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Physics at RHIC Results from the RHIC STAR Experiment

Physics at RHIC Results from the RHIC STAR Experiment. Motivation for studying Relativistic Heavy Ion Collisions RHIC and the STAR experiment Soft Physics from STAR Hard Physics from STAR Summary. Outline. Why heavy ion collisions?. The “little bang”.

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Physics at RHIC Results from the RHIC STAR Experiment

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  1. Physics at RHIC Results from the RHIC STAR Experiment

  2. Motivation for studying Relativistic Heavy Ion Collisions RHIC and the STAR experiment Soft Physics from STAR Hard Physics from STAR Summary Outline

  3. Why heavy ion collisions? The “little bang” • Study bulk properties of nuclear matter • Extreme conditions (high density/temperature) expect a transition to new phase of matter… • Quark-Gluon Plasma (QGP) • partons are relevant degrees of freedom over large length scales (deconfined state) • believed to define universe until ~ ms • Study of QGP crucial to understanding QCD • low-q (nonperturbative) behaviour • confinement (defining property of QCD) • nature of phase transition • Heavy ion collisions ( “little bang”) • the only way to experimentally probe deconfined state

  4. Stages of the collision The “little bang” • pre-equilibrium (deposition of initial energy) • rapid (~1 fm/c) thermalization (?) • high-pT observables probe this stage QGP formation (?) hadronization transition (very poorly understood) hadronic rescattering • Chemical freeze-out: end of inelastic scatterings • Kinetic freeze-out: end of all scatterings • low-pT hadronic observables probe this stage Does “end result” look about the same whether a QGP was formed or not??? time temperature

  5. The Phase Space Diagram TWO different phase transitions at work! – Quarks and gluons roam freely over a large volume – Quarks behave as though they are massless 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 Quark-Gluon Plasma Chiral transition Hadrons Tc ~ 150-170 MeV ec ~ 0.5-0.7 GeV/fm

  6. Beam energy up to 100 GeV/A (250 GeV for p); • Two independent rings (asymmetric beam collisions are possible); • Beam species: from proton to Au; • Six interaction points: STAR, PHENIX, PHOBOS and BRAHMS

  7. RHIC Data-Taking Year 2000: Au + Au @ 130 GeV 2 weeks Year 2001: Au + Au @ 200 GeV 15 weeks Au + Au @ 20 GeV 1 day p + p @ 200 GeV 5 weeks Year 2003: 1st of January d + Au @ 200 GeV 10 weeks p + p @ 200 GeV (5) + 3 weeks    

  8. The STAR Collaboration The Ohio State U. Group Profs: PostDocs: Students: T.Humanic D.Majestro S.Bekele M.Lisa B. Nilsen M.Lopez- Noriega E.Sugarbaker I. Kotov R.Wells R.Willson Brazil: Universidade de Sao Paolo China:IHEP - Beijing, IPP - Wuhan England:University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt Poland:Warsaw University, Warsaw University of Technology Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino U.S. Labs:Argonne, Berkeley, Brookhaven National Labs U.S. Universities:Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale Institutions: 36 Collaborators: 415

  9. The STAR Detector

  10. The STAR Detector Magnet Time Projection Chamber Coils Silicon Vertex Tracker * TPC Endcap & MWPC ZCal ZCal FTPCs (1 +1) Endcap Calorimeter Vertex Position Detectors Barrel EM Calorimeter Central Trigger Barrel + TOF patch RICH * yr.1 SVT ladder • Year 2000, year 2001,year-by-year until 2003, installation in 2003

  11. STAR Time Projection Chamber (TPC) On-board FEE Card: Amplifies, samples, digitizes 32 channels • Active volume: Cylinder r=2 m, l=4 m • 139,000 electronics channels sampling drift in 512 time buckets • active volume divided into 70M 3D pixels

  12. Triggering/Centrality Spectators – Definitely going down the beam line Participants – Definitely created moving away from beamline Several meters Spectators Zero-Degree Calorimeter Participants Impact Parameter Spectators

  13. Triggering/Centrality • “Minimum Bias” ZDC East and West thresholds set to lower edge of single neutron peak. REQUIRE: Coincidence ZDC East and West • “Central” CTB threshold set to upper 15% REQUIRE: Min. Bias + CTB over threshold ~30K Events |Zvtx| < 200 cm Spectators – Definitely going down the beam line Participants – Definitely created moving away from beamline Several meters Spectators Zero-Degree Calorimeter Participants Impact Parameter Spectators

  14. Au-Au Event at 130 A-GeV Peripheral Event From real-time Level 3 display.

  15. Au- Au Event 130 A-GeV Mid-Central Event From real-time Level 3 display.

  16. Au -Au Event 130 A-GeV Central Event From real-time Level 3 display.

  17. STAR Pertinent Facts (130 GeV) Field: 0.25 T (Half Nominal value)  worse resolution at higher p  lower pt acceptance TPC: Inner Radius – 50cm (pt>75 MeV/c) Length – ± 200cm ( -1.5< h < 1.5) Events: ~300,000 “Central” Events –top 8% multiplicity ~160,000 “Min-bias” Events

  18. Needle in the Hay-Stack! How do you do tracking in this regime? Solution: Build a detector so you can zoom in close and “see” individual tracks high resolution Clearly identify individual tracks Pt (GeV/c) Good tracking efficiency

  19. Particle ID Techniques - dE/dx

  20. Particle ID Techniques - dE/dx dE/dx dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

  21. Particle ID Techniques - dE/dx Resolution: dE/dx No calibration 9 % With calibration 7.5% Design 6.7% Even identified anti-3He ! dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

  22. Particle ID Techniques - Topology X+ Decayvertices Ks p + + p - L  p + p - L  p + p + X-  L + p - X+L + p + W  L + K- L Vo “kinks”: K  + 

  23. Physics Measurements(ones in red will be shown) • dN/dh for h- (|h|<= ~1.5) particle density, entropy • Elliptic flow early dynamics, pressure • p/p, L/L stopping • Particle spectra temperature, radial flow • Particle ratioschemistry • Particle correlations geometry, collective flow • High Pt jet quenching _ _ • Neutral particle decays L,K0s, X strangeness production

  24. Transverse Energy Phenix Electromagnetic Calorimeter measures transverse energy in collisions Central Events: Lattice predicts transition at PHENIX Preliminary ~ 5.0 GeV/fm3 ecritical ~ 0.5-0.7 GeV/fm3 Have the Energy Density!!

  25. Soft Physics (pT < 2 GeV/c)

  26. Soft Physics (pT < 2 GeV/c) 99.5% The majority of produced particles are low pT. Do they interact and exhibt collective behaviour? What are the bulk dynamics ?

  27. Is there Thermalization? Look at “Elliptic” Flow Origin: spatial anisotropy of the system when created and rescattering of evolving system probe of the early stage of the collision Almond shape overlap region in coordinate space

  28. Elliptic Flow of Pions and Protonsfrom STAR (130 GeV) • Hydrodynamic calculations: P. Huovinen, P. Kolb and U. Heinz Mass dependence of v2(pt) shows a behavior in agreement with hydro calculations, which assumes a system in equilibrium

  29. Charged particle elliptic flow 0< pt< 4.5 GeV/c from STAR(130 GeV) Around pt > 2 GeV/c the data starts to deviate from hydro. However, v2 stays large. Only statistical errors Systematic error 10% - 20% for pt = 2 – 4.5 GeV/c

  30. Kinetic Freeze-out and Radial Flow Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

  31. Kinetic Freeze-out and Radial Flow 1/mt d2N/dydmt Look at mt = (pt2 + m2 )distribution A thermal distribution gives a linear distribution dN/dmt  e-(mt/T) mt Slope = 1/T Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

  32. Kinetic Freeze-out and Radial Flow 1/mt d2N/dydmt Look at mt = (pt2 + m2 )distribution A thermal distribution gives a linear distribution dN/dmt  e-(mt/T) mt Slope = 1/T If there is transverse flow Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

  33. Kinetic Freeze-out and Radial Flow 1/mt d2N/dydmt Look at mt = (pt2 + m2 )distribution A thermal distribution gives a linear distribution dN/dmt  e-(mt/T) mt Slope = 1/T If there is transverse flow Slope = 1/Tmeas ~ 1/(Tfreeze out + 0.5mo<bflow>2) Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion

  34. First RHIC spectra - an explosive source • various experiments agree well • different spectral shapes for particles of differing mass strong collective radial flow data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz

  35. First RHIC spectra - an explosive source purely thermal source light 1/mT dN/dmT heavy mT T • various experiments agree well • different spectral shapes for particles of differing mass strong collective radial flow data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz

  36. First RHIC spectra - an explosive source purely thermal source light 1/mT dN/dmT heavy mT explosive source light T,b T 1/mT dN/dmT heavy mT • various experiments agree well • different spectral shapes for particles of differing mass strong collective radial flow data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz

  37. First RHIC spectra - an explosive source purely thermal source light 1/mT dN/dmT heavy mT explosive source light T,b T 1/mT dN/dmT heavy mT • various experiments agree well • different spectral shapes for particles of differing mass strong collective radial flow • good agreement with hydrodynamiccalculations data: STAR, PHENIX, QM01 model: P. Kolb, U. Heinz

  38. Increase with collision centrality  consistent with radial flow: Tfreeze out=0.12 GeV, bflow=0.6c mt slopes vs. Centrality mid-rapidity Tp = 565 MeV TK = 300 MeV Tp = 190 MeV

  39. We’ve shown so far that for RHIC collisions: • Some evidence that source is thermalized • Particles kinetically freeze-out with common T • Large transverse flow - • common to all particle species

  40. “HBT 101” - probing source geometry p1 r1 x1 YT = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2} + U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1} p source r(x) 1 m x2 r2 p2 5 fm q = p2 – p1 Integrate Y*Y over r(x) e.g. r ~ exp(-r2/2R2)  C = 1 + lexp(-q2R2)

  41. “HBT 101” - probing source geometry 1-particle probability 2-particle probability p1 r1 x1 YT = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2} + U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1} p source r(x) 1 m x2 r2 p2 5 fm q = p2 – p1 Integrate Y*Y over r(x) e.g. r ~ exp(-r2/2R2)  C = 1 + lexp(-q2R2)

  42. “HBT 101” - probing source geometry 1-particle probability 2-particle probability F.T. of pion source Measurable! p1 r1 x1 YT = U(x1,p1)exp{i(r1-x1)p1}U(x2,p2)exp{i(r2-x2)p2} + U(x1,p2)exp{i(r2-x1)p2}U(x2,p1)exp{i(r1-x2)p1} p source r(x) 1 m x2 r2 p2 5 fm q = p2 – p1 Integrate Y*Y over r(x) e.g. r ~ exp(-r2/2R2)  C = 1 + lexp(-q2R2)

  43. “HBT 101” - probing the timescale of emission K Rout Rside beware this “helpful” mnemonic! Decompose q into components: qLong: in beam direction qOut : in direction of transverse momentum qSide:  qLong & qOut RO2 = <(xOut - bTt)2> RS2 = < xSide2 > RL2 = <(xLong – bLt)2> (beam is into board)

  44. HBT and the Phase Transition with transition without transition “e” ec Generic prediction of 3D hydrodynamic models ~ emission timescale Rischke & Gyulassy NPA 608, 479 (1996) PrimaryHBT “signature” of QGP Phase transition  longer lifetime; Rout/Rside ~ 1 + (bt)/Rside

  45. Correlation function for identical bosons: 1d projections of 3d Bertsch-Pratt 12% most central out of 170k events Coulomb corrected |y| < 1, 0.125 < pt < 0.225 Two-pion interferometry (HBT)from STAR qout STAR preliminary qlong STAR preliminary

  46. Radii dependence on centrality and kt y (fm) x (fm) central collisions low kT p- p+ STAR preliminary • Radii increase with multiplicity - Just geometry (?) • Radii decrease with kt – Evidence of flow (?) “multiplicity”

  47. Hydro attempts to reproduce data generic hydro Rlong: model waits too long before emitting Rout model emission timescale too long • KT dependence approximately reproduced correct amount of collective radial flow • Right dynamic effect / wrong space-time evolution??? the “RHIC HBT Puzzle” Rside

  48. HBT excitation function midrapidity, low pTp- from central AuAu/PbPb • decreasing l parameter partially due to resonances • saturation in radii • geometric or dynamic (thermal/flow) saturation • the “action” is ~ 10 GeV (!) • no jump in effective lifetime • NO predicted Ro/Rs increase(theorists: data must be wrong) • Lower energy running needed!? STAR Collab., PRL 87 082301 (2001)

  49. time Before collision (heavy nuclei) Evolution of a heavy-ion collision After collision: QM formation?? Hadronization In order to study QM/hadronization stage of collision from freezeout hadrons, need to understand rescattering stage first! Strong hadronic rescattering “Freezeout” (hadrons freely stream to detectors)

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