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 Nuclear Physics Group, UCD  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, P295 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

3. 3 Manuel Calderón de la Barca, P295 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

4. 4 Manuel Calderón de la Barca, P295 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)

5. 5 Manuel Calderón de la Barca, P295 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, P295 Seminar Expectations from Lattice QCD Computer calculations  /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, P295 Seminar The phase diagram of QCD, the strong force. Temperature baryon density Neutron stars Early universe nuclei nucleon gas hadron gas colour superconductor quark-gluon plasma TcTc 00 critical point ? vacuum CFL

8. 8 Manuel Calderón de la Barca, P295 Seminar The phase diagram of water Analogous graphs superfluids superconductors metal/insulator …

9. 9 Manuel Calderón de la Barca, P295 Seminar How can we study the thermodynamics of the STRONG force?

10. 10 Manuel Calderón de la Barca, P295 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!

11. 11 Manuel Calderón de la Barca, P295 Seminar 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– the Relativistic Heavy Ion Collider (RHIC) Producing “Bulk” nuclear Matter in the laboratory. 1000’s of particles produced in each collision

12. 12 Manuel Calderón de la Barca, P295 Seminar What tools do we use? How fast? How massive? How long? Detectors and accelerators are our “bread-and- butter”.

13. 13 Manuel Calderón de la Barca, P295 Seminar RHIC as seen by the Landsat-4 satellite… A large collider Useful for: Heavy Ions Polarized Protons

14. 14 Manuel Calderón de la Barca, P295 Seminar RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) 2 km v = 0.99995  c = 186,000 miles/sec

15. 15 Manuel Calderón de la Barca, P295 Seminar RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) STAR ~500 Collaborators

16. 16 Manuel Calderón de la Barca, P295 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

17. 17 Manuel Calderón de la Barca, P295 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, P295 Seminar So, what have we seen?

19. 19 Manuel Calderón de la Barca, P295 Seminar Experimental search for “interesting” phenomena Look at elementary p+p collisions –Measure an observable (e.g. angular correlations) Look at Au+Au collisions –Measure the same observable you did in p+p Compare them, is there something new?

20. 20 Manuel Calderón de la Barca, P295 Seminar “Jets” and angular correlations p+p  dijet “Jets” in p+p are generated by a collision between fast quarks and gluons (partons). The outgoing quark or gluon can’t exist in the vacuum (confinement!) and “fragments” into a spray of particles. (Also happens in e+e-, ep, p+pbar…) The particles can be seen in the detector, they are very close in angle, like a “jet” of water drops coming out of a hose. nucleon parton jet

21. 21 Manuel Calderón de la Barca, P295 Seminar Angular correlations: observing jets p+p  dijet Look at the fastest particles in the collision. (p T trigger > 4 GeV/c)  distribution: 2 GeV/c<p T <p T trigger trigger Phys Rev Lett 90, 082302 Particles from the same jet will be close in angle, difference is ~ 0 °. Particles from the opposite-side jet will be close to ~180° apart.

22. 22 Manuel Calderón de la Barca, P295 Seminar Jets at RHIC nucleon parton jet p+p  jet+jet (STAR@RHIC) Find this … Au+Au  ??? (STAR@RHIC) in this !!!

23. 23 Manuel Calderón de la Barca, P295 Seminar What happens to the particle jets in a head-on Nuclear collision? Quarks and gluons lose energy in dense medium generated in collision Speeding Nuclei Hot, Dense region… QGP? Jets have to pass through Hot, Dense Zone!

24. 24 Manuel Calderón de la Barca, P295 Seminar How do you tell a “head-on” collision? b = 0  “central collision” many particles produced “peripheral collision” fewer particles produced

25. 25 Manuel Calderón de la Barca, P295 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

26. 26 Manuel Calderón de la Barca, P295 Seminar What might all this mean? ? Conjecture: core of reaction volume is opaque to jets  we mainly see jets close to the surface of the hot zone  Jets in the core are LOST! Consequences: near-side corrleations unchanged suppression of back-to-back correlations less high momentum particles (also measured!) Evidence for dissipative behavior in the most violent head-on collisions.

27. 27 Manuel Calderón de la Barca, P295 Seminar Experiment: At high momentum we see strong suppression of the number of particles “back-to-back” correlations dissappear! The effects are more dramatic when the collision is head-on. We have evidence that effects 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…

28. 28 Manuel Calderón de la Barca, P295 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 Lots of interesting things to do! http://nuclear.ucdavis.edu/~calderon/ We have yet to show that: dissipation and collective behavior both occur at the level of quarks and gluons the system is deconfined (quarks and gluons are free) and thermalized a transition occurs: can we turn the effects off ? This represents significant progress in our understanding of the strong nuclear force

29. 29 Manuel Calderón de la Barca, P295 Seminar Extra slides

30. 30 Manuel Calderón de la Barca, P295 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 #

31. 31 Manuel Calderón de la Barca, P295 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

32. 32 Manuel Calderón de la Barca, P295 Seminar

33. 33 Manuel Calderón de la Barca, P295 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,…

34. 34 Manuel Calderón de la Barca, P295 Seminar Partonic energy loss via leading hadrons - Energy loss  softening of fragmentation  suppression of leading hadron yield Binary collision scalingp+p reference

35. 35 Manuel Calderón de la Barca, P295 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,…

36. 36 Manuel Calderón de la Barca, P295 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

37. 37 Manuel Calderón de la Barca, P295 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

38. 38 Manuel Calderón de la Barca, P295 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 )

39. 39 Manuel Calderón de la Barca, P295 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

40. 40 Manuel Calderón de la Barca, P295 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

41. 41 Manuel Calderón de la Barca, P295 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

42. 42 Manuel Calderón de la Barca, P295 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

43. 43 Manuel Calderón de la Barca, P295 Seminar PHENIX, PHOBOS, BRAHMS find similar results Phys Rev Lett 91, 072302/3/5 (2003) PHENIX BRAHMS PHOBOS

44. 44 Manuel Calderón de la Barca, P295 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

45. 45 Manuel Calderón de la Barca, P295 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 ?