Axel Drees, University Stony Brook, PHY 551 S2003 Heavy Ion Physics at Collider Energies I.Introduction to heavy ion physics II.Experimental approach and RHIC at BNL III.Global observables, hadron spectra, strangeness production IV.Lepton pairs and heavy flavor V.High pt phenomena

Axel Drees Fundamental Forces in Nature Gravitygeneral relativity Electro-weakstandard model Strong interaction QCD weak electro magnetic Although we have fundamental theories for all forces we need ~20 parameters, constants of unknown origin to describe nature. Two outstanding puzzles: unseen quarks broken symmetries  existence of massive particles Both connected to complex structure of vacuum

Axel Drees Vacuum low resolution

Axel Drees Vacuum high resolution Vacuum is see of  qq pairs (+ gg pairs +..) Vacuum expectation value for u or d quarks ~ - (230 MeV) 3 Vacuum density of u and d pairs ~ 3 fm -3

Axel Drees l Quarks and gluons carry color the charge of QCD l In nature only color neutral objects exist l Bag model: Confinement qqqbaryons  qqmesons 0.8 fm Pressure of vacuum (B) compensated by internal pressure bag constant B 1/4 ~ 200 MeV

Axel Drees String Models  r String with tension  ~ 1 GeV/fm QCD potential: Need infinite energy to separate quarks  confinement V QCD r r < r bag r > r bag  r  1/r (relation to ??) 1 fm 1S 2S 3S 4S  bb 1S 1P 2S  cc charmonuim and bottonium states explore QCD potential

Axel Drees Chiral Symmetry l Chirality (handedness) or helicity for massless particles chirality is conserved l QCD with 3 massless quarks (flavors) symmetry q R does not couple to q L l Masses break symmetry if mass  0 q R couples to q L spin momentum spsp spsp left handed right handed

Axel Drees Masses of Quarks l spontaneous breaking of electro-weak interaction  current mass of quark for u & d quarksm o u ~ m o d ~ 5 MeV s quark m o s ~ 175 MeV explicitly breaking of chiral symmetry l spontaneous breaking of chiral symmetry  constituent mass of quarks for u & d quarksm u ~ m d ~ 300 MeV (~1/3 m proton ) s quark m o s ~ 500 MeV spontaneous breaking of chiral symmetry  qq q q coupling G q couples to  qq see

Axel Drees Symmetry Breaking l Spontaneously l Explicit external force V V ground state potential symmetric ground state symmetric potential symmetric symmetry broken for ground state massless Goldstone bosons here       (2 flavors) massive       V potential asymmetric Mass small ~ 140 MeV

Axel Drees 1) all hadrons have well defined parity chiral symmetry  q R q R =  q L q L  expect J  doublets 2)characteristic mass scale of hadrons 1 GeV mass gap to quark condensate except pseudoscaler mesons Goldstone bosons:  and  Consequences of Spontaneous Symmetry Breaking 11 1 + a 1 (1270 MeV) 1 -  (770 MeV)

Axel Drees Fundamental Puzzles of Hadrons l Confinement l Quarks do not exist as free particles l Large hadron masses l Free quark mass ~ 5-7 MeV l Quarks become “fat” in hadrons constituent mass ~ 330 MeV l Complex structure of hadrons l Sea quarks and anti quarks l Gluons l “spin crisis” Spin of protons not carried by quarks! These phenomena must have occurred with formation of hadrons nuclear matter p, n

Axel Drees ~ 10  s after Big Bang Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV/c 2 ~ 100 s after Big Bang Nucleon Synthesis strong force binds protons and neutrons bind in nuclei

Axel Drees ~ 10  s after Big Bang T ~ 200 MeV Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV/c 2 ~ 100 ps after Big BangT ~ GeV Electroweak Transition explicit breaking of chiral symmetry inflation Planck scale T ~ GeV End of Grand Unification

Axel Drees “Travel” Back in Time l QGP in Astrophysics early universe after ~ 10  s l possibly in neutron stars l Quest of heavy ion collisions l create QGP as transient state in heavy ion collisions l verify existence of QGP l Study properties of QGP l study QCD confinement and how hadrons get their masses neutron stars Quark Matter Hadron Resonance Gas Nuclear Matter Color Superconductor SIS AGS SPS RHIC & LHC early universe BB T T C ~170 MeV 940 MeV MeV baryon chemical potential temperature

Axel Drees Phase Diagram of Nuclear (QCD) Matter T >>  QCD : weak coupling  deconfined phase (Quark Gluon Plasma) T <<  QCD : strong coupling  confinement  phase transition at T~  QCD ? e.g. two massless flavors (Rajagopal and Wilczek, hep-ph/ )

Axel Drees Estimating the Critical Energy Density normal nuclear matter  0 critical density: naïve estimation nucleons overlap R~r n nuclear matter p, n Quark-Gluon Plasma q, g density or temperature distance of two nucleons: 2 r 0 ~ 2.3 fm size of nucleon r n ~ 0.8 fm

Axel Drees Critical Temperature and Degrees of Freedom l In thermal equilibrium relation of pressure P and temperature T l Assume deconfinement at mechanical equilibrium l Internal pressure equal to vacuum pressure B = (200 MeV) 4 l Energy density in QGP at critical temperature T c Noninteracting system of 8 gluons with 2 polarizations and 2 flavor’s of quarks (m=0, s=1/2) with 3 colors

Axel Drees Critical energy  C = 6  2 T C 4 critical temperature T C QCD calculations l perturbative QCD calculations applicable only for large momentum transfer  small coupling l for small momentum transfer  large coupling only solution numerical QCD calculations on lattice results from lattice QCD establish the QCD phase transition T C ~ MeV  C ~ GeV/fm 3  jump in energy density:

Axel Drees The QCD phase transition Change of order parameter: deconfinement: Polyakov loop L ~ e -F q chiral symmetry: Quark condensate  qq  chiral restoration and deconfinement at same critical temperature T C ~ 170 MeV temperature deconfinement chiral symmetry restoration Polyakov loop response function chiral susceptibility different quark mass m q 165 MeV 175 MeV

Axel Drees QCD Potential from Lattice Calculations As temperature increases towards T C the QCD potential vanishes at large distances

Axel Drees Restoration of Chiral Symmetry l Temperature axis: l sharp transition at T C (similar to lattice QCD results) l baryon density axis: l smooth transition l at nuclear matter density In hot and dense matter chiral symmetry is restored model calculation (Nambu, Jona-Lasinio) approaching of chiral symmetry restoration should strongly modify hadron properties like  and m

Axel Drees The Hadron Level Diagram Degeneracy Mass (MeV)  K  f o  Increasing number of hadrons

Axel Drees Hagedorn’s Limiting Temperature (1965) Discovery of the QCD phase transition before quarks were understood as underlying constituents requires T < T H Fit to this form with T H = 163 MeV

Axel Drees

Au-Au Event in STAR summer 2001

Axel Drees Detecting the QGP “matter box” l “ideal” experiment Rutherford experiment   atomdiscovery of nucleus SLAC electron scattering e  protondiscovery of quarks l Experiments with QGP not quite that simple l QGP created in nucleus-nucleus collisions can not be put in “box” l Thousands of particles produced during collision vacuum QGP penetrating beam absorption or scattering pattern

Axel Drees Space-time Evolution of Collisions e  space time Hard Scattering Au  Expansion  Hadronization  Freeze-out jet J/  QGP Thermaliztion  e p K   