Introduction to Relativistic Heavy Ion Collision Physics Huan Z. Huang 黄黄黄 Department of Physics and Astronomy University of California, Los Angeles Oct 2006 @Tsinghua http://hep.tsinghua.edu.cn/talks/Huang/
Jan 09, 2016
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