Exploring Strong Force and Quarks in High-Energy Collisions

1. How does the strong force, QCD, behave at high energy? • How can we study it? • What tools do we use? • What have we learned? Hotquarks! : 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

2. Forces and structures in Nature • Gravity • one “charge” (mass) • force decreases with distance m1 m2 • 2) Electric (& Magnetic) • two “charges” (+/-) • force decreases with distance Atom + - + + Manuel Calderón de la Barca, P295 Seminar

3. quark 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 neutron proton Manuel Calderón de la Barca, P295 Seminar

4. To understand the strong force and the phenomenon of confinement: Create and study a system of deconfined colored quarks (and gluons) quark-antiquark pair created from vacuum An analogy… and a difference! to study structure of an atom… electron …separate constituents Imagine our understanding of atoms or QED if we could not isolate charged objects!! nucleus neutral atom Confinement: fundamental & crucial (but not understood!) feature of strong force - colored objects (quarks) have  energy in normal vacuum quark Strong color field Energy grows with separation !!! “white” 0 (confined quarks) E=mc2 ! “white” proton (confined quarks) “white” proton Manuel Calderón de la Barca, P295 Seminar

5. Generating a deconfined state • Present understanding of Quantum Chromodynamics (QCD) • heating • compression •  deconfined color matter ! Hadronic Matter (confined) Nuclear Matter (confined) Quark Gluon Plasma deconfined ! Manuel Calderón de la Barca, P295 Seminar

6. Expectations from Lattice QCD Computer calculations /T4 ~ # degrees of freedom confined: few d.o.f. deconfined: many d.o.f. TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core] Manuel Calderón de la Barca, P295 Seminar

7. The phase diagram of QCD, the strong force. Early universe quark-gluon plasma critical point ? Tc Temperature colour superconductor hadron gas nucleon gas nuclei CFL r0 Neutron stars vacuum baryon density Manuel Calderón de la Barca, P295 Seminar

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

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

10. 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! Manuel Calderón de la Barca, P295 Seminar

11. Producing “Bulk” nuclear Matter in the laboratory. • We must create/compress/heat a bulk (geometrically large) system • freeze/melt a single H20 molecule? • fundamental distinction from particle physics • Only achievable through collisions of the heaviest nuclei (Au, Pb) at the highest available energy– the RelativisticHeavy Ion Collider (RHIC) 1000’s of particles produced in each collision Manuel Calderón de la Barca, P295 Seminar

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

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

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

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

16. Solenoidal Tracker At RHIC goal: track “all” charged hadrons (bags of quarks) emitted in each collision 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) Manuel Calderón de la Barca, P295 Seminar

17. One collision seen by STAR TPC Momentum determined by track curvature in magnetic field… …and by direction relative to beam Manuel Calderón de la Barca, P295 Seminar

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

19. 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? Manuel Calderón de la Barca, P295 Seminar

20. nucleon nucleon jet parton “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. Manuel Calderón de la Barca, P295 Seminar

21. trigger Angular correlations: observing jets • Look at the fastest particles in the collision. (pTtrigger > 4 GeV/c) • Df distribution: 2 GeV/c<pT<pTtrigger p+p  dijet • 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. Phys Rev Lett 90, 082302 Manuel Calderón de la Barca, P295 Seminar

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

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

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

25. Au+Au central Azimuthal distributions in Au+Au Au+Au peripheral pedestal and flow subtracted Phys Rev Lett 90, 082302 Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au Manuel Calderón de la Barca, P295 Seminar

26. ? 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. Manuel Calderón de la Barca, P295 Seminar

27. What are the main lessons so far… • Experiment: • At high momentum • we see strong suppressionof 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 tofinal-state interactions with the dense mediumgenerated in such collisions. • Theory:pQCD models which reproduce the inclusive suppression in central Au+Au collisions requirecolor charge densities a factor 30-50 greater than that of cold nuclear matter RHIC is generating very high energy density matter Manuel Calderón de la Barca, P295 Seminar

28. 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 This represents significant progress in our understanding of the strong nuclear force 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 ? Lots of interesting things to do! http://nuclear.ucdavis.edu/~calderon/ Manuel Calderón de la Barca, P295 Seminar

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

30. Electric field ..generating a cluster of liberated electrons Anode wires with +HV sitting ~5 mm above pads Copper pads ~ 1cm2 “Avalanche” as electrons approach anode wire... V ADC t Amplifying and digitizing electronics connected to each pad bucket # Operation of a Time Projection Chamber Charged particle flies through TPC gas… DAQ SCA/ADC ..capacitively inducing a signal on nearby pads... ..which is amplified, digitized, and recorded for later analysis Manuel Calderón de la Barca, P295 Seminar

31. jet parton Jets in high energy collisions • 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 Free quarks and gluons not observed: high transverse energy partons fragment into collimated sprays (jets) of hadrons nucleon nucleon Manuel Calderón de la Barca, P295 Seminar

32. Manuel Calderón de la Barca, P295 Seminar

33. Partonic energy loss in dense matter Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,… Multiple soft interactions: Gluon bremsstrahlung Opacity expansion: • Strong dependence of energy loss on gluon density glue: • measure DE color charge density at early hot, dense phase Manuel Calderón de la Barca, P295 Seminar

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

35. Leading hadrons at lower energy Central Pb+Pb collisions at CERN SPS (s=20 GeV) p+A collisions: SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow,… Multiple scattering in initial state(“Cronin effect”) Manuel Calderón de la Barca, P295 Seminar

36. nucl-ex/0305015, PRL in press PRL 89, 202301 Au+Au and p+p: inclusive charged hadrons p+p reference spectrum measured at RHIC Manuel Calderón de la Barca, P295 Seminar

37. Suppresion of inclusive hadron yield RAA Au+Au relative to p+p RCP Au+Au central/peripheral nucl-ex/0305015 • central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT Manuel Calderón de la Barca, P295 Seminar

38. Is suppression initial or final state effect? Initial state? Final state? partonic energy loss in dense mediumgenerated in collision strong modification of Au wavefunction initial jet production rates suppressedfor heavy nuclei (e.g. gluon saturation at low xBjorken) Manuel Calderón de la Barca, P295 Seminar

39. RCP nucl-ex/0305015 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 Final state Initial state pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching Manuel Calderón de la Barca, P295 Seminar

40. How to discriminate? Turn off final state  d+Au collisions Is suppression initial or final state effect? Initial state? Final state? partonic energy loss gluon saturation Manuel Calderón de la Barca, P295 Seminar

41. d+Au vs. p+p: Theoretical expectations Inclusive spectra RAB If Au+Au suppression is final state 1.1-1.5 1 If Au+Au suppression is initial state (KLM saturation: 0.75) ~2-4 GeV/c pT High pT hadron pairs broadening? pQCD: no suppression, small broadening due to Cronin effect 0 saturation models: suppression due to mono-jet contribution? suppression? /2  0  (radians) All effects strongest in central d+Au collisions Manuel Calderón de la Barca, P295 Seminar

42. STAR d+Au inclusive yields relative to binary-scaled p+p Phys Rev Lett 91, 072304 • d+Au : • enhancement • Au+Au: • strong suppression pT Suppression of the inclusive yield in central Au+Au is a final-state effect Manuel Calderón de la Barca, P295 Seminar

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

44. pedestal and flow subtracted Azimuthal distributions Phys Rev Lett 91, 072304 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 Manuel Calderón de la Barca, P295 Seminar

45. Summary of STAR high pT measurements • hadrons at pT>~3 GeV/c are jet fragments • central Au+Au: • strong suppression of inclusive yield at pT>5 GeV/c • suppression factor ~ constant for 5<pT<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 ? Manuel Calderón de la Barca, P295 Seminar