Introduction to Relativistic Heavy Ion Collision Physics
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Transcript of Introduction to Relativistic Heavy Ion Collision Physics
Introduction to Relativistic Heavy Ion Collision Physics
Huan Z. Huang
黄焕中Department of Physics and Astronomy
University of California, Los Angeles
Oct 2006 @Tsinghua
http://hep.tsinghua.edu.cn/talks/Huang/
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 PancakesLorentz =100
Nuclei pass thru each other
< 1 fm/c
Huge StretchTransverse ExpansionHigh Temperature (?!)
The Last Epoch:Final Freezeout--
Large Volume
Au+Au Head-on Collisions 40x1012 eV ~ 6 micro-Joule
Human Ear Sensitivity ~ 10-11 erg = 10-18 Joule
A very loud Bang, indeed, if E Sound……
Vacuum Engineering !
initial state
pre-equilibrium
QGP andhydrodynamic expansion
hadronization
hadronic phaseand 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
)ln(2
1
Z
Z
PE
PEy
)2
ln(tan)ln(2
1
Z
Z
PP
PP
22YXT ppp
20
2 mpm TT
Useful Expressions
Edydp
y
ymp
ymE
z
z
TZ
T
tanh
sinh
cosh
2
*
*max
*
S
LLF
p
p
px
M
Qx
EE
qQppqfi
fi
2
)(
;)(
2
2222
Feymann xF:
Bjorken x:
Light-cone x+:beamz
z
pE
pEx
)(
)(
Cross Sections
Total = Number of Reactions
Number of Beam Particles X Scattering Center / Area
Dimension [L2]
Total = inel + el
inel= SD + ND
SD: Singly Diffractive ND: Non-Diffractive
Differential Cross Section:
dddppdpdpdppd
pd
d
zyx sin23
3
3
Question: differential cross section vs total cross section?
Invariant Multiplicity Density:
E d3n/d3p
Invariant Cross Sections
dydmm
d
dydpp
d
TT
TT
2
22
2Invariant Differential Cross Section:
E d3/d3p
dydmmN
Nd
dydppN
Nd
TTev
TTev
2
22
2
Experimental Considerations: Efficiency, Acceptance, Decay Correction, Target-out Correction.
Order of Magnitude
Geometrical CS: pp r2 = (1fm)2 = 32 mb
Au+Au Collisions: Rau = 1.2 A1/3 = 6.98 fm bmax=(2R)2 = 6 barn
1 barn = 10-24 cm2
Regge Theory: total=XS0.0808 + YS-0.4525
p-pbar 21.70 98.39 mbp-p 21.70 56.08 mb
Pomeron f,a,….
HIJING: minijet production
Luminosity at Collider
L = NB
2 B v / UA
B Number of bunches per beamNB Number of particles per bunchv velocity of particlesU circumference of the ringA beam cross section at the collision
Relativistic Heavy Ion Collider:
*
2
2
3
N
Brev
NBfL
N Invariant Transverse 95% Emittance the beta function
RHIC Numbers
RHIC Design:Au Beam proton Beam
B 57 NB 109 1011
L 2x1026 1x1031 cm-2s-1
200 GeV 500 GeVNNs
Collision Rate: L x Hz 0.7 MHz
RHIC Complex
STAR
Relativistic Heavy Ion Collider --- RHIC
Au+Au 200 GeV N-N CM energyPolarized p+p up to 500 GeV CM energy
Building Blocks of Hadron World
Proton Neutron
(uud) (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/volumenormal nucleus 0 ~ 0.15 /fm3 ~ 0.25x1015 g/cm3
Temperature, MeV ~ 1.16 x 1010 K10-6 second after the Big Bang T~200 MeV
Nucleus
Hyperons
(s…)
Energy Scale and Phase Transition
Entity Energy Dimension Physics Bulk Property P/T
Atom 10’s eV 10-10 m Ionization e/Ion Plasma No
Nucleus 8 MeV 10-14 m Multifrag. Liquid-Gas Y(?)
QCD 200 MeV 10-15 m Deconfine. QGP Y(?)
EW 100 GeV 10-18 m P/CP Baryon Asymmetry Y(?)
GUT 1015-16 GeV Supersymmetry
TOE 1019 GeV Superstring
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 q q
q qq q
Quark pairs can be produced from vacuumNo free quark can be observedMomentum Transfer
Co
up
lin
g S
tren
gth
Shorter distance
(GeV)
QCD on Lattice
Transition from quarks to hadrons – DOF !QGP – not an ideal Boltzmann gas !
Lattice: current statusLattice: current status• technical progress: finer mesh size, physical quark masses, improved
fermion actions phase-transition: smooth, rapid cross-over EoS at finite μB: in reach, but with large systematic uncertainties
critical temperature: TC180 MeV
Rajagopal & Wilczek, hep-ph/0011333
Fodor & Katz, hep-lat/0110102
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 starquark star
High Temperature Vacuum -- high energy heavy ion collisions -- the Big Bang
Deconfinement
QCD Phase Transition
Baryonic Potential B [MeV]
Chem
ical Tem
pera
ture
Tch
[M
eV
]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
SPS
RHIC quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinementchiral restoration
Lattice QCD
atomic nuclei
What do experimental data points indicate and how were these points obtained ?
Nuclear Collision Geometry
Number of Participants
Impact Parameter
Particle Production is assumed to be directlyrelated to the impact parameter or number of
participant nucleons.
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 NPART
Best approach (for fixed target!):– Directly measure in a “zero degree calorimeter”– (for A+A collisions)
– Strongly (anti)-correlated with produced transverse energy:
PerNucleon
ZDCPART E
EAN 2
ET
ET
EZDC
NA50
Number of Participant Nucleonsc) 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
mT spectrum: d2n/(2mT)dmTdy vs (mT-m0)pT spectrum: d2n/(2pT)dpTdy vs pT
Spectrum Fit
Boltzmann mT Fit:d2n/(2mT)dmTdy ~ mT exp(-mT/slp)
slp Slope Parameter
Why is this Boltzmann?d3n/d3p ~ exp(-E/T)
Invariant Multiplicity Density:Ed3n/d3p ~ E exp(-E/T)E = mTcosh(y-ycm)d2n/(2mT)dmTdy ~ mT cosh(y-ycm) exp(-mT cosh(y-ycm)/T)
Slp depends on rapidity for an isotropic thermal fireballslp = T/cosh(y-ycm)
dn/dy =
2
2
2
)(2
2)2
( y
cmyy
TTTT
edmmdydmm
nd
y ~ 0.7-0.8
Naïve Expectations• Thermal Isotropic Source mT Scaling
, K and proton have the same slope parameter e-E/T
T = 190 MeV
T = 300 MeV
Tp = 565 MeV
mid-rapidity
Data show a large difference among these particles Expansion
Naïve Expectation 2
Slope parameter TemperatureRapidity density dn/dy entropy or energy density
First Order Phase Transition:
dn/dy
<pT>
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
vz = z/t or y = 0.5 ln ((t+z)/(t-z))
22 zt
Bjorken Energy Density
Energy density = E x N
A x z
E average energy per particleA transverse area of the collision volumez longitudinal intervalN number of particles in z interval
vz = z/t = tanh y; z = sinh yz = cosh y yE = mT cosh y
= mT cosh y N
A cosh y ymT
Adn/dy
Initial Energy Density EstimatePRL 85, 3100 (00); 91, 052303 (03); 88, 22302 (02), 91, 052303 (03)
PHOBOS
hminus:Central Au+Au <pT>=0.508GeV/cpp: 0.390GeV/c
Pseudo-rapidityWithin ||<0.5 the total transverse momentum created is 1.5x650x0.508 ~ 500 GeV from an initial transverse overlap area of R2 ~ 153 fm2 !
Energy density ~ 5-30 0 at early time =0.2-1 fm/c !
19.6 GeV
130 GeV200 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-82-059 (1982) X.N. Wang and M. Gyulassy, PRL 68, 1480 (1992))
Quark/gluon
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/.
c c Charm quarks c-c may not bindInto 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/0302050)
Models of Neutron StarsF. Weber J.Phys. G27 (2001) 465
“Strangeness" of dense matter ?In-medium properties of hadrons ?Compressibility of nuclear matter ? Deconfinement at high baryon densities ?
1st 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
2nd year detectors installation in 2002 installation in 2003
ZDC
The STAR Detector