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Heavy Ion Collisions – A Trillion Degrees in the Shade

Heavy Ion Collisions – A Trillion Degrees in the Shade. Akron Physics Club, Oct 27, 2008. Outline. Why collide heavy nuclei at high energy? RHIC: Machine and experiments Physics from the first ten years of RHIC Soft physics Hard physics.

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Heavy Ion Collisions – A Trillion Degrees in the Shade

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  1. Heavy Ion Collisions – A Trillion Degrees in the Shade Akron Physics Club, Oct 27, 2008

  2. Outline • Why collide heavy nuclei at high energy? • RHIC: Machine and experiments • Physics from the first ten years of RHIC • Soft physics • Hard physics

  3. “In high-energy physics we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions. In order to study the question of ‘vacuum’, we must turn to a different direction; we should investigate some ‘bulk’ phenomena by distributing high energy over a relatively large volume.” T.D. Lee (Nobel Laureate – Parity violation) Rev. Mod. Phys. 47 (1975) 267.

  4. Quantum Chromodynamics • Quantum Chromodynamics (QCD) is the established theory of strongly interacting matter. • Gluons hold quarks together to from hadrons: • Gluons and quarks, or partons, typically exist in a color singlet state: confinement. meson baryon

  5. quark-antiquark pair created from vacuum An analogy… and a difference! An atom has substructure – we want to study its constituents and their interactions electron Separate constituents: EM interaction vanishes & we can study constituents/interactions separately Imagine our understanding of atoms or QED if we could not isolate charged objects!! nucleus Confinement: fundamental & crucial (but not well-understood!) feature of QCD colored objects (partons) have  energy in normal vacuum would love to study deconfined partonic system! quark Strong color field Energy grows with separation F~kr (!) 0 proton proton

  6. But how? • Use feature of asymptotic freedom • Bring nucleons as close as possible • Strong force gets weaker • Partons would be free to move around • A ‘phase transition’ to parton ‘plasma’ should occur

  7. Collective Phenomenology & QGP • Present understanding of Quantum Chromodynamics (QCD) • heating • compression •  deconfined color matter ! Hadronic Matter (confined) Nuclear Matter (confined) Quark Gluon Plasma deconfined !

  8. QCD on Lattice RHIC LHC 1) Large increase in , a fast cross cover ! • 2) Does not reach ideal, • non-interaction S. Boltzmann • limit ! • many body interactions • Collective modes • Quasi-particles are necessary • Y. Aoki, Z. Fodor, S.D. Katz, K.K. Szabo, • PLB643 46(06); hep-lat/0609068 • Z. Fodor et al, JHEP 0203:014(02) • Z. Fodor et al, hep-lat/0204029 • C.R. Allton et al, PRD66, 074507(02) • F. Karsch, Nucl. Phys. A698, 199c(02). Lattice calculations predict TC ~ 160 ± 20 MeV

  9. The Phase Diagram TWO different phase transitions at work! – Particles roam freely over a large volume – Masses change (vanish) Calculations show that these occur at approximately the same point Two sets of conditions: High Temperature High Baryon Density Lattice QCD calc. Predict: Deconfinement transition QGP Early Universe 1012 oK Chiral transition Normal Nuclear matter Nuclear Collisions can do both Density - “Compress” Tc ~ 150-170 MeV ec ~ 1 GeV/fm3

  10. Hadronization and Freeze-out Initial high Q2 interactions Parton matter - QGP - The hot-QCD Initial conditions Experimental approaches: • Energy loss - ‘jet-quenching’ • Collectivity - v2, radial flow • Charm - productions plus the combination (1) and (2) Hard scattering production - QCD prediction Interactions with medium - deconfinement/thermalization Initial parton density Initial condition in high-energy nuclear collisions Cold-QCD-matter, small-x, high-parton density - parton structures in nucleon / nucleus High-energy Nuclear Collisions S. Bass

  11. The ‘Little Bang’ in the Laboratory Kinetic freezeout: elastic scattering stops Spectra freeze Chemical freezeout: inelastic scattering stops Particle yields (ratios) fixed Initial scattering: Parton cascade Et production Baryon transport v2 saturates

  12. Crossing a New Threshold Relativistic Heavy Ion Collider Manhattan

  13. - New Scientist "Big Bang machine could destroy Earth" -The Sunday Times – July ‘99 most dangerous event in human history: - ABC News –Sept ‘99 the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99 No… the experiment will not tear our region of space to subatomic shreds. - Washington Post – Sept ‘99 Apocalypse2 – ABC News – Sept ‘99 Will Brookhaven Destroy the Universe? – NY Times – Aug ‘99 REVIEW OF SPECULATIVE 'DISASTER SCENARIOS' AT RHIC.W. Busza, R.L. Jaffe, J. Sandweiss, F. Wilczek, Sep 1999 Rev.Mod.Phys.72 (2000) and hep-ph/9910333

  14. PHOBOS BRAHMS RHIC PHENIX STAR AGS TANDEMS RHIC & its experiments 1 km

  15. PHOBOS BRAHMS RHIC PHENIX STAR AGS TANDEMS STAR ~500 Collaborators The STAR experiment

  16. STAR Main Detector

  17. Time Projection Chamber FTPCs (1 +1) Vertex Position Detectors The STAR Detector Magnet 1st year (130 GeV), 2nd year (200 GeV) Coils Silicon Vertex Tracker TPC Endcap & MWPC ZCal ZCal Endcap Calorimeter Barrel EM Calorimeter Central Trigger Barrel + TOF patch RICH * yr.1 SVT ladder

  18. Older tracking detectors: 2-D projections

  19. TPC - true tracking in 3-D

  20. 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 thru TPC gas… DAQ SCA/ADC ..capacitively inducing a signal on nearby pads... ..which is amplified, digitized, and recorded for later analysis

  21. Getting z-coord from drift time in TPC

  22. STAR ZDC • Each of the RHIC experiments has a pair of Zero Degree Calorimeters for beam monitoring, triggering, and locating interaction vertices. • ZDCs detect neutrons emitted along beam directions and measure their total energy (multiplicity) . • Baseline ZDCs have no transverse segmentation, which motivates upgrade.

  23. RICH dE/dx dE/dx PID range: [s (dE/dx) = .08] p  ~ 0.7 GeV/c for K/  ~ 1.0 GeV/c for p/p RICH PID range: 1 - 3 GeV/c for K/ 1.5 - 5 GeV/c for p/p f from K+ K- pairs dn/dm background subtracted Topology Decay vertices Ks p + + p - L  p + p - L  p + p + X - L + p - X +L + p + W  L + K - Combinatorics Ks p + + p - f  K + + K - L  p + p - L  p + p + [ r  p + + p -] [D  p + p -] m inv kaons protons dn/dm deuterons K+ K- pairs pions same event dist. mixed event dist. Vo m inv electrons STAR “kinks”: K  +  Particle ID in STAR

  24. STAR STRANGENESS K+ L̅ f K0s L X- X̅+ K*

  25. How to Observe QGP in Nuclear Collisions • Nuclear collisions are highly dynamic, no first-principles theory • Emphasis shifts – Experience and Surprises • Some tools to distinguishdeconfined QGPfromdense hadron gas: • High energy density: interaction of jets with medium • High temperature: direct photons • Quasi-equilibrium at early stage: flow • Rapid equilibration, mass shifts: strangeness enhancement • Threshold behavior: must be able to turn effects off • s, centrality of collision, mass of system • Need p-p, d-Au reference data or central/peripheral event selection QGP must emerge as most reasonable picture from many different observables simultaneously

  26. 5% Central ZDC ZDC Au Au Central Multiplicity Detectors Event (Centrality) Selection PRL 86, (2001) 402 nch = primary tracks in || < 0.75 Produced particles Spectators Spectators

  27. Soft Physics: pT< 2 GeV/c Goal: Characterize the bulk of the event What do we know (or we think we do)? • High apparent energy density ~5 GeV/fm3 (lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3) PRL 86, 112303 (2001)

  28. Bjorken Energy Density • Bjorken ’83: ideal 1+1 D relativistic hydrodynamics • boost invariance  h~0 (R ~ A1/3, t = 1 fm/c) • Central Au+Au @ sNN=130: • PHENIX ET:e = 4.6 GeV/fm3 (nucl-ex/0104015) • STAR charged particles: e ~ 4.5 GeV/fm3 • Compare NA49 Pb+Pb@SPS: e ~ 3 GeV/fm3 (t = 1 fm/c) • Critical issues: • Has equilibrium been achieved? (i.e. hydrodynamics valid?) • If so, what is formation time t?

  29. Soft Physics: pT< 2 GeV/c Goal: Characterize the bulk of the event What do we know (or we think we do)? • High apparent energy density ~5 GeV/fm3 (lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3) PRL 86, 112303 (2001) • Chemical equilibrium(?): T~175 MeV, B<40 MeV  near lattice phase boundaryPRC 66, 061901(R) (2002); PRL 89, 092301 (2002); PRC 65, 041901(R) (2002); nucl-ex/0211024; nucl-ex/0206008

  30. Chemical freezeout Particle Ratios in Central Collisions • Simple thermal model: • Partition fn, spectrum of hadrons • Parameters T, mB, ms • Fit to ratios of antiparticle/particles: , K, p, , , K*0

  31. data Thermal model fits Tch = 163 ± 4 MeV B = 24 ± 4 MeV Yields Ratio Results • In central collisions, thermal model fit well with S = 1. The system is thermalized at RHIC. • Short-lived resonances show deviations. There is life after chemical freeze-out. RHIC white papers - 2005, Nucl. Phys. A757, STAR: p102; PHENIX: p184.

  32. early universe P. Braun-Munzinger, nucl-ex/0007021 250 RHIC quark-gluon plasma Chemical Temperature Tch [MeV] 200 SPS AGS Lattice QCD deconfinement chiral restoration 150 thermal freeze-out SIS 100 hadron gas neutron stars atomic nuclei 50 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] 0 Phase Diagram at Chemical Freezeout Put parameters from Thermal Model fits on the phase diagram: • parameters near phase boundary • (strangeness) equilibration time for hadronic gas very long (~50 fm/c) • do we have more direct evidence of early equilibration?

  33. Soft Physics: pT< 2 GeV/c Goal: Characterize the bulk of the event What do we know (or we think we do)? • High apparent energy density ~5 GeV/fm3 (lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3) PRL 86, 112303 (2001) • Chemical equilibrium: T~175 MeV, B<40 MeV  near lattice phase boundary PRC 66, 061901(R) (2002); PRL 89, 092301 (2002); PRC 65, 041901(R) (2002); nucl-ex/0211024; nucl-ex/0206008 • Hydrodynamics works wellPRL 90, 032301 (2003); PRC 66, 034904 (2002); PRL 89, 132301 (2002); PRL 87, 182301 (2001); PRL 86, 402 (2001) • (With some trouble with HBT correlations)

  34. Non-central Collisions z x y Elliptic Flow Geometry of Heavy Ion Collisions Reaction plane

  35. Anisotropic flow: v1, v2, v4 backward midrapidity forward h~3 h~-3 h~0 Reaction plane Spectators Spectators Directed flow Elliptic flow

  36. Time evolution at finite b 1) Superposition of independent NN: momenta pointed at random relative to reaction plane

  37. Time evolution at finite b 1) Superposition of independent NN: high density / pressure at center momenta pointed at random relative to reaction plane 2) Evolution as a bulksystem Pressure gradients (larger in-plane) push bulk “out”  “flow” “zero” pressure in surrounding vacuum more, faster particles seen in-plane

  38. N N   0 0 /4 /4 /2 /2 3/4 3/4 -RP (rad) -RP (rad) Time evolution at finite b 1) Superposition of independent NN: momenta pointed at random relative to reaction plane 2) Evolution as a bulksystem Pressure gradients (larger in-plane) push bulk “out”  “flow” more, faster particles seen in-plane

  39. Elliptic Flow: ultra-cold Fermi-Gas • Li-atoms released from an optical trap exhibit elliptic flow analogous to what is observed in ultra-relativistic heavy-ion collisions • Elliptic flow is a general feature of strongly interacting systems!

  40. Resulting azimuthal distributions STAR, PRL90 032301 (2003) b ≈ 6.5 fm b ≈ 4 fm “central” collisions midcentral collisions

  41. Resulting azimuthal distributions STAR, PRL90 032301 (2003) b ≈ 10 fm b ≈ 6.5 fm b ≈ 4 fm peripheral collisions

  42. “v2” Elliptic flow observed momentum anisotropy is largely elliptic deformation; its amplitude is denoted v2 RHIC v2 reaches large values yielded by hydro (unlike lower energies) STAR, PRL90 032301 (2003) Hydrodynamic calculation of system evolution

  43. Elliptic flow - sensitive to early stages Zhang, Gyulassy, Ko, Phys. Lett. B455, 45 (1999) ZPC parton cascade

  44. PRL 90, 032301 (2003) s = 130 GeV Azimuthal anisotropy of leading hadrons • pT<2 GeV: detailed agreement with hydrodynamics • pT >4 GeV: flattening of data

  45. Elliptic flow - sensitive to QGP/Hadronic EOS P. Huovinen et al.

  46. The bottom line: It produces copious mesons and baryons with yield ratios and flow properties that suggest their formation via coalescence of valence quarks from a hot thermal bath.

  47. H2O The universal tendency of flow to be dissipated due to the fluid’s internal friction results from a quantity known as the shear viscosity. All fluids have non-zero viscosity. The larger the viscosity, the more rapidly small disturbances are damped away. Quantum limit: h/sAdS/CFT ~ 1/4p pQCD limit: ~ 1 At RHIC: ideal (h/s = 0) hydrodynamic model calculations fit to data  Perfect Fluid at RHIC?! N2 He Viscosity and the Perfect Fluid hadronic partonic Caption: The viscosity to entropy ratio versus a reduced temperature. Lacey et al. PRL 98:092301(07) hep-lat/0406009 hep-ph/0604138 APS top physics story in 2005

  48. Soft Physics: pT< 2 GeV/c Goal: Characterize the bulk of the event What do we know (or we think we do)? • Baryon/antibaryon ratios ~0.6-1 close to baryon-free PRL 86, 4778 (2001) • High apparent energy density ~5 GeV/fm3 (lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3) PRL 86, 112303 (2001) • Chemical equilibrium: T~175 MeV, B<40 MeV  near lattice phase boundary PRC 66, 061901(R) (2002); PRL 89, 092301 (2002); PRC 65, 041901(R) (2002); nucl-ex/0211024; nucl-ex/0206008 • Hydrodynamics works well PRL 90, 032301 (2003); PRC 66, 034904 (2002); PRL 89, 132301 (2002); PRL 87, 182301 (2001); PRL 86, 402 (2001) • (With some trouble with HBT correlations) • Explosive dynamics – Huge radial flow Overall picture: system appears to be in equilibrium but explodes and hadronizes rapidly  high initial pressure

  49. Low-pT dynamics — one (naïve?) interpretation:rapid evolution and a “flash” RHIC 130 GeV Au+Au K-p K* yield Disclaimer: all numbers (especially time) are approximate

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