Power Week pQCD+Energy Loss Introduction Marco van Leeuwen, Utrecht University

2 Hard probes of QCD matter Use the strength of pQCD to explore QCD matter Hard-scatterings produce ‘quasi-free’ partons  Initial-state production known from pQCD  Probe medium through energy loss Heavy-ion collisions produce ‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ MeV Sensitive to medium density, transport properties

3 Plan of the next few days Perturbative QCD tools: –PDFs, matrix elements, Fragmentation DGLAP evolution and Monte-Carlo showers Geometry –Woods-Saxon geometry, tools Energy loss models –Using Quenching weights; multiple gluon radiation Goals: Provide hands-on experience with all ingredients of a simple energy loss model Increase understanding of the models, the assumptions and uncertainties Provide a basic knowledge and experience that allows you to answer your own questions Plus introduction to MC techniques

4 Hard processes in QCD Hard process: scale Q >>  QCD Hard scattering High-p T parton(photon) Q~p T Heavy flavour production m >>  QCD Cross section calculation can be split into Hard part: perturbative matrix element Soft part: parton density (PDF), fragmentation (FF) Soft parts, PDF, FF are universal: independent of hard process QM interference between hard and soft suppressed (by Q 2 /  2 ‘Higher Twist’) Factorization parton densitymatrix elementFF

5 Jet Quenching 1)How is does the medium modify parton fragmentation? Energy-loss: reduced energy of leading hadron – enhancement of yield at low p T ? Broadening of shower? Path-length dependence Quark-gluon differences Final stage of fragmentation outside medium? 2)What does this tell us about the medium ? Density Nature of scattering centers? (elastic vs radiative; mass of scatt. centers) Time-evolution? High-energy parton (from hard scattering) Hadrons

6  0 R AA – high-p T suppression Hard partons lose energy in the hot matter  : no interactions Hadrons: energy loss R AA = 1 R AA < 1  0 : R AA ≈ 0.2  : R AA = 1

7 A simple model `known’ from e + e - known pQCDxPDF extract Parton spectrum Fragmentation (function) Energy loss distribution This is where the information about the medium is P(  E) combines geometry with the intrinsic process – Unavoidable for many observables Notes: This formula is the simplest ansatz – Independent fragmentation after E-loss assumed Jet,  -jet measurements ‘fix’ E, removing one of the convolutions We will explore this model during the week; was ‘state of the art’ 3-5 years ago

8 Two extreme scenarios p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T Scenario I P(  E) =  (  E 0 ) ‘Energy loss’ Shifts spectrum to left Scenario II P(  E) = a  (0) + b  (E) ‘Absorption’ Downward shift (or how P(  E) says it all) P(  E) encodes the full energy loss process R AA not sensitive to energy loss distribution, details of mechanism

9 Energy loss distribution Brick L = 2 fm,  E/E = 0.2 E = 10 GeV Typical examples with fixed L  E/E> = 0.2 R 8 ~ R AA = 0.2 Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to  E/E = 1) Theory expectation: mix of partial transmission+continuous energy loss – Can we see this in experiment?

10 Geometry Density profile Profile at  ~  form known Density along parton path Longitudinal expansion dilutes medium  Important effect Space-time evolution is taken into account in modeling (Glauber geometry)

11 Some existing calculations Bass et al, PRC79, ASW: HT: AMY: Large density: AMY: T ~ 400 MeV Transverse kick: qL ~ GeV All formalisms can match R AA, but large differences in medium density At RHIC:  E large compared to E, differential measurements difficult After long discussions, it turns out that these differences are mostly due to uncontrolled approximations in the calculations  Best guess: the truth is somewhere in-between This week: looking behind the scenes for such calculations

12 R AA at LHC R AA at LHC: increase with p T  first sign of sensitivity to P(  E) Nuclear modification factor By the way: R AA is also p T -dependent at RHIC?

13 Comparing to theory Many theory calculations available Ingredients: -pQCD production -Medium density profile tuned to RHIC data, scaled -Energy loss model Large spread of predictions: Will be narrowed down by discussion/thought Need to understand models/calculations to sort it out All calculations show increase with p T

14 Path length dependence: R AA vs L PHENIX, PRC 76, In Plane Out of Plane 3<p T <5 GeV/c R AA as function of angle with reaction plane Suppression depends on angle, path length Relation between RAA(  ) and v 2 :

15 Path length dependence and v 2 PHENIX PRL105, v 2 at high p T due to energy loss Most calculations give too small effect Path length dependence stronger than expected? Depends strongly on geometry – stay tuned

16 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

17 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: energy loss High-p T hadron production in Au+Au dominated by (di-)jet fragmentation Di-hadrons at high-p T : recoil suppression

18 Di­hadron yield suppression 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? Near side associated trigger Away side associated trigger

19 Path length II: ‘surface bias’ Near side trigger, biases to small E-loss Away-side large L Away-side suppression I AA samples longer path-lengths than inclusives R AA

20 L scaling: elastic vs radiative T. Renk, PRC76, R AA : input to fix density Radiative scenario fits data; elastic scenarios underestimate suppression Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions Confirms L 2 dependence  radiative loss dominates

21 Factorisation in perturbative QCD Parton density function Non-perturbative: distribution of partons in proton Extracted from fits to DIS (ep) data Matrix element Perturbative component Fragmentation function Non-perturbative Measured/extracted from e+e- Factorisation: non-perturbative parts (long-distance physics) can be factored out in universal distributions (PDF, FF)

22 PYTHIA (by Adam Kocoloski) gg qq gq Subprocesses and quark vs gluon p+pbar dominantly from gluon fragmentation?

23 PRL 97, (2006) STAR Preliminary, QM08 Curves: X-N. Wang et al PRC70(2004) Baryon & meson NMF STAR Preliminary Comparing quark and gluon suppression Protons less suppressed than pions, not more No sign of large gluon energy loss

24 Quark vs gluon suppression + renk plot WHDG Renk and Eskola, PRC76, GLV formalism BDMPS formalism Quark/gluon difference larger in GLV than BDMPS (because of cut-off effects  E < E jet ?) ~10% baryons from quarks, so baryon/meson effect smaller than gluon/quark Are baryon fragmentation functions under control? Conclusion for now: some homework to do... Day 1, 3 of this week

25 Equalibration of rare probes Rare probes: not chemically equilibrated in the jet spectrum. Example 1: flavor not contained in the medium, but can be produced off the medium (e.g. photons) –Need enough yield to outshine other sources of N rare. Example 2: flavor chemically equilibrated in the medium –E.g. strangeness at RHIC –Coupling of jets (flavor not equilibrated) to the equilibrated medium should drive jets towards chemical equilibrium. R. Fries, QM09

26 Determining the initial energy `known’ from e + e - known pQCDxPDF extract Parton spectrum Fragmentation (function) Energy loss distribution This is where the information about the medium is P(  E) combines geometry with the intrinsic process Jet,  -jet measurements ‘fix’ E, removing one of the convolutions Allows to study energy loss as function of E (at least in principle)

27 Generic expectations from energy loss Longitudinal modification: –out-of-cone  energy lost, suppression of yield, di-jet energy imbalance –in-cone  softening of fragmentation Transverse modification –out-of-cone  increase acoplanarity k T –in-cone  broadening of jet-profile kT~kT~ E jet fragmentation after energy loss?

28 Fragmentation functions Qualitatively: Fragmentation functions sensitive to P(  E) Distinguish GLV from BDMPS?

29 Modified fragmentation functions Small-z enhancement from gluon fragments (only included in HT, not important for R AA ) Differences between formalisms large, both magnitude of supresion and z-dependence Can we measure this directly? Jet reconstruction A. Majumder, MvL, arXiv:

30 Jet shapes Energy distribution in sub-jets Energy loss changes radial distribution of energy Several ‘new’ observables considered Discussion: sensitivity  viability … ongoing q-Pythia, Eur Phys J C 63, 679

31 Fixing the parton energy with  -jet events T. Renk, PRC74,  -jet: know jet energy  sensitive to P(  E) R AA insensitive to P(  E) Nuclear modification factor Away-side spectra in  -jet E  = 15 GeV Away-side spectra for  -jet are sensitive to P(  E)  Input energy loss distribution

32 I AA (z T ) = D AA (z T ) D pp (z T ) Direct-  recoil suppression Large suppression for away-side: factor 3-5 Reasonable agreement with model predictions  8 < E T,  < 16 GeVSTAR, arXiv: NB: gamma p T = jet p T still not very large

33 Jet reconstruction algorithms Two categories of jet algorithms: Sequential recombination k T, anti-k T, Durham –Define distance measure, e.g. d ij = min(p Ti,p Tj )*R ij –Cluster closest Cone –Draw Cone radius R around starting point –Iterate until stable ,  jet = particles For a complete discussion, see: Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors Jet is an object defined by jet algorithm If parameters are right, may approximate parton

34 Jets at LHC LHC: jet energies up to ~200 GeV in Pb+Pb from 1 ‘short’ run Large energy asymmetry observed for central events

35 Jets at LHC Centrality ATLAS, arXiv: (PRL) Jet-energy asymmetry Large asymmetry seen for central events N.B. only measures reconstructed di-jets Does not show ‘lost’ jets Large effect on recoil: qualitatively consistent with RHIC jet I AA Energy losses: tens of GeV, ~ expected from BDMPS, GLV etc beyond kinematic reach at RHIC

36 Jets at LHC CMS, arXiv: CMS sees similar asymmetries

37 Jet R CP R=0.2R=0.4 R CP < 1: jet production suppressed, even at high p T  Out-of-cone radiation with R=0.4 significant NB: Jet-measurements are difficult: important experimental questions about (trigger) bias and background fluctuations

38 Jet imbalance calculations Qin, Muller, arXiv: Radiation plus evolution Parton transport (brick) Coleman-Smith, Qin, Bass, Muller, arXiv: MARTINI: AMY+MC Young, Schenke, Jeon, Gale, arXiv: Several calculations describe measured imbalance Need to keep track of all fragments: Various approximations made Most natural approach: parton showers (qPYTHIA, qHERWIG, JEWEL ?)

39 Fragmentation and parton showers large Q 2 Q ~ m H ~  QCD FF Analytical calculations: Fragmentation Function D(z,  ) z=p h /E jet Only longitudinal dynamics High-energy parton (from hard scattering) Hadrons MC event generators implement ‘parton showers’ Longitudinal and transverse dynamics

40 Getting ready Software set-up: ROOT LHAPDF Fragmentation function libraries AliFastGlauber AliQuenchingWeights Day 1 See also Day 2 Day 3 Make sure that you have the code and that the test macros work Questions/problems  See me or Andreas

41 Extra slides

42 Seeing quarks and gluons In high-energy collisions, observe traces of quarks, gluons (‘jets’)