HeavyIon Physics Raimond Snellings XXIII Physics in Collision
- Slides: 47
Heavy-Ion Physics Raimond Snellings XXIII Physics in Collision Zeuthen, Germany June 26 -28, 2003
Outline l l l Brief introduction to Heavy-Ion Physics CERN SPS: a new state of matter BNL Relativistic Heavy Ion Collider BRAHMS, PHOBOS, PHENIX and STAR (a few selected) RHIC results from year 1 -3 Summary
l Collisions of “Large” nuclei convert beam energy to temperatures above 200 Me. V or 1, 500, 000, 000 K l l ~100, 000 times higher temperature than the center of our sun. “Large” as compared to mean-free path of produced particles.
QCD Phase Diagram l We normally think of 4 phases: l l Plasma Gas Liquid Solid Phase diagram of nuclear matter Phase diagram of water
QCD on the Lattice F. Karsch, hep-lat/0106019 • Z. Fodor and S. D. Katz, hep-lat/01060002
Schematic Space-Time Diagram of a Heavy Ion Collision
Schematic Time Evolution p J/Y L K e e Hadronization si on ------ Freeze-out QGP? an g jet Ex p g Thermalization? Hard Scattering time space Au Au
CERN SPS: A New State of Matter? NA 50 Are hadronic scenarios ruled out? Co-mover absorption? canonical suppression? l l l J/Y suppression indication of deconfinement? Strangeness enhancement Melting of the r
SPS, NA 49: Indications of a Phase Transition at ≈ 30 Ge. V ?
l l A New Era for Heavy Ion Physics: 120 bunches/ring The Relativistic Heavy Ion Collider 106 ns crossing time at BNL Capable of colliding 3. 83 km circumference Two independent rings l l l ~any nuclear species on ~any other species l Energy: 200 Ge. V for Au-Au (per N-N collision) 500 Ge. V for p-p l Luminosity l l l ` Au-Au: 2 x 1026 cm-2 s-1 p-p : 2 x 1032 cm-2 s-1 (polarized)
l Hadron PID over broad rapidity acceptance l Two conventional beam line spectrometers Magnets, Tracking Chambers, TOF, RICH l
l l Charged Hadrons in Central Spectrometer Nearly 4 p coverage multiplicity counters Silicon Multiplicity Rings Magnetic field, Silicon Pad Detectors, TOF
l Electrons, Muons, Photons and Hadrons Measurement Capabilities l Focus on Rare Probes: J/y, high-p. T l l Two central spectrometers with tracking and electron/photon PID Two forward muon spectrometers
l Hadronic Observables over a Large Acceptance l l Event-by-Event Capabilities Solenoidal magnetic field Large coverage Time-Projection Chamber Silicon Tracking, RICH, EMC, TOF Online Level 3 Trigger Display
Heavy-ion Physics at RHIC • RHIC different from previous (fixed target) heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction) decreases by ~ same factor • Study pp, p. A to AA • Comprehensive set of detectors • All final state particles measured with overlap between the detectors • Study QCD at high density with probes generated in the medium • If QGP produced at RHIC most likely to live longer than at the SPS and therefore easier to observe and study its properties
Event Characterization l l Cannot directly measure the impact parameter b! but we can distinguish peripheral collisions from central collisions! • b Ncoll STAR Npart 5% Central Impact Parameter (b)
Soft Physics l l l Particle Yields Spectra shapes Elliptic Flow
Particle distributions (PHOBOS) d. Nch/dh 19. 6 Ge. V 130 Ge. V 200 Ge. V PHOBOS Preliminary Central Peripheral h • central collisions at 130 Ge. V: 4200 charged particles ! • mid rapidity plateau
Energy Density (Bjorken estimate) Bjorken Estimate 503± 2 Ge. V (130 Ge. V) R to PRL 87 (2001) • preliminary
Particle spectra at RHIC l l Superimposed on thermal (~Boltzmann) distributions: l Collective velocity fields from l Momentum spectra ~ l ‘Test’ by investigating description for different mass particles: Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061)
Particle spectra at the SPS Rather well described by Hydro motivated fit
Particle ratios: chemical potentials and freeze-out temperature l Assume distributions described by one temperature T and one ( baryon) chemical potential m l One ratio (e. g. , p / p ) determines m / T l l A second ratio (e. g. , K / p ) provides T m Then predict all other hadronic yields and ratios:
Where is RHIC on the phase diagram?
Three Forms of Collective Motion l l Only type of transverse flow in central collision (b=0) is transverse flow. Integrates pressure history over complete expansion phase y Elliptic flow, caused by anisotropic initial overlap region (b > 0). More weight towards early stage of expansion. y x x x l Directed flow, sensitive to earliest collision stage (pre-equilibrium, b > 0) z
What makes elliptic flow an unique probe? coordinate space l y x Momentum space l l py px Non central collisions coordinate space configuration anisotropic (almond shape). However, initial momentum distribution isotropic (spherically symmetric). Only interactions among constituents (mean free path small) generate a pressure gradient which transforms the initial coordinate space anisotropy into the observed momentum space anisotropy Multiple interactions lead to thermalization -> limiting behavior hydrodynamic flow
Elliptic Flow at the SPS (NA 49 and CERES) • Clearly deviates from ideal hydrodynamic model calculations
Integrated Elliptic Flow Hydrodynamic limit STAR PHOBOS Compilation and Figure from M. Kaneta First time in Heavy-Ion Collisions a system created which at low p t is in quantitative agreement with hydrodynamic model predictions for v 2 up to mid-central collisions
Differential Elliptic Flow l l Typical pt dependence Heavy particles more sensitive to velocity distribution (less effected by thermal smearing) therefore put better constrained on EOS Hydro calculation: P. Huovinen et. al.
Soft Physics l l Energy density estimate well above critical Lattice values Particle yields are well described in a thermal model Spectra shapes are consistent with thermal boosted distributions Elliptic flow reaches hydrodynamical model predictions l l l First time in heavy-ion collisions Observables consistent with strong early partonic interactions and approaching early local equilibrium However, size measurements (HBT) are not completely understood yet
Hard probes and the produced medium
Hard probes l p+p-> 0 + X At RHIC energies different mechanisms are responsible for different regions of particle production. Hard Scattering Thermallyshaped Soft Production l Rare process (Hard Scattering or “Jets”), a calibrated probe “Well Calibrated” hep-ex/0305013 S. S. Adler et al.
Hard Probes and the Produced Medium l l Hard scatterings in nucleon collisions produce jets of particles. In the presence of a colordeconfined medium, the partons strongly interact losing much of their energy “Jet Quenching” schematic view of jet production hadrons leading particle q q hadrons leading particle
Jets at RHIC p+p jet+jet (STAR@RHIC) find this Au+Au X (STAR@RHIC) in this
Find partonic energy loss with leading hadrons Energy loss softening of fragmentation suppression of leading hadron yield Binary collision scaling p+p reference
Measurements of jet suppression BRAHMS preliminary Relative to UA 1 p+p nucl-ex/0304022 Binary scaling Participant scaling nucl-ex/0305015
Elliptic Flow at higher-pt M. Gyulassy, I. Vitev and X. N. Wang STAR preliminary • R. S, A. M. Poskanzer, S. A. Voloshin, • STAR note, nucl-ex/9904003
Back to back “jets” at the SPS (CERES) • Centrality 24 -30% • Centrality 11 -15% Cronin Effect: Multiple Collisions broaden high PT spectrum • SPS. CERES: Away side jet broadening, no disappearance
Disappearance of back to back “jets” near side PRL 90, 082302 (2003) away side peripheral central • In central Au+Au collisions the away-side “jet” disappears !!
High-pt phenomena: Initial state or final state effect? nucl-ex/0305015 Final state Initial state p. T>5 Ge. V/c: well described by KLM saturation model (up to 60% central) and p. QCD+jet quenching
Theory expectations for d+Au RAB Inclusive spectra If Au+Au suppression is final state If Au+Au suppression is initial state (KLM saturation: 0. 75) 1. 1 -1. 5 1 ~2 -4 Ge. V/c High p. T hadron pairs broadening? p. QCD: no suppression, small broadening due to Cronin effect saturation models: suppression due to mono-jet contribution? p. T 0 0 /2 (radians) suppression? All effects strongest in central d+Au collisions
Comparison of Au+Au to d+Au (PHOBOS and BRAHMS) PHOBOS d+Au: nucl-ex/0306025 central Au+Au
Comparison of Au+Au to d+Au (PHENIX and STAR) l l Dramatically different behavior of Au+Au observables compared to d+Au observables. Jet Suppression is clearly a final state effect.
Back to back “jets” in d+Au Central Au+Au d+Au ? “PHENIX Preliminary” results, consistent with STAR data in submitted paper
Summary l l High-pt probes are a new unique tool at RHIC to understand heavy-ion collisions New phenomena have been found: l l l Suppression of the inclusive yields (“jet quenching”) Large elliptic flow Disappearance of the away-side “jet” Pointing at very dense (≈ 30 x nuclear densities) and strongly interacting matter Low-pt (bulk) and high-pt observables consistent with expectations from a QGP (but not as proof, still more work to be done. RHIC program just started)
Thanks l Many figures on the slides are “borrowed” from: l W. Zajc, P. Steinberg, N. Xu, P. Jacobs, F. Laue, P. Kolb, U. Heinz, T. Hemmick, G. Roland, I. Bearden, M. van Leeuwen and many others
Time Evolution in a Hydro Calculation: P. Kolb, J. Sollfrank and U. Heinz l Elliptic Flow reduces spatial anisotropy -> shuts itself off
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