The Quark-Gluon Plasma and Jet Quenching Marco van Leeuwen

2 QCD and hadrons Quarks and gluons are the fundamental particles of QCD (feature in the Lagrangian) However, in nature, we observe hadrons: Color-neutral combinations of quarks, anti-quarks Baryon multiplet Meson multiplet Baryons: 3 quarks I 3 (u,d content) S strangeness I 3 (u,d content) Mesons: quark-anti-quark ‘Red + Green + Blue = white’ ‘Red + anti-Red = white’

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

4 How does it fit together? S. Bethke, J Phys G 26, R27 Running coupling:  s decreases with Q 2 Pole at  =   QCD ~ 200 MeV ~ 1 fm -1 Hadronic scale

5 Asymptotic freedom and pQCD At large Q 2, hard processes: calculate ‘free parton scattering’ At high energies, quarks and gluons are manifest + more subprocesses

6 Low Q 2 : confinement Lattice QCD potential  large, perturbative techniques not suitable Lattice QCD: solve equations of motion (of the fields) on a space-time lattice by MC Bali, hep-lat/ No free color charges can exist: would take infinite energy field generates quark-anti-quark pairs

7 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 Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)  c ~ 1 GeV/fm 3 g : deg of freedom Nuclear matter Quark Gluon Plasma

8 QCD phase diagram Temperature Confined hadronic matter Quark Gluon Plasma (Quasi-)free quarks and gluons Nuclear matter Neutron stars Elementary collisions (accelerator physics) High-density phases? Early universe Critical Point Bulk QCD matter: T and  B drive phases

9 Heavy ion collisions Collide large nuclei at high energy to generate high energy density  Quark Gluon Plasma Study properties RHIC: Au+Au  s NN = 200 GeV Lac Leman Lake Geneva Geneva airport CERN Meyrin site LHC: Pb+Pb √s NN ≤ 5.5 TeV 27 km circumference

10 ALICE Central tracker: |  | < 0.9 High resolution TPC ITS Particle identification HMPID TRD TOF Forward muon arm -4 <  < : 20M hadronic Pb+Pb events, 300M p+p MB events EM Calorimeters EMCal PHOS

11 Heavy ion Collision in ALICE

12 Heavy ion collisions ‘Hard probes’ Hard-scatterings produce ‘quasi-free’ partons  Probe medium through energy loss p T > 5 GeV Heavy-ion collisions produce ‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ MeV ‘Bulk observables’ Study hadrons produced by the QGP Typically p T < 1-2 GeV Two basic approaches to learn about the QGP 1)Bulk observables 2)Hard probes

13 Centrality examples This is what you really measure... and this is what you see in a presentation central mid-centralperipheral

14 Centrality Peripheral Central Density, Temperature, Pressure (Almost) Circular Volume, ‘Number of participants’ Initial shape Elliptic Lifetime ‘QGP effects’

15 Hard Probes of Heavy Ion Collisions Use this ALICE Pb+Pb event To probe this

16 Participants and Collisions b N part : n A + n B (ex: = 9 + …) N bin : n A x n B (ex: 4 x 5 = 20 + …) Two limits: - Complete shadowing, each nucleon only interacts once,   N part - No shadowing, each nucleon interact with all nucleons it encounters,  N bin Soft processes: long timescale, large   tot  N part Hard processes: short timescale, small ,  tot  N bin

17 Testing volume (N coll ) scaling in Au+Au PHENIX Direct  spectra Scaled by N coll PHENIX, PRL 94, Direct  in A+A scales with N coll Centrality A+A initial state is incoherent superposition of p+p for hard probes

18 Fragmentation and parton showers In the vacuum (no QGP) 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

19 Medium-induced radiation propagating parton radiated gluon Landau-Pomeranchuk-Migdal effect Formation time important Radiation sees length ~  f at once Energy loss depends on density: and nature of scattering centers (scattering cross section) Transport coefficient C R : color factor (q, g) : medium density L: path length m: parton mass (dead cone eff) E: parton energy Path-length dependence L n n=1: elastic n=2: radiative (LPM regime) n=3: AdS/CFT (strongly coupled) Energy loss

20  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

21 Nuclear modification factor p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T ‘Energy loss’ Shifts spectrum to left ‘Absorption’ Downward shift ‘What you plot is what you get’ Measured R AA is a ratio of yields at a given p T The physical mechanism is energy loss; shift of yield to lower p T

22 Nuclear modification factor (pre-QM) PHENIX run-4 data RHIC √s NN =200 GeV ALICE: arXiv: CMS: arXiv: LHC √s NN =2.76 TeV LHC: increase of R AA with p T RHIC: no p T dependence ? ASW: HT: AMY: Model curves: density fit to data Model curves: Density scaled from RHIC Some curves fit well, others don’t  Handle on E-loss mechanism(s)

23 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

24 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

25 Jets in Pb+Pb Out-of-cone radiation: suppression of jet yield: R AA jets < 1 In-cone radiation: softening and/or broadening of jet structure Main motivation: integrate radiated energy; Determine ‘initial parton energy’ First question: is out-of-cone radiation significant?

26 PbPb jet spectra Charged jets, R=0.3 Jet spectrum in Pb+Pb: charged particle jets Two cone radii, 4 centralities M. QM R CP, charged jets, R=0.3 Jet reconstruction does not ‘recover’ much of the radiated energy

27 Pb+Pb jet R AA Jet R AA measured by ATLAS, ALICE, CMS R AA < 1: not all produced jets are seen; out-of-cone radiation and/or ‘absorption’ For jet energies up to ~250 GeV; energy loss is a very large effect ATLAS+CMS: hadron+EM jets ALICE: charged track jets Good agreement between experiments Despite different methods:

28 , hadrons, jets compared , hadrons Jets Suppression of hadron (leading fragment) and jet yield similar

29 Model comparison M. QM2012 JEWEL: K. Zapp et al, Eur Phys J C69, 617 U. Hadron R AA Jet R AA Schukraft et al, arXiv: At least one model calculation reproduces the observed suppression  Understand mechanism for out-of-cone radiation?

30 Jet broadening: R dependence Ratio of spectra with different R Larger jet cone: ‘catch’ more radiation  Jet broadening ATLAS, A. Angerami, QM2012 However, R = 0.5 still has R AA < 1 – Hard to see/measure the radiated energy

31 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?

32 The End

33 Summary Elementary particles of the strong interaction (QCD): quarks and gluon Bound states: p, n, , K (hadrons) Bulk matter: Quark-Gluon-Plasma –High T~200 MeV Heavy ion collisions: –Produce and study QGP –Elliptic flow –Parton energy loss

34 Extra slides

35 Centrality dependence of hard processes d  /dN ch 200 GeV Au+Au Rule of thumb for A+A collisions (A>40) 40% of the hard cross section is contained in the 10% most central collisions Binary collisions weight towards small impact parameter Total multiplicity: soft processes

36 Elementary particles Atom Electron elementary, point-particle Protons, neutrons Composite particle  quarks up charm top down strange bottom Quarks: Electrical charge Strong charge (color) electron Muon Tau    Leptons: Electrical charge Force carriers: photon EM force gluon strong force W,Z-boson weak force Standard Model: elementary particles +anti-particles EM force binds electrons to nucleus in atom Strong force binds nucleons in nucleus and quarks in nucleons

37 Quarks, gluons, jets Jets: Signature of quarks, gluons in high-energy collisions large Q 2 Q ~ m H ~  QCD High-energy parton Hadrons Quarks, gluons radiate/split in vacuum to hadronise

38 R AA at LHC Larger dynamic range at LHC very important: sensitive to P(  E;E) Nuclear modification factor LHC: R AA rises with p T  relative energy loss decreases Au+Au  s NN = 200 GeVPb+Pb  s NN = 2760 GeV

39 Jet broadening: transverse fragment distributions PbPb PbPb CMS PAS HIN CMS, P. Jet broadening: Soft radiation at large angles

40 Time evolution All observables intregrate over evolution Radial flow integrates over entire ‘push’