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On the Trail of a Dense Plasma at RHIC (a detector builder’s point of view) Jim Thomas

On the Trail of a Dense Plasma at RHIC (a detector builder’s point of view) Jim Thomas Lawrence Berkeley Laboratory Cornell University Journal Club 5/21/2004. The Phase Diagram for Nuclear Matter. One of the goals of RHIC is to understand the QCD in the context of the many body problem

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On the Trail of a Dense Plasma at RHIC (a detector builder’s point of view) Jim Thomas

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  1. On the Trail of a Dense Plasma at RHIC (a detector builder’s point of view) Jim Thomas Lawrence Berkeley Laboratory Cornell University Journal Club 5/21/2004

  2. The Phase Diagram for Nuclear Matter • One of the goals of RHIC is to understand the QCD in the context of the many body problem • Another goal is to discover and characterize the Quark Gluon Plasma • RHIC is a place where fundamental theory and experiment can meet after many years of being apart The goal is to explore nuclear matter under extreme conditions – T > mpc2 and r > 10 * r0 K. Rajagopol

  3. { W. Zajc QCD is a Rich Theory with Many Features We have a theory of the strong interaction Low(er) energy nuclear physics uses OPEP or descriptions in terms of a pion gas. These worked because QCD is a theory with a mass gap. This gap is a manifestation of the approximate SU(2)R x SU(2)Lchiral symmetry of QCD with pions as the Nambu-Goldstone bosons Hadron “level” Diagram

  4. TC ~ 170 15 MeV eC ~ 0.7 GeV/fm3 e0 ~ 0.16 GeV/fm3 Energy Density Karsch QM2004 Temperature Lattice QCD predictions Stephan Boltzman limits for a free Quark Gluon gas TC ~ 170 MeV, a very stable prediction over time & technologies (F. Karsch, hep-lat/0106019, edited by JT)

  5. BRAHMS PHOBOS PHENIX Long Island Who is RHIC and What Does He Do? RHIC • Two independent rings • 3.83 km in circumference • Accelerates everything, from p to Au Ös L p-p 500 1032 Au-Au 200 1026 (GeV and cm-2 s-1) • Polarized protons • Two Large and two small detectors have been built h

  6. RHIC Physics is Relativistic Nuclear Physics

  7. The Initial State is Important • Only a few of the nucleons participate as determined by the impact parameter • There is multiple scattering in the initial state before the hard collisions take place • Cronin effect • The initial state is Lorentz contracted • Cross-sections become coherent. • The uncertainty principle allows wee partons to interact with the front and back of the nucleus • The interaction rate for wee partons saturates ( ρσ = 1 ) • The intial state is even time dilated • A color glass condensate •proton•neutron•delta•pionstring

  8. Time Projection Chamber Magnet Coils Silicon Tracker SVT & SSD TPC Endcap & MWPC FTPCs Endcap Calorimeter Beam Beam Counters Barrel EM Calorimeter Central Trigger Barrel & TOF How do we ‘see’ RHIC physics? Not Shown: pVPDs, ZDCs, PMD, and FPDs 4.2 meters A TPC lies at the heart of STAR

  9. Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm Voltage : - 28 kV at the central membrane 135 V/cm over 210 cm drift path 420 CM TPC Gas Volume & Electrostatic Field Cage Self supporting Inner Field Cage:Al on Kapton using Nomex honeycomb; 0.5% rad length

  10. Pixel Readout of a Pad Plane Sector A cosmic ray + deltaelectron 3 sigma threshold

  11. f from K+ K- pairs dn/dm background subtracted m inv dn/dm K+ K- pairs same event dist. mixed event dist. m inv Particle ID using Topology & Combinatorics Secondary vertex: Ks p + p L  p + p X  L + p W  L + K g  e++e- Ks p + + p - f  K + + K - L  p + p - r  p + + p - “kinks” K  + 

  12. Au on Au Event at CM Energy ~ 130 GeV*A Data taken June 25, 2000. The first 12 events were captured on tape! Real-time track reconstruction Pictures from Level 3 online display. ( < 70 mSec )

  13. Au on Au Event at CM Energy ~ 130 GeV*A A Central Event Typically 1000 to 2000 tracks per event into the TPC Two-track separation 2.5 cm Momentum Resolution < 2% Space point resolution ~ 500 mm Rapidity coverage –1.8 < h < 1.8

  14. Boost invariant hydrodynamics: Bjorken Estimate of Initial Energy Density ~ 6.5 fm t~ 0.2 - 1 fm/c time to thermalize the system pR2 Bjorken Estimate of Initial Energy Density Cold nuclear matter: r0~ 0.16 GeV/fm3 30xr0 nucl-ex/0311017 PRL 87 (01) 52301

  15. Rapidity vs xf • xf = pz / pmax • A natural variable to describe physics at forward scattering angles • Rapidity is different. It is a measure of velocity but it stretches the region around v = c to avoid the relativistic scrunch • Rapidity is relativistically invariant and cross-sections are invariant β Rapidity is the natural kinematic variable for HI collisions ( y is approximately the lab angle where y = 0 at 90 degrees )

  16. Bose-Einstein fits mt exponential fits K- p+ Identified Particle Spectra at 200 GeV p+, p-, K+, K- spectra versus centrality PRL 92 (2004) 171801 and nucl-ex/0206008

  17. gaussian fits p 200 GeV data 130 GeV data Anti-Proton Spectra at 200 & 130 GeV / N Au + Au  p + X p p andp spectra versus centrality PRL 92 (2004) 171801 and PRL 87 (2001) 262302

  18. Anti-Baryon/Baryon Ratios versus sNN • In the early universe • p/ p ratio = 0.999999 • At RHIC, pair-production increases with s • Mid-rapidity region is not yet baryon-free! • Pair production is larger than baryon transport • 80% of protons from pair production • 20% from initial baryon number transported over 5 units of rapidity In HI collisions at RHIC, more baryons are pair produced than are brought in by the initial state

  19. K+/K- ratios  p/p ratios Anti-Particle to Particle Ratios STAR results on thep/p ratio • p/p = 0.11 ± 0.01 @ 20 GeV • p/p = 0.71 ± 0.05 @ 130 GeV • p/p = 0.80 ± 0.05 @ 200 GeV Excellent agreement between experiments at y = 0, Ös = 130

  20. Thermal model fits Compare to QCD on Lattice: Tc = 154 ± 8 MeV (Nf=3) Tc = 173 ± 8 MeV (Nf=2) (ref. Karsch QM01) Chemical Freeze-out – from a thermal model • The model assumes a thermally and chemically equilibrated fireball at hadro-chemical freeze-out • Works great, but there is not word of QCD in the analysis. Done entirely in a color neutral Hadronic basis! input: measured particle ratiosoutput: temperature T and baryo-chemical potential B

  21. Lattice results Neutron STAR Putting RHIC on the Phase Diagram • Final-state analysis suggests RHIC reaches the phase boundary • Hadron spectra cannot probe higher temperatures • Hadron resonance ideal gas (M. Kaneta and N. Xu, nucl-ex/0104021 & QM02) • TCH = 175 ± 10 MeV • B = 40 ± 10 MeV • <E>/N ~ 1 GeV • (J. Cleymans and K. Redlich, PRL 81, p. 5284, 1998 ) We know where we are on the phase diagram but now we want to know what other features are on the diagram

  22. time Kinetic freeze-out Chemical freeze-out elastic interactions inelastic interactions space blue beam yellow beam Chemical and Kinetic Freeze-out • Chemical freeze-out (first) • End of inelastic interactions • Number of each particle species is frozen • Useful data • Particle ratios • Kinetic freeze-out (later) • End of elasticinteractions • Particle momenta are frozen • Useful data • Transverse momentum distributions • and Effective temperatures

  23. T ≈ 215 MeV The transverse radial expansion of the source (flow) adds kinetic energy to the particle distribution. So the classical expression for ETot suggests a linear relationship T ≈ 310 MeV T ≈ 575 MeV Slopes decrease with mass. <pT> and the effective temperature increase with mass. Transverse Flow Au+Au at 200 GeV  - STAR Preliminary K- p

  24. <r> [c] PHENIX Tth [GeV] STAR Kinetic Freezeout from Transverse Flow STAR Preliminary <ßr> (RHIC) = 0.55 ± 0.1 c TKFO(RHIC) = 100 ± 10 MeV Thermal freeze-out determinations are done with the blast-wave model to find <pT> Explosive Transverse Expansion at RHIC  High Pressure

  25. Peripheral Collisions z y x Anisotropic Flow py px Anisotropic (Elliptic) Transverse Flow • The overlap region in peripheral collisions is not symmetric in coordinate space • Almond shaped overlap region • Easier for particles to emerge in the direction of x-z plane • Larger area shines to the side • Spatial anisotropy  Momentum anisotropy • Interactions among constituents generates a pressure gradient which transforms the initial spatial anisotropy into the observed momentum anisotropy • Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane • v2 is the 2nd harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane

  26. Hydro predictions PRL 86, (2001) 402 more central  v2 vs. Centrality • v2 is large • 6% in peripheral collisions • Smaller for central collisions • Hydro calculations are in reasonable agreement with the data • In contrast to lower collision energies where hydro over-predicts anisotropic flow • Anisotropic flow is developed by rescattering • Data suggests early time history • Quenched at later times Anisotropic transverse flow is large at RHIC

  27. PRL 86, 402 (2001) & nucl-ex/0107003 v2 vs. pT and Particle Mass • The mass dependence is reproduced by hydrodynamic models • Hydro assumes local thermal equilibrium • At early times • Followed by hydrodynamic expansion D. Teaney et al., QM2001 Proc.P. Huovinen et al., nucl-th/0104020 Hydro does a surprisingly good job!

  28. qn Coalescence V2 at high Pt shows meson / baryon differences Bulk PQCD Hydro Asym. pQCD Jet Quenching

  29. The Recombination Model ( Fries et al. PRL 90 (2003) 202303 ) The flow pattern in v2(pT) for hadrons is predicted to be simple if flow is developed at the quark level pT → pT /n v2 → v2 / n , n = (2, 3) for (meson, baryon)

  30. What does this mean? • Hadrons are created by the recombination of quarks and this appears be the dominant mechanism for hadron formation at intermediate PT • Baryons and Mesons are produced with equal abundance at intermediate PT • The collective flow pattern of the hadrons appears to reflect the collective flow of the constituent quarks. Partonic Collectivity

  31. Initial state Final state Au + Au d + Au p + p Lets look at some collision systems in detail …

  32. Binary collision scaling p+p reference Partonic energy loss via leading hadrons Energy loss  softening of fragmentation  suppression of leading hadron yield

  33. nucl-ex/0305015, PRL in press PRL 89, 202301 Au+Au and p+p: inclusive charged hadrons p+p reference spectrum measured at RHIC

  34. Suppresion of inclusive hadron yield Au+Au relative to p+p RAA Au+Au central/peripheral RCP nucl-ex/0305015 • central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT

  35. PHENIX observes a similar effect (p0s) nucl-ex/0304022 lower energy Pb+Pb lower energy a+a Factor ~5 suppression for central Au+Au collisions

  36. The Suppression occurs in Au-Au but not d-Au

  37. Jet Physics … it is easier to find one in e+e- Jet event in e+e-collision STAR Au+Au collision

  38. Identifying jets on a statistical basis in Au-Au • You can see the jets in p-p data at RHIC • Identify jets on a statistical basis in Au-Au • Given a trigger particle with pT > pT (trigger), associate particles with pT > pT (associated) STAR Data Au+Au @ 200 GeV/c 0-5% most central 4 < pT(trig) < 6 GeV/c 2 < pT(assoc.) < pT(trig)

  39. Angular Distribution: Peripheral Au+Au data vs. pp+flow • Ansatz: • A high pT triggered • Au+Au event is a superposition of a high pT triggered • p+p event plus anisotropic transverse flow • v2 from reaction plane analysis • “A” is fit in non-jet region (0.75<||<2.24)

  40. Angular Distribution: Central Au+Au data vs. pp+flow

  41. 3/4 /4 Out-of-plane y x in-plane -3/4 -/4 Dj correlations vs the reaction plane pTtrigger=4-6 GeV/c, 2<pTassociated<pTtrigger, ||<1 Back-to-back suppression out-of-plane stronger than in-plane Effect of path length on suppression is experimentally accessible

  42. What does it mean? Surface emission • The backward going jet is missing in central Au-Au collisions when compared to p-p data + flow • The backward going jet is not suppressed in d-Au collisions • These data suggest opaque nuclear matter and surface emission of jets Suppression of back-to-back correlations in central Au+Au collisions

  43. Exp: Conclusions About Nuclear Matter at RHIC • Its hot • Chemical freeze out at 175 MeV • Thermal freeze out at 100 MeV • The universal freeze out temperatures are surprisingly flat as a function of s • Its fast • Transverse expansion with an average velocity greater than 0.55 c • Large amounts of anisotropic flow (v2) suggest hydrodynamic expansion and high pressure at early times in the collision history • Its opaque • Saturation of v2 at high pT • Suppression of high pT particle yields relative to p-p • Suppression of the away side jet • And its not inconsistent with thermal equilibrium • Excellent fits to particle ratio data with equilibrium thermal models • Excellent fits to flow data with hydrodynamic models that assume equilibrated systems

  44. ‘They say …’ • ‘Some theorists say’ we have discovered the Quark Gluon Plasma at RHIC • Evidence for PQCD via vn bulk collective flow of 104p, K,p,L, X, W • Evidence for pQCD jet quenching in Au+Au at RHIC • Evidence jet un-quenching in D+Au = Null Control • The experimental community is somewhat more cautious because, in part, its not clear what the words mean • Web definition for plasma: A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality. • In the collision of two heavy ions, we have found a high density plasma containing quarks and gluons. So whats the problem with the experimentalists?

  45. Are the theories a unique explanation for the data?

  46. Au+Au Datasets • This year: extremely successful run, all goals met • Greater than order of magnitude increase in dataset

  47. Rcp ratios At  =0 the central events have the ratio systematically above that of semi-central events. We see a reversal of behavior as we study events at =3.2 1/<Ncollcentral> NABcentral(pT,h) Rcp= 1/<Ncoll periph> NABperiph(pT,h)

  48. RdAu ratios Cronin like enhancement at =0. Clear suppression as  changes up to 3.2 Same ratio made with dn/d follows the low pT RdAu 1 d2Nd+Au/dpTdh RdA= <Ncoll> d2Nppinel/dpTdh where < Ncoll> = 7.2±0.3

  49. Current State of Affairs • A theory is something nobody believes, except the person who made it. • An experiment is something everybody believes, except the person who made it. • attributed to Albert Einstein

  50. Recombination Tested The complicated observed flow pattern in v2(pT)d2n/dpTdf ~ 1 + 2 v2(pT) cos (2 f) is predicted to be simple at the quark level underpT → pT / n , v2 → v2 / n , n = 2,3 for meson,baryon if the flow pattern is established at the quark level Compilation courtesy of H. Huang

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