1. 1 Hot quarks! : What do we see in Relativistic Heavy Ion collisions “ How to cook the primordial soup” Manuel Calderón de la Barca Sánchez Indiana University  How does the strong force, QCD, behave at high energy?  How can we study it?  What tools do we use?  What have we learned?

2. 2 Manuel Calderón de la Barca, P408/508 Seminar Disclaimer many of the concepts I will present are “works in very active progress” an unavoidable consequence of exciting, cutting edge science

3. 3 Manuel Calderón de la Barca, P408/508 Seminar Heavy Ions: How does nuclear matter look at high temperature?  ~ 1-3 GeV/fm 3 High Density QCD Matter in Laboratory Determine its properties QCD Prediction: Phase Transitions Deconfinement to Q-G Plasma Chiral symmetry restoration Relevance to other research areas? Quark-hadron phase transition in early Universe Cores of dense stars High density QCD

4. 4 Manuel Calderón de la Barca, P408/508 Seminar The phase diagram of water Analogous graphs superfluids superconductors metal/insulator …

5. 5 Manuel Calderón de la Barca, P408/508 Seminar Generating a deconfined state Nuclear Matter (confined) Hadronic Matter (confined) Quark Gluon Plasma deconfined ! Present understanding of Quantum Chromodynamics (QCD) heating compression  deconfined color matter !

6. 6 Manuel Calderón de la Barca, P408/508 Seminar Expectations from Lattice QCD  /T 4 ~ # degrees of freedom confined: few d.o.f. deconfined: many d.o.f. T C ≈ 173 MeV ≈ 2  10 12 K ≈ 130,000  T[Sun’s core]

7. 7 Manuel Calderón de la Barca, P408/508 Seminar How can we study it?

8. 8 Manuel Calderón de la Barca, P408/508 Seminar Imagine… You know that ice exists… Your theory friends with huge computers tell you that there is something called water… You don’t have a way to heat ice… So you put millions of ice cubes in an ice- accelerator Send them at 99.995% of the speed of light to collide Generating thousands of ice-cube+ice-cube collisions per second… And you watch it all from the vicinity of Mars!

9. 9 Manuel Calderón de la Barca, P408/508 Seminar Key Idea: Bulk Matter We must create/compress/heat a bulk (geometrically large) system –freeze/melt a single H 2 0 molecule? –fundamental distinction from particle physics Only achievable through collisions of the heaviest nuclei (Au, Pb) at the highest available energy– at Relativistic Heavy Ion Collider (RHIC) 1000’s of particles produced in each collision

10. 10 Manuel Calderón de la Barca, P408/508 Seminar What tools do we use? How fast? How massive? How long? Detectors and accelerators are our “bread-and- butter”.

11. 11 Manuel Calderón de la Barca, P408/508 Seminar “RHIC is big” as seen by the Landsat-4 satellite… big facility big detectors big collaborations “big” collisions

12. 12 Manuel Calderón de la Barca, P408/508 Seminar RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) 1 km v = 0.99995  c = 186,000 miles/sec

13. 13 Manuel Calderón de la Barca, P408/508 Seminar RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) PHENIX ~ 500 collaborators

14. 14 Manuel Calderón de la Barca, P408/508 Seminar RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) STAR ~500 Collaborators

15. 15 Manuel Calderón de la Barca, P408/508 Seminar The STAR Experiment STAR: Solenoidal Tracker at RHIC multipurpose detector system for hadronic measurements large coverage (geometrical acceptance) tracking of charged particles in high multiplicity environment measure correlations of observables study of hard processes (jet physics) Solenoidal Tracker At RHIC goal: track “all” charged hadrons (bags of quarks) emitted in each collision

16. 16 Manuel Calderón de la Barca, P408/508 Seminar Operation of a Time Projection Chamber SCA/ADC DAQ Copper pads ~ 1cm 2 Amplifying and digitizing electronics connected to each pad Anode wires with +HV sitting ~5 mm above pads Charged particle flies through TPC gas…..generating a cluster of liberated electrons Electric field “Avalanche” as electrons approach anode wire.....capacitively inducing a signal on nearby pads.....which is amplified, digitized, and recorded for later analysis V t ADC bucket #

17. 17 Manuel Calderón de la Barca, P408/508 Seminar One collision seen by STAR TPC Momentum determined by track curvature in magnetic field… …and by direction relative to beam

18. 18 Manuel Calderón de la Barca, P408/508 Seminar Particle momentum from tracking… … how to get particle space-time information??

19. 19 Manuel Calderón de la Barca, P408/508 Seminar Impact parameter & Reaction plane b = 0  “central collision” many particles produced “peripheral collision” fewer particles produced

20. 20 Manuel Calderón de la Barca, P408/508 Seminar Impact parameter & Reaction plane b = 0  “central collision” many particles produced “peripheral collision” fewer particles produced

21. 21 Manuel Calderón de la Barca, P408/508 Seminar Collision Geometry and Centrality x z y Non-central Collisions Reaction plane  s NN = 200 GeV Multiplicity central peripheral 5% Central STAR Multiplicity (arb. units) ZDC Paddles/BBC ZDC Au Paddles/BBC Central Multiplicity Detectors

22. 22 Manuel Calderón de la Barca, P408/508 Seminar What sorts of things have we learned? We hoped to create bulk matter, –do we see evidence for collective behaviour? We hoped to create high density matter, –do we see evidence for dissipative behaviour?

23. 23 Manuel Calderón de la Barca, P408/508 Seminar How do semi-central collisions evolve? 1) Superposition of independent p+p: momenta pointed at random relative to reaction plane

24. 24 Manuel Calderón de la Barca, P408/508 Seminar How do semi-central collisions evolve? 1) Superposition of independent p+p: 2) Evolution as a bulk system momenta pointed at random relative to reaction plane high density / pressure at center “zero” pressure in surrounding vacuum Pressure gradients (larger in-plane) push bulk “out”  “flow” more, faster particles seen in-plane

25. 25 Manuel Calderón de la Barca, P408/508 Seminar How do semi-central collisions evolve? 1) Superposition of independent p+p: 2) Evolution as a bulk system momenta pointed at random relative to reaction plane Pressure gradients (larger in-plane) push bulk “out”  “flow” more, faster particles seen in-plane N  -  RP (rad) 0  /2   /43  /4 N  -  RP (rad) 0  /2   /43  /4

26. 26 Manuel Calderón de la Barca, P408/508 Seminar Azimuthal distributions at RHIC STAR, PRL90 032301 (2003) b ≈ 4 fm “central” collisions b ≈ 6.5 fm midcentral collisions

27. 27 Manuel Calderón de la Barca, P408/508 Seminar Azimuthal distributions at RHIC STAR, PRL90 032301 (2003) b ≈ 4 fm b ≈ 6.5 fm b ≈ 10 fm peripheral collisions

28. 28 Manuel Calderón de la Barca, P408/508 Seminar Elliptic flow – collectivity & sensitivity to early system STAR, PRL90 032301 (2003) “v 2 ” “Elliptic flow” evidence of collective motion quantified by v 2 geometrical anisotropy  momentum anisotropy sensitive to early pressure evidence for early thermalization QGP in early stage Hydrodynamic calculation of system evolution

29. 29 Manuel Calderón de la Barca, P408/508 Seminar A more direct handle? elliptic flow (v 2 ) and other measurements (in a few slides)  evidence towards QGP at RHIC –indirect connection to geometry Are there more direct handles on the space-time geometry of collisions? –yes ! Even at the 10 -15 m / 10 -23 s scale ! What can they tell us about the QGP and system evolution? –Let’s put Quantum Mechanics to good use!

30. 30 Manuel Calderón de la Barca, P408/508 Seminar probing source geometry through interferometry Measurable! F.T. of pion source Creation probability  (x,p) = U * U 5 fm 1 m  source  (x) r1r1 r2r2 x1x1 x2x2 p1p1 p2p2 The Bottom line… if a pion is emitted, it is more likely to emit another pion with very similar momentum if the source is small experimentally measuring this enhanced probability: quite challenging

31. 31 Manuel Calderón de la Barca, P408/508 Seminar Correlation functions for different colliding systems C 2 (Q inv ) Q inv (GeV/c) STAR preliminary p+p R ~ 1 fm d+Au R ~ 2 fm Au+Au R ~ 6 fm Different colliding systems studied at RHIC Interferometry probes extremely small scales!

32. 32 Manuel Calderón de la Barca, P408/508 Seminar beam direction More detailed geometry Relative momentum between pions is a vector  can extract 3D shape information p2p2 p1p1 q R long R side R out R long – along beam direction R out – along “line of sight” R side –  “line of sight”

33. 33 Manuel Calderón de la Barca, P408/508 Seminar Timescales Evolution of source shape –suggests system lifetime is shorter than otherwise-successful theory predicts Is there a more direct handle on timescales?

34. 34 Manuel Calderón de la Barca, P408/508 Seminar Disintegration timescale Relative momentum between pions is a vector  can extract 3D shape information p2p2 p1p1 q R out R long – along beam direction R out – along “line of sight” R side –  “line of sight” R side  increases with emission timescale

35. 35 Manuel Calderón de la Barca, P408/508 Seminar Disintegration timescale - expectation 3D 1-fluid Hydrodynamics Rischke & Gyulassy, NPA 608, 479 (1996) with transition with transition “”“” “”“” Long-standing favorite signature of QGP: increase in , R OUT /R SIDE due to deconfinement  confinement transition expected to “turn on” as QGP energy threshold is reached

36. 36 Manuel Calderón de la Barca, P408/508 Seminar Disintegration timescale - observation 4 6 8 4 6 8 1.0 1.25 1.5 R O (fm) R S (fm) R O / R S increasing collision energy RHIC no threshold effect seen R O /R S ~ 1

37. 37 Manuel Calderón de la Barca, P408/508 Seminar Disintegration timescale - observation no threshold effect seen R O /R S ~ 1 toy model calculations suggest very short timescales

38. 38 Manuel Calderón de la Barca, P408/508 Seminar What have we learned (part I)? There is evidence that the system behaves collectively! Space-momentum correlations consistent with –a very rapidly expanding source –developed in extremely short timescales ~1fm (so short that no models can account for it) Do we know that the system is at high densisty?

39. 39 Manuel Calderón de la Barca, P408/508 Seminar How can you know it’s high density? High energy collisions: access to high- momentum transfer processes –High pT particle production –High Mass particle production (Heavy Flavors!) Name of the game: –Theorists can calculate this for p+p collisions –Check how it is modified in Au+Au collisions

40. 40 Manuel Calderón de la Barca, P408/508 Seminar Jets in high energy collisions nucleon parton jet Free quarks and gluons not observed: high transverse energy partons fragment into collimated sprays (jets) of hadrons fundamental expectation of QCD occurs in all high energy collisions: e + +e -, p+pbar, Au+Au,… Hard scattering of partons: quarks or gluons drawn from wavefunction of colliding projectiles

41. 41 Manuel Calderón de la Barca, P408/508 Seminar

42. 42 Manuel Calderón de la Barca, P408/508 Seminar Partonic energy loss in dense matter Multiple soft interactions: Strong dependence of energy loss on gluon density  glue  measure   color charge density at early hot, dense phase Gluon bremsstrahlung Opacity expansion: Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,…

43. 43 Manuel Calderón de la Barca, P408/508 Seminar Jets at RHIC nucleon parton jet p+p  jet+jet (STAR@RHIC) Find this … Au+Au  ??? (STAR@RHIC) in this !!!

44. 44 Manuel Calderón de la Barca, P408/508 Seminar Partonic energy loss via leading hadrons - Energy loss  softening of fragmentation  suppression of leading hadron yield Binary collision scalingp+p reference

45. 45 Manuel Calderón de la Barca, P408/508 Seminar Leading hadrons at lower energy Multiple scattering in initial state(“Cronin effect”) p+A collisions: Central Pb+Pb collisions at CERN SPS (  s=20 GeV) SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow,…

46. 46 Manuel Calderón de la Barca, P408/508 Seminar Au+Au and p+p: inclusive charged hadrons p+p reference spectrum measured at RHIC PRL 89, 202301 nucl-ex/0305015, PRL in press

47. 47 Manuel Calderón de la Barca, P408/508 Seminar Suppresion of inclusive hadron yield central Au+Au collisions: factor ~4-5 suppression p T >5 GeV/c: suppression ~ independent of p T nucl-ex/0305015 Au+Au relative to p+p Au+Au central/peripheral R AA R CP

48. 48 Manuel Calderón de la Barca, P408/508 Seminar PHENIX observes a similar effect (  0 s) nucl-ex/0304022 Factor ~5 suppression for central Au+Au collisions lower energy Pb+Pb lower energy 

49. 49 Manuel Calderón de la Barca, P408/508 Seminar PHOBOS (  ~ 0.8) and BRAHMS (  ~ 2.0) PHOBOS: nucl-ex/0302015 BRAHMS: nucl-ex/0307003 Also have  = 0

50. 50 Manuel Calderón de la Barca, P408/508 Seminar High-pt Evidence The yield of high pt particles is significantly suppressed in central Au+Au collisions.

51. 51 Manuel Calderón de la Barca, P408/508 Seminar Jets and two-particle azimuthal distributions p+p  dijet trigger: highest p T track, p T >4 GeV/c  distribution: 2 GeV/c<p T <p T trigger normalize to number of triggers trigger Phys Rev Lett 90, 082302

52. 52 Manuel Calderón de la Barca, P408/508 Seminar Azimuthal distributions in Au+Au Au+Au peripheral Au+Au central Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au pedestal and flow subtracted Phys Rev Lett 90, 082302

53. 53 Manuel Calderón de la Barca, P408/508 Seminar High-pt Evidence The yield of high pt particles is significantly suppressed in central Au+Au collisions. We see an away side jet in p+p, but not in central Au+Au collisions.

54. 54 Manuel Calderón de la Barca, P408/508 Seminar What might all this mean? ? Conjecture: core of reaction volume is opaque to jets  surface emission Consequences: near-side corrleations unchanged suppression of back-to-back correlations suppression of inclusive rates azimuthal correlations with reaction plane Compelling picture, but is it right?

55. 55 Manuel Calderón de la Barca, P408/508 Seminar Is suppression initial or final state effect? Initial state? Final state? partonic energy loss in dense medium generated in collision strong modification of Au wavefunction  initial jet production rates suppressed for heavy nuclei (e.g. gluon saturation at low x Bjorken )

56. 56 Manuel Calderón de la Barca, P408/508 Seminar nucl-ex/0305015 R CP Inclusive suppression: theory vs. data pQCD-I: Wang, nucl-th/0305010 pQCD-II: Vitev and Gyulassy, PRL 89, 252301 Saturation: KLM, Phys Lett B561, 93 p T >5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching Final state Initial state

57. 57 Manuel Calderón de la Barca, P408/508 Seminar Is suppression initial or final state effect? Initial state? Final state? gluon saturation partonic energy loss How to discriminate? Turn off final state  d+Au collisions

58. 58 Manuel Calderón de la Barca, P408/508 Seminar d+Au vs. p+p: Theoretical expectations All effects strongest in central d+Au collisions pQCD: no suppression, small broadening due to Cronin effect 0  /2   (radians) 0 High p T hadron pairs saturation models: suppression due to mono- jet contribution? suppression? broadening? If Au+Au suppression is initial state (KLM saturation: 0.75) 1.1-1.5 pTpT R AB 1 Inclusive spectra If Au+Au suppression is final state ~2-4 GeV/c

59. 59 Manuel Calderón de la Barca, P408/508 Seminar d+Au inclusive yields relative to binary-scaled p+p Phys Rev Lett 91, 072304 STAR d+Au : enhancement Au+Au: strong suppression Suppression of the inclusive yield in central Au+Au is a final-state effect pTpT

60. 60 Manuel Calderón de la Barca, P408/508 Seminar PHENIX, PHOBOS, BRAHMS find similar results Phys Rev Lett 91, 072302/3/5 (2003) PHENIX BRAHMS PHOBOS

61. 61 Manuel Calderón de la Barca, P408/508 Seminar Azimuthal distributions pedestal and flow subtracted Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back correlation in central Au+Au is a final-state effect Phys Rev Lett 91, 072304

62. 62 Manuel Calderón de la Barca, P408/508 Seminar Summary of STAR high p T measurements  hadrons at p T >~3 GeV/c are jet fragments  central Au+Au:  strong suppression of inclusive yield at p T >5 GeV/c  suppression factor ~ constant for 5<p T <12 GeV/c  strong suppression of back-to-back hadron pairs  Possible interpretation:  Hard scattered partons (or their fragments) interact strongly with medium  Observed fragments are emitted from the surface of the hot & dense zone created in the collision ?

63. 63 Manuel Calderón de la Barca, P408/508 Seminar Experiment: the strong suppression of the inclusive yield and back-to-back correlations at high p T observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions. Theory: pQCD models which reproduce the inclusive suppression in central Au+Au collisions require color charge densities a factor 30-50 greater than that of cold nuclear matter RHIC is generating very high energy density matter What are the main lessons so far…

64. 64 Manuel Calderón de la Barca, P408/508 Seminar Have we found the Quark Gluon Plasma at RHIC? We now know that Au+Au collisions generate a medium that is dense (pQCD theory: many times cold nuclear matter density) is dissipative exhibits strong collective behavior :we do create bulk matter Not yet, there is still work to do We have yet to show that: dissipation and collective behavior both occur at the partonic stage the system is deconfined and thermalized a transition occurs: can we turn the effects off ? This represents significant progress in our understanding of strongly interacting matter

65. 65 Manuel Calderón de la Barca, P408/508 Seminar Extra Slides

66. 66 Manuel Calderón de la Barca, P408/508 Seminar “observe” the source from all angles relative to the reaction plane expect oscillations in radii for non-round sources big R SIDE small R SIDE Source shape R SIDE  -  RP (rad) 0  /2   /43  /4 reaction plane

67. 67 Manuel Calderón de la Barca, P408/508 Seminar STAR, PRL93 012301 (2004) Measured final source shape central collisions mid-central collisions peripheral collisions Expected evolution:

68. 68 Manuel Calderón de la Barca, P408/508 Seminar “Big” and “Small” – in meters 10 0 10 6 10 12 10 18 10 24 10 -6 10 -12 10 -18 1,000,000,000,000,000,000,000,000 m 0.000000000000000001 m

69. 69 Manuel Calderón de la Barca, P408/508 Seminar “Short” and “long” – in seconds Today’s lecture 10 0 10 6 10 12 10 18 10 24 10 -6 10 -12 10 -18 10 -24 as many yoctoseconds (10 -24 s ~ 3 fm/c) in a second as seconds in 10 thousand trillion years

70. 70 Manuel Calderón de la Barca, P408/508 Seminar How can we check if it’s deconfined?

71. 71 Manuel Calderón de la Barca, P408/508 Seminar The Original Idea … Matsui & Satz (PLB 178 (1986) 416 J/  suppression by QGP Color screening of static potential between heavy quarks:

72. 72 Manuel Calderón de la Barca, P408/508 Seminar The Original Idea … Matsui & Satz (PLB 178 (1986) 416 J/  suppression by QGP Color screening of static potential between heavy quarks: D. Kharzeev and H. Satz, Phys. Lett. B477 (2000): Hard gluons needed for breaking J/  not available in hadron gas S. Digal et al., Phys. Rev. D 64, 094015 (2001) C.-Y. Wong, Phys. Rev. C 65, 034902 (2002) Calculations using lattice potentials: sequential suppression  ’,  c dissolves below T C J/  dissolves at 1.2 T C

73. 73 Manuel Calderón de la Barca, P408/508 Seminar Heavy Quarks in the Lattice Profile of the action density between static quarks at distance 1.5 fm, Phys. Rev. D51 (1995) 5165

74. 74 Manuel Calderón de la Barca, P408/508 Seminar Static free energy in full QCD 3F: Improved staggered Asqtad action, lattices K. Petrov, P.P, PRD 70 (04) 054503 String breaking screening Vacuum physics 2F: Improved staggered p4 action lattices, O. Kaczmarek et al, Progr. Theor. Phys. Suppl 153 (04) 287 P.Petrevczky

75. 75 Manuel Calderón de la Barca, P408/508 Seminar Taking into account the entropy and the internal energy Kaczmarek, Karsch, P.P., Zantow, hep-lat/0309121 see talk by Kaczmarek Potential energy In the Lattice shows screening!

76. 76 Manuel Calderón de la Barca, P408/508 Seminar … and its Confirmation … CERN/SPS  s  17 GeV mid-rapidity large statistics (beyond RHIC)

77. 77 Manuel Calderón de la Barca, P408/508 Seminar Electron PID with MRPC TOF/TPC and EMC EMC 1.use TPC for p and dE/dx 2.use Tower E  p/E 3.use SMD shape to reject hadrons 4.e/h discrimination power ~ 10 5 5.works for p T > 1.5 GeV/c ToF 1.use TPC and ToF PID 2.works for p T < 3 GeV/c

78. 78 Manuel Calderón de la Barca, P408/508 Seminar Quarkonium Trigger Development Use calorimeter for triggering on 2 high towers with large invariant mass J/  Trigger for pp and d+Au  Trigger even for Au+Au

79. 79 Manuel Calderón de la Barca, P408/508 Seminar

80. 80 Manuel Calderón de la Barca, P408/508 Seminar Azimuthal anisotropy in non- central collisions x y z Initial spatial anisotropy pypy pxpx Final momentum anisotropy Reaction plane defined by “soft” (low p T ) particles Elliptic term Reaction plane azimuthal angle

81. 81 Manuel Calderón de la Barca, P408/508 Seminar Elliptic flow at high p T : theory Snellings; Gyulassy, Vitev and Wang (nucl-th/00012092) Jet propagation through anisotropic matter (non-central collisions) Finite v 2 : high p T hadron correlated with reaction plane defined by the “soft” part of event (p T <2 GeV/c) jet

82. 82 Manuel Calderón de la Barca, P408/508 Seminar Elliptic flow at high p T : dataSTAR PRL 90, 032301 p T <2 GeV: detailed agreement with hydrodynamics p T >4 GeV: finite v 2 in qualitative agreement with jet quenching expectations STAR preliminary STAR

83. 83 Manuel Calderón de la Barca, P408/508 Seminar More information Relative momentum between pions is a vector  can extract 3D shape information p2p2 p1p1 R out R long – along beam direction R out – along “line of sight” R side –  “line of sight” R side study as K grows…

84. 84 Manuel Calderón de la Barca, P408/508 Seminar Why do the radii fall with increasing momentum ??

85. 85 Manuel Calderón de la Barca, P408/508 Seminar Why do the radii fall with increasing momentum ?? It’s collective flow !! Direct geometrical/dynamical evidence for bulk behaviour!!! Amount of flow consistent with p-space

86. 86 Manuel Calderón de la Barca, P408/508 Seminar Disintegration timescale - observation N(  ) Heinz & Kolb, hep-ph/0204061 no threshold effect seen R O /R S ~ 1 toy model calculations suggest very short timescales rapid, explosive evolution too explosive for “real” models which explain all other data An important space-time “puzzle” at RHIC - actively under study

87. 87 Manuel Calderón de la Barca, P408/508 Seminar Forces and structures in Nature 1) Gravity one “charge” (mass) force decreases with distance m1m1 m2m2 2) Electric (& Magnetic) two “charges” (+/-) force decreases with distance +- ++ Atom

88. 88 Manuel Calderón de la Barca, P408/508 Seminar Atomic nuclei and the “nuclear” force Nuclei composed of: protons (+ electric charge) neutrons (no electric charge) Does not blow up!?  “nuclear force” overcomes electrical repulsion determines nuclear reactions (stellar burning, fusion…) arises from fundamental strong force (#3) acts on color charge of quarks proton neutron quark

89. 89 Manuel Calderón de la Barca, P408/508 Seminar Strong color field Energy grows with separation !!! E=mc 2 ! An analogy… and a difference! to study structure of an atom… “white” proton …separate constituents Imagine our understanding of atoms or QED if we could not isolate charged objects!! nucleus electron quark quark-antiquark pair created from vacuum “white” proton (confined quarks) “white”  0 (confined quarks) Confinement: fundamental & crucial (but not understood!) feature of strong force - colored objects (quarks) have  energy in normal vacuum neutral atom To understand the strong force and the phenomenon of confinement: Create and study a system of deconfined colored quarks (and gluons)