Introduction to Relativistic Heavy Ion Collision Physics Huan Z. Huang 黄焕中 Department of Physics and Astronomy University of California, Los Angeles Oct

Two Puzzles of Modern Physics Missing Symmetry – all present theories are based on symmetry, but most symmetry quantum numbers are NOT conserved. Unseen Quarks – all hadrons are made of quarks, yet NO individual quark has been observed. -- T.D.Lee

Vacuum As A Condensate Vacuum is everything but empty! The complex structure of the vacuum and the response of the vacuum to the physical world breaks the symmetry. Vacuum can be excited! We do not understand vacuum at all !

A Pictorial View of Micro-Bangs at RHIC Thin Pancakes Lorentz  =100 Nuclei pass thru each other < 1 fm/c Huge Stretch Transverse Expansion High Temperature (?!) The Last Epoch: Final Freezeout-- Large Volume Au+Au Head-on Collisions  40x10 12 eV ~ 6 micro-Joule Human Ear Sensitivity ~ erg = Joule A very loud Bang, indeed, if E  Sound…… Vacuum Engineering !

initial state pre-equilibrium QGP and hydrodynamic expansion hadronization hadronic phase and freeze-out High Energy Nucleus-Nucleus Collisions Physics: 1) Parton distributions in nuclei 2) Initial conditions of the collision 3) a new state of matter – Quark-Gluon Plasma and its properties 4) hadronization

Rapidity: Pseudo-rapidity: Transverse Momentum: Transverse Mass: Kinematic Variables

Useful Expressions Feymann x F : Bjorken x: Light-cone x + :

Cross Sections  Total = Number of Reactions Number of Beam Particles X Scattering Center / Area Dimension [L 2 ]  Total =  inel +  el  inel =  SD +  ND SD: Singly Diffractive ND: Non-Diffractive Differential Cross Section: Question: differential cross section vs total cross section?

Invariant Multiplicity Density: E d 3 n/d 3 p  Invariant Cross Sections Invariant Differential Cross Section: E d 3  /d 3 p  Experimental Considerations: Efficiency, Acceptance, Decay Correction, Target-out Correction.

Order of Magnitude Geometrical CS: pp  r 2 =  (1fm) 2 = 32 mb Au+Au Collisions: R au = 1.2 A 1/3 = 6.98 fm  b max =  (2R) 2 = 6 barn 1 barn = cm 2 Regge Theory:  total =XS YS p-pbar mb p-p mb Pomeron  f,a,…. HIJING: minijet production

Luminosity at Collider L = N B 2 B v / U A B  Number of bunches per beam N B  Number of particles per bunch v  velocity of particles U  circumference of the ring A  beam cross section at the collision Relativistic Heavy Ion Collider:  N  Invariant Transverse 95% Emittance    the beta function

RHIC Numbers RHIC Design: Au Beamproton Beam B 57 N B L 2x x10 31 cm -2 s GeV 500 GeV Collision Rate: L x   Hz0.7 MHz

RHIC Complex

STAR Relativistic Heavy Ion Collider --- RHIC Au+Au 200 GeV N-N CM energy Polarized p+p up to 500 GeV CM energy

Building Blocks of Hadron World ProtonNeutron (uud)(uud)(udd)(udd) Mesons (q-q) Exotics (qqqq-q,…) Molecules Atoms Electrons Strong interaction is due to color charges and mediated by gluons. Gluons carry color charges too. Baryon Density:  = baryon number/volume normal nucleus  0 ~ 0.15 /fm 3 ~ 0.25x10 15 g/cm 3 Temperature, MeV ~ 1.16 x K second after the Big Bang T~200 MeV Nucleus Hyperons (s…)

Energy Scale and Phase Transition Entity Energy Dimension PhysicsBulk PropertyP/T Atom10’s eV mIonizatione/Ion PlasmaNo Nucleus 8 MeV mMultifrag.Liquid-GasY(?) QCD200 MeV mDeconfine.QGPY(?) EW100 GeV mP/CP Baryon AsymmetryY(?) GUT GeVSupersymmetry TOE10 19 GeVSuperstring

Salient Feature of Strong Interaction Asymptotic Freedom: Quark Confinement: 庄子天下篇 ~ 300 B.C. 一尺之棰，日取其半，万世不竭 Take half from a foot long stick each day, You will never exhaust it in million years. QCD qq qq q q Quark pairs can be produced from vacuum No free quark can be observed Momentum Transfer Coupling Strength Shorter distance  (GeV)

QCD on Lattice Transition from quarks to hadrons – DOF ! QGP – not an ideal Boltzmann gas !

Lattice: current status technical progress: finer mesh size, physical quark masses, improved fermion actions  phase-transition: smooth, rapid cross-over  EoS at finite μ B : in reach, but with large systematic uncertainties  critical temperature: T C  180 MeV Rajagopal & Wilczek, hep-ph/ Fodor & Katz, hep-lat/

Quark-Hadron Phase Transition

QGP – micro-second after the Big Bang

The Melting of Quarks and Gluons -- Quark-Gluon Plasma -- Matter Compression:Vacuum Heating: High Baryon Density -- low energy heavy ion collisions -- neutron star  quark star High Temperature Vacuum -- high energy heavy ion collisions -- the Big Bang Deconfinement

QCD Phase Transition Baryonic Potential  B [MeV] Chemical Temperature T ch [MeV] AGS SIS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restoration Lattice QCD atomic nuclei What do experimental data points indicate and how were these points obtained ?

Nuclear Collision Geometry

a)Geometrical Interpretation of Observables A monotonic relation between the observable and collision centrality is assumed b) Estimate from direct measurement missing energy from Zero-degree calorimeter from dn/dy of protons…. Number of Participant Nucleons

Directly Determining N PART Best approach (for fixed target!): –Directly measure in a “zero degree calorimeter” – (for A+A collisions) –Strongly (anti)-correlated with produced transverse energy: ETET ETET E ZDC NA50

Number of Participant Nucleons c) Dynamical Model Tune to fit experimental measurement From model to convert measurement to impact parameter and number of participant nucleons ++ Fluctuations are included - - Physical picture is biased to begin with

m T spectrum: d 2 n/(2  m T )dm T dy vs (m T -m 0 ) p T spectrum: d 2 n/(2  p T )dp T dy vs p T Spectrum Fit Boltzmann m T Fit: d 2 n/(2  m T )dm T dy ~ m T exp(-m T /slp) slp  Slope Parameter Why is this Boltzmann? d 3 n/d 3 p ~ exp(-E/T) Invariant Multiplicity Density: Ed 3 n/d 3 p ~ E exp(-E/T) E = m T cosh(y-y cm ) d 2 n/(2  m T )dm T dy ~ m T cosh(y-y cm ) exp(-m T cosh(y-y cm )/T) Slp depends on rapidity for an isotropic thermal fireball slp = T/cosh(y-y cm ) dn/dy =  y ~

Naïve Expectations Thermal Isotropic Source  m T Scaling , K and proton have the same slope parameter e -E/T T  = 190 MeV T  = 300 MeV T p = 565 MeV mid-rapidity Data show a large difference among these particles  Expansion

Naïve Expectation 2 Slope parameter  Temperature Rapidity density dn/dy  entropy or energy density First Order Phase Transition: dn/dy hadron QGP Mixed Collision dynamics much more complicated !!

Collision Dynamics

Bjorken Scaling Bjorken Ansatz: “…… at sufficient high energy there is a ‘central-plateau’ structure for the particle production as a function of the rapidity variable.” y dn/dy Physics must be invariant under Lorentz-boost: 1) Local thermodynamic quantity must be a function of proper time only. 2) Longitudinal velocity v z = z/t or y = 0.5 ln ((t+z)/(t-z))

Bjorken Energy Density Energy density  = E x  N A x  z E  average energy per particle A  transverse area of the collision volume  z  longitudinal interval  N  number of particles in  z interval v z = z/t = tanh y; z =  sinh y  z =  cosh y  y E = m T cosh y  = m T cosh y  N A  cosh y  y mTmT AA dn/dy

Initial Energy Density Estimate PRL 85, 3100 (00); 91, (03); 88, (02), 91, (03) PHOBOS hminus: Central Au+Au =0.508GeV/c pp: 0.390GeV/c Pseudo-rapidity Within |  |<0.5 the total transverse momentum created is 1.5x650x0.508 ~ 500 GeV from an initial transverse overlap area of  R 2 ~ 153 fm 2 ! Energy density  ~ 5-30  0 at early time  =0.2-1 fm/c ! 19.6 GeV 130 GeV 200 GeV

Ideas for QGP Signatures Strangeness Production: (J.Rafelski and B. Muller PRL 48, 1066 (1982)) s-s quark pair production from gluon fusions in QGP leads to strangeness equilibration in QGP  most prominent in strange hyperon production (  and anti-particles). Parton Energy Loss in a QCD Color Medium: (J.D. Bjorken Fermilab-pub (1982) X.N. Wang and M. Gyulassy, PRL 68, 1480 (1992)) Quark/gluon dE/dx in color medium is large!

Ideas for QGP Signatures Chiral Symmetry Restoration: T = 0, m(u,d,s) > 0 – Spontaneous symmetry breaking T> 150 MeV, m=0 – Chiral symmetry restored Mass, width and decay branching ratios of resonances may be different in dense medium. QCD Color Screening: (T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986)) A color charge in a color medium is screened similar to Debye screening in QED  the melting of J/ . cc Charm quarks c-c may not bind Into J/  in high T QCD medium The J/  yield may be increased due to charm quark coalescence at the final stage of hadronization (e.g., R.L. Thews, hep-ph/ )

Models of Neutron Stars F. Weber J.Phys. G27 (2001) 465 “Strangeness" of dense matter ? In-medium properties of hadrons ? Compressibility of nuclear matter ? Deconfinement at high baryon densities ?

1 st year detectors Silicon Vertex Tracker Central Trigger Barrel FTPCs Time Projection Chamber Barrel EM Calorimeter Vertex Position Detectors Endcap Calorimeter Magnet Coils TPC Endcap & MWPC RICH + TOF Silicon Strip Detector ZDC 2 nd year detectors installation in 2002 installation in 2003 ZDC The STAR Detector