RHIC and our understanding of hot hadronic systems

Wojciech Broniowski

Institute of Nuclear Physics, Cracow andInstitute of Physics, Świętokrzyska Academy, Kielce

Ljubljana, 18 September 2006

1. A short story of quarks

• 1964 Gell-Mann: Phys. Lett. 8 (1964) 214-215, „A schematic model of baryons and mesons” Zweig: simultaneous CERN preprint• 1965 Greenberg, Han, Nambu: introduction of color, confinement• For many years treated as a purely theoretical concept!• 60’/70’ Friedman + Kendall + Taylor: SLAC-MIT experiment – deep inelastic scattering, • 1968-69 Bjorken + Feynman: parton model• 1973 Fritch + Gell-Mann: QCD Gross + Wilczek + Politzer: asymptotic freedom, partons=quarks• 1979 three-jet events at PETRA/DESY, gluons • 70’- Current and constituent quarks, effective quark models

[from BNL web page]

2. Quark-gluon plasma

• 1975 Collins + Perry: „our basic picture then is that matter at densities higher than nuclear consists of a quark soup”

• 1975 Cabibbo + Parisi: phase transition at the Hagedorn temperature

• 1978 Shuryak first to use quark-gluon plasma (QGP) [Yad. Fiz. 28 (1978) 796]• 1978 Chin and 1979 Kapusta consider quantitatively the

possibility of QGP production in relativistic heavy ion collisions

• 1983 Bjorken: estimation of energy available for QGP production

3. Relativistic heavy-ion collisions

• 70’ BEVALAC (LBL) + SYNCHROPHASOTRON (DUBNA), 1-4 GeV/nucleon

• 1986 AGS (BNL) (14 GeV/n, Si) and SPS (CERN) (60 and 200 GeV/n, O i S)• 1992 AGS (11 GeV/n, Au)• 1995 SPS (158 GeV/n, Pb)• 2000 RHIC (BNL) collider: 200 GeV/nucleon pair • 1999 - 2003 SPS (NA49) collisions of various nuclei at

energies of 20, 30, 40 and 80 GeV/n

[from BNL web page]

4. Signatures of QGP

• Enhanced strangeness production, J/Ψ suppression, modified dilepton production, collective behavior, ...

• After 15 years of experiments CERN on 10 February 2000 issued an official statement:

„a compelling evidence now exists for the formation of a new state of matter”

• Doubts remain, circumstances, models not referring to QGP or phase transition in many instances describe the data

• Since 2000 new data from RHIC at 200 GeV/nucleon pair in CM frame – against expectations!

5. Elliptic flowPhenomenon known for 20 years ...

[film by J. Mitchell]

• not a superposition of independent NN colisions

• all particles flow together: pions, kaons, protons, heavy flavor (!)

• flow develops very early

• QGP leads to flow!

6. Jet quenching

[film by J. Mitchell]

Evidence for very dense medium, many scattering centers• Bjorken’s formula indicates density where hadrons largely

overlay - quarks and gluons are the proper degrees of freedom• Parton interactions lead to flow• Simulation of partonic cascades and the success of

hydrodynamic calculations with zero viscosity indicate very strong interaction of partons

Shuryak: QGP sQGP(nearly perfect fluid, 1000 times less viscous than water!)

Cross sections 50-100 times larger than in perturbative QCD!

7. The fate of sQGP

• Hard scatterings, strings, color glass condensate, ...

sQGP forms

• sQGP expands and gradually changes into a gas of hadrons

• Due to expansion the mean free path of hadrons increases and at some point the particles become free - freeze-out

• The chemical composition (pions, kaon, nucleons, hyperons, K*) corresponds to full chemical equilibrium with T = 165 MeV, transverse momentum spectra have a Boltzmann shape with T = 110 MeV and average velocity of transverse expansion of v = 0.5 c

8. The Cracow freeze-out model PRL 87 (2001) 272302, WB + W. Florkowski

All resonance decays included, simple description of freeze-out

+ P. Bożek, M. Michalec, A. Baran,

M. Chojnacki, B. Biedroń

Spectra of hyperons

(predictions compared to STAR)

Pion pair distribution in invariant mass M (WB+WF+ Brigitte Hiller), compared to STAR

[plot by P. Fachini]

Charge balance function (WB+WF+ Piotr Bożek) - correlation measure of positive and negative pions compared to the results of STAR

1log

2

E py

E p

ratio of protons to antiprotons

[prepared by B. Biedroń]

baryonic

strange

Topography of the fireball

Spectra at different rapidities compared to BRAHMSThe

rmal

desc

riptio

n

works

rem

arka

bly!

[by M. Chojnacki]

9. The freeze-out curve

RHIC

HADRONS

QGP

[adapted from F. Becattini]

Phase diagram of hadronic matter

first-order transition

crossover

Phase diagram of water

• sQGP - thermalized matter

• freeze-out – successful description of many phenomena

• to come: LHC – an order of magnitude larger energies, RHIC at SPS energies, NA49 – future, new facility at GSI – low temperature and high density

10. Summary and outlook

• Question on the nature of collision – transition (Bjorken) or stopping and explosion (Landau)

• No plateau in rapidity!

BRAHMS @200GeV

• Search and investigation of the critical point in the QCD phase diagram

• More precise scan at lower energies –

RHIC „overshoots” (Gaździcki plots)

• Fundamental theoretical question: what makes QGP sQGP?

• What happens in the earliest phase?

• Correlation studies, HBT, event-by-event fluctuations cluster picture of fireball

[WB, B. Hiller, W. Florkowski, P. Bożek]