)‏ M. Horner, M. van Leeuwen, et al Trigger particle Note also: not shown is large background – some non-trivial may be hiding there?

19 p T evolution of correlations Increase trigger p T arXiv: A.Hanks et al, WWND08 PHENIX arXiv Low p T : recoil clear double-peak Effect reduces with increasing p T ?

20 Conclusion High-p T hadron production at RHIC probes jet structure and parton energy loss Data/theory comparisons becoming quantitative –Qhat ~ 10 GeV 2 /fm, dN/dy ~ 1500  medium density 30-50x nuclear matter ‘Golden probe’  -jet: –First results consistent with hadron measurements –Can we extract P(  E)? Statistics needed … Intermediate p T GeV, new phenomena: –Ridge:  -  assymetry, yield at large  –Broad recoil distribution Luminosity still increase … so is theoretical understanding More to come!

% 4.0 < p T trig < 6.0 GeV/c 6.0 < p T trig < 10.0 GeV/c 3.0 < p T trig < 4.0 GeV/c Preliminary Au+Au 0-12% 1.3 < p T assoc < 1.8 GeV/c Low p T trig : broad shape, two peaksHigh p T trig : broad shape, single peak Away-side shapes Fragmentation becomes ‘cleaner’ as p T trig goes up Suggests kinematic effect? M. Horner, M. van Leeuwen, et al

22 Background level for di-hadrons Signal is few per cent So is v 2 -modulation Δ  12 2-Part Correlation Flow background “Jetty”sig nal C. Pruneau, QM06

23 Energy loss in a QCD medium + alternative scenarios, e.g. shock wave Energy loss and fragmentation Unmodified fragmentation after energy loss Fragmentation in the medium completely modified A more complete picture FragmentationIn-medium energy loss Or in-medium fragmentation Or Time-scales matter Hadron formation time Lower p T assoc : measure radiation fragments Lower p T trig : explore timescale

24 Baryon enhancement Large baryon/meson ratio in Au+Au ‘intermediate p T ‘ Hadronisation by coalescence? 3-quark p T -sum wins over fragmentation M. Konno, QM06 High p T : Au+Au similar to p+p  Fragmentation dominates p/  ~ 1,  /K ~ 2

25 Hadronisation through coalescence fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Fries, Muller et al Hwa, Yang et al Baryon p T =3p T,parton Meson p T =2p T,parton ‘Shower-thermal’ recombination will result in larger associated B/M at intermediate p T Recombination of thermal (‘bulk’) partons produces baryons at larger p T  No associated yield (Hwa, Yang)‏ Recombination enhances baryon/meson ratios Hard parton Hot matter

26 Associated yields from coalescence Baryon p T =3p T,parton Meson p T =2p T,parton Expect large baryon/meson ratio associated with high-p T trigger No associated yield with baryons from coalescence: Expect reduced assoc yield with baryon triggers 3<p T <4 GeV (Hwa, Yang) Hard parton Hot matter Baryon p T =3p T,parton Meson p T =2p T,parton Hard parton Hot matter Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination

27 Baryon/meson ratios in ‘jets’ Shape similar for mesons and baryons –provides constraint on models describing modification of away-side Baryon to Meson ratio similar to the bulk –inconsistent with vacuum fragmentation –consistent with jet induced medium excitation PHENIX arXiv, A. Hanks WWND 08

28 STAR Preliminary Associated baryon/meson ratios STAR Preliminary p/  ratio in jet-peak < inclusivep/  ratio in ridge > inclusive Ridge and jet-peak have different hadro-chemistry, different production mechanism Jet-peakRidge region p T trig > 4.0 GeV/c 2.0 < p T Assoc < p T trig

29 Jet-like peak: ( Λ+Λ) /2K 0 S ≈0.5 STAR Preliminary Associated baryon/meson ratios STAR Preliminary Ridge: ( Λ+Λ) /2K 0 S ≈ 1 Note: systematic error due to v 2 not shown Similar to p+p inclusive ratio Baryon/meson enhancement in the ridge? L. Gaillard, J. Bielcikova, C. Nattras et al. No shower-thermal contribution?

30 Separating jet and ridge: p T -spectra Jet spectra Yield (p t,assoc > p t,assoc,cut )‏ Ridge spectra Yield (p t,assoc > p t,assoc,cut )‏ p t,assoc,cut Jet (peak) spectra harden with p T,trig Peak dominated by jet fragmentationRadiated gluons ‘thermalise’ in the medium? Jet and ridge  separate dynamics inclusive Ridge yield and spectra independent of p T,trig Slope of spectra similar to inclusives J. Putschke, M. van Leeuwen, et al inclusive

31 Properties of medium at RHIC Broad agreement between different observables, and with theory pQCD: 2.8 ± 0.3 GeV 2 /fm (Baier)‏   23 ± 4 GeV/fm 3 T  400 MeV Transport coefficient Total E T Viscosity (model dependent)‏   = 0.3-1fm/c  ~ GeV/fm 3 T ~ MeV (Bjorken)‏ From v 2 (Majumder, Muller, Wang)‏ Lattice QCD:  /s < 0.1 A quantitative understanding of hot QCD matter is emerging (Meyer)‏

32 Fixing the jet energy:  -jet events T. Renk, PRC74,  -jet: monochromatic source  sensitive to P(  E) Expectations for different P(  E)‏ E  = 15 GeV   -jet events are rare, need large luminosity First results from 2007 RHIC run p+p

33 RHIC summary Jets interact strongly with medium at RHIC High p T : yield suppression, but no change in shapes –Fragmentation after energy loss Lower p T : enhancement, strongly modified shapes –Gluon fragments, medium response, etc Large Baryon/Meson ratio suggests coalescence of ‘free’ quarks –Test shower-thermal contribution by di-hadron correlations 2.8 ± 0.3 GeV 2 /fm Transport coefficient from high-p T results:

34 Extra slides

35 Extracting the transport coefficient Zhang, H et al, nucl-th/ Di-hadrons provide stronger constrain on density Extracted transport coefficient from singles and di-hadrons consistent 2.8 ± 0.3 GeV 2 /fm  2 -minimum narrower for di-hadrons Di-hadron suppression Inclusive hadron suppression Di-hadrons Inclusive hadrons

36 Theory vs. data II HEDP/HEDLA meeting APS St. Louis Apr 08 Jet probes of the QGP S g g AMY GeV/fm9.1ZOWW 1400 dy dN WHDG 1400 dy dN GLV fm/GeV2.13 ˆ PQM/ASW                  q Model parameters are constrained within ~20% Values are large: ~30-50 times cold nuclear matter density! Additional assumptions → different models are broadly consistent (except PQM – much larger than others) Strong conclusions: initially generated medium is highly opaque to energetic partons very dense, high temperature matter has been created PHENIX ’08; J. Nagle WWND08

37 Two extreme scenarios p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T Scenario I P(  E) =  (  E 0 )‏ ‘typical energy loss’ Shifts spectrum to left Scenario II P(  E) = a  (0) + b  (E)‏ ‘partial transmission’ Downward shift (or how P(  E) says it all)‏ P(  E) encodes the full energy loss process R AA cannot distinguish those two extreme scenarios … need more differential probes

38 Radiative energy loss: calculational frameworks 38 A. Majumder, nucl-th/ GLV (Gyulassy, Levai, Vitev): systematic expansion in small number of scatterings (“opacity”) n=L/ ASW (Armesto, Salgado, Wiedemann): multiple soft interactions ZOWW (Zhang, Owens, Wang Wang): medium-enhanced power corrections to vacuum fragmentation function (higher twist) AMY (Arnold, Moore, Yaffe): finite temperature effective field theory (Hard Thermal Loops) at small coupling

39 The extremes of QCD This is the basic theory, but what is the phenomenology? Small coupling Quarks and gluons are quasi-free Calculable with pQCD Two basic regimes in which QCD theory gives quantitative results: Hard scattering and bulk matter QCD Lagrangian Nuclear matterQuark Gluon Plasma High density Quarks and gluons are quasi-free Bulk QCD matter Calculable with Lattice QCD Hard scattering

40 Expectations for hot QCD matter Bernard et al. hep-lat/ T c ~ MeV Energy density from Lattice QCD Deconfinement transition: sharp rise of energy density at T c  Measure energy density   does not reach Boltzmann limit Signals remaining interactions, structure? Measure transport properties -Viscosity  -Transport coefficient  c ~ 1 GeV/fm 3

41 Bulk QCD matter in heavy ion collisions Azimuthal anisotropy: v2v2 Elliptic flow p T (GeV)‏ We create bulk QCD matter at RHIC Initial state pressure accelerates matter v 2 = 0 free streaming Au+Au event dN ch /dy  600 For central Au+Au at √s NN = 200 GeV Low p T : Qualitative agreement with hydrodynamics: viscosity, mean free path small

42 Experimental probes of energy loss Particle spectra –High statistics –Integrates over all production mechanisms Di-hadron correlations –Probe jet-structure –Some control over parton kinematics Identified particles –Probe hadronisation mechanisms –Heavy flavours: systematics of energy loss Focus of this talk

43 Model dependence of Different calculational frameworks C. Loizides hep-ph/ v2 2.8 ± 0.3 GeV 2 /fm Di-hadrons Inclusive hadrons Zhang, H et al, nucl-th/ Multiple soft scattering (BDMPS, Wiedemann, Salgado,…)‏ Twist expansion (Wang, Wang,…)‏ Different approximations to the theory give significantly different results Main uncertainties: -Formalism for QCD radiation -Geometry (density profile)‏

44 ALICE 2008: p+p 14 TeV 2009: Pb+Pb 5.5 TeV 3 Large general purpose detectors ALICE dedicated to Heavy Ion Physics, PID p,K,  out to p T > 10 GeV Large Hadron Collider at CERN ATLAS CMS

45 From RHIC to LHC RHIC  s=200 GeV Au+Au  s=5.5 TeV Pb+Pb LHC -Larger p T -reach: typical parton energy > typical  E -New observables e.g. jet reconstruction  fix parton energy Larger initial density  = GeV/fm 3 at RHIC  ~ 100 GeV/fm 3 at LHC 10k/year Large cross sections for hard processes Including heavy flavours Validate understanding of RHIC data Direct access to energy loss dynamics, P(  E)‏ And others, e.g. gluon saturation

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

47 Jet modifications at LHC Radial profile Fragmentation function PQM with fragmentation of radiated gluons (A. Morsch)‏ Energy loss depletes high-z and populates low-z Low-z fragments from gluon radiation at large R In-medium energy loss redistributes momenta in jets Measure these modifications to extract P(  E), medium properties Expectations from QCD+jet quenching Jet reconstruction E jet = 125 GeV  = ln( E Jet / p hadron )‏ z

48 ALICE EMCal Lead-scintillator sampling calorimeter |  |<0.7,  =110 o ~13k towers (  x  ~0.014x0.014)‏ ALICE-EMCal project: -Approved in Full detector by 2011 US-France-Italy project Testbeam: Support frame installed EMCal module Improves jet energy resolution Provides jet triggers

49 Full jet reconstruction performance Simulation input reference Medium modified (APQ)‏ Simulated result Full jet reco in ALICE is sensitive to modification of fragmentation function E >  E, explore dynamics of energy loss process

50 Conclusion Large effects of medium on parton fragmentation –Lower p T : various effects Large baryon/meson ratio Near-side ridge Away-side broadening –High p T : fragmentation after energy loss Quantitative understanding –Transport coefficient –High energy density  ~ GeV/fm 3 –Low viscosity  /s ~ 0.1 Clear picture of in-medium energy loss and medium properties at RHIC developing Future at RHIC and LHC: Direct measurements of energy loss  -jet -High-p T : E >  E -Full jet reconstruction Crucial tests of energy loss theory 2.8 ± 0.3 GeV 2 /fm

51 Thank you for your attention

52 M. Lamont (STAR), J.Phys.G32:S105-S114,2006 J. Bielcikova (STAR), v: [nucl-ex] (  Λ+Λ) /2K 0 S Baryon/meson ratio in jets, ridge and inclusive 2 < p T,trig < 3 GeV

53 Preliminary Near side yield |  |>0.9 Away side yield |  |<0.9 8 < p T trig < 15 GeV 8 < p T < 15 GeV z T =p T assoc /p T trig Energy loss in action Both near- and away-side show yield enhancement at low p T Possible interpretation: di-jet → di-jet (lower Q 2 ) + gluon fragments ‘primordial process’ High-p T fragments as in vacuum Near side: ridge Away-side: broadening M. Horner, M. van Leeuwen, et al Au+Au / d+Au 8 < p T < 15 GeV Near side yield ratio z T =p T assoc /p T trig Lower p T trig Preliminary Away side yield ratio z T =p T assoc /p T trig Au+Au / d+Au M. Horner, M. van Leeuwen, et al Lower p T trig

54 Results and interpretation Extraction of direct  away-side yields R=Y  -rich+h /Y  0+h near Y  +h = (Y  -rich+h - RY  0+h )/1-R away Assume no near-side yield for direct  then the away-side yields per trigger obey

55 Results and interpretation Direct  away-side yields The away-side yield of the associated particle per trigger in  -jet is suppressed.