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Che-Ming Ko

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Searching for the Quark-Gluon Plasma in Relativistic Heavy Ion Collisions Che-Ming Ko Teaxs A&M University Introduction: concepts and definitions – PowerPoint PPT presentation

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Title: Che-Ming Ko


1
Searching for the Quark-Gluon Plasma in
Relativistic Heavy Ion Collisions
Che-Ming Ko Teaxs AM University
  • Introduction concepts and definitions
  • - Quark-gluon plasma (QGP)
  • - Heavy ion collisions (HIC)
  • Experiments and theory
  • - Signatures of QGP
  • - Experimental observations

Largely based on slides by Vincenzo Greco
2
Big Bang
  • e. m. decouple (T 1 eV , t 3.105 ys)
  • thermal freeze-out
  • but matter opaque to e.m. radiation
  • Atomic nuclei (T100 KeV, t 200s)
  • chemical freeze-out
  • Hadronization (T 0.2 GeV, t 10-2s)
  • Quark and gluons

Well never see what happened at t lt 3 .105 ys
(hidden behind the curtain of the cosmic
microwave background)
HIC can do it!
3
Little Bang
Freeze-out Hadron Gas Phase
Transition Plasma-phase Pre-Equilibrium
4
Heuristic QGP phase transition
Free massless gas
Bag Model
Pressure exceeds the Bag pressure -gt quark
liberation
Extension to finite mB , mI
5
Quantum ChromoDynamics
  • Similar to QED, but much richer structure
  • SU(3) gauge symmetry in color space
  • Approximate Chiral Symmetry in the light sector
  • which is broken in the vacuum.
  • UA(1) ciral
  • Scale Invariance broken by quantum effects

6
Enhancement of the degrees of freedom towards
the QGP
Noninteracting massless partons
Gap in the energy density (Ist order or cross
over ?)
7
QCD phase diagram
From high rB regime to high T regime
AGS
SPS
RHIC
We do not observe hadronic systems with Tgt 170
MeV (Hagedon prediction)
8
Definitions and concepts in HIC
  • Kinematics
  • Observables
  • Language of experimentalist

9
The RHIC Experiments
10
Soft and Hard
SOFT (non-pQCD) string fragmentation in ee-
, pp or (pTlt2 GeV) string melting
in AA (AMPT, HIJING, NEXUS)
HARD minijets from first NN collisions
Independent Fragmentation pQCD
phenomenology
  • Small momentum transfer
  • Bulk particle production
  • How ? How many ? How are they distributed?
  • Only phenomenological descriptions available
    (pQCD doesnt work)

11
Collision Geometry - Centrality
Spectators
Participants
S. Modiuswescki
For a given b, Glauber model predicts Npart
and Nbinary
0 N_part
394
12
Additive like Galilean velocity
Transverse mass
Angle with respect to beam axis
Rapidity -pseudorapidity
13
Energy Density
Estimate e for RHIC
Particle streaming from origin
dET/dy 720 GeV
? Tinitial 300-350 MeV
14
Some definitions I radial collectiv flows
Slope of transverse momentum spectrum is due to
folding temperature with radial collective
expansion ltbTgt from pressure.
Absence
Slopes for hadrons with different masses allow
to separate thermal motion from collective flow
Tf (120 10) MeV ltbTgt (0.5 0.05)
15
Collective flow II Elliptic Flow
Fourier decomposition of particle momentum
distribution in x-y plane
v2 is the 2nd Fourier coeff. of particle
transverse moment distribution
Measure of the pressure gradient
Good probe of early pressure
16
Anisotropic flow
Anisotropic flow vn
Sine terms vanish because of the symmetry ???? in
AA collisions
Initial spatial anisotropy
x
17
Statistical Model
Maximum entropy principle
Is there a dynamical evolution that leads to
such values of temp. abundances?
Hydro adds radial flow freeze-out
hypersurface for describing the differential
spectrum
Yes, but what is Hydro?
18
HYDRODYNAMICS
5 partial diff. eq. for 6 fields (p,e,n,u)
Equation of State p(e,nB)
Local conservation Laws
  • No details about collision dynamics (mean free
    path -gt0)

19
Transport Model
Follows time evolution of particle
distribution from initial non-equilibrium
partonic phase
To be treated - Multiparticle collision
(elastic and inelastic) - Quantum transport
theory (off-shell effect, ) - Mean field or
condensate dynamics
20
Elliptic Flow
Hydro
Transport
Spectra still appear thermal
rapidity
rapidity
21
  • Chemical equilibrium with a limiting Tc 170MeV
  • Thermal equilibrium with collective behavior
  • - Tth 120 MeV and ltbTgt 0.5
  • Early thermalization (t lt 1 fm/c, e 10 GeV/fm3)
  • - very large v2

We have not just crashed 400 balls to get
fireworks, but we have created a transient state
of plasma
A deeper understanding of the system is
certainly needed!
22
Signatures of quark-gluon plasma
  • Dilepton enhancement (Shuryak, 1978)
  • Strangeness enhancement (Meuller Rafelski,
    1982)
  • J/? suppression (Matsui Satz, 1986)
  • Pion interferometry (Pratt Bertsch, 1986)
  • Elliptic flow (Ollitrault, 1992)
  • Jet quenching (Gyulassy Wang, 1992)
  • Net baryon and charge fluctuations (Jeon Koch
    Asakawa, Heinz Muller, 2000)
  • Quark number scaling of hadron elliptic flows
    (Voloshin 2002)

23
Dilepton spectrum at RHIC
  • Low mass thermal dominant
  • (calculated by Rapp in kinetic model)
  • Inter. mass charm decay

No signals for thermal dileptons yet
24
Strangeness Enhancement
  • Basic Idea
  • Production threshold is lowered in QGP

In the QGP
  • Equilibration timescale? How much time do we
    have?

25
QGP Scenario
Hadronic Scenario
Decreasing threshold in a Resonance Gas
To be weighted with the abundances
npQCD calculation with quasi particle picture and
hard-thermal loop still gives t5-10 fm/c
26
How one calculates the Equilibration Time
Similarly in hadronic case but more channels
Reaction dominated by gg
6 fm/c
  • (pQCD) Equilibration time in
  • QGP teq 10 fm/c gt tQGP
  • Hadronic matter teq 30 fm/c

27
Experimental results
Strangeness enhancement 1
Strangeness enhancement 2
Schwinger mechanism
28
J/Y suppression
  • In a QGP enviroment
  • Color charge is subject to screening in QGP
  • -gt qq interaction is weakened
  • Linear string term vanishes in the deconfined
    phase
  • s(T) -gt 0 deconfinement

29
Screening Effect
  • Abelian
  • Non Abelian
  • (gauge boson self-interaction)

One loop pQCD
TBound is not Tc !
In HIC at vs SPS, J/Y should be suppressed !
30
Lattice result for V channel (J/y)
A(w) w2r (w)
J/y (p 0) disappears between 1.62Tc and 1.70Tc
31
Suppression respect to extrapolation from pp
J/Y Initial production
Dissociation In the plasma
Recombine with light quarks
  • For light quarks rBohr 4 fm gtgt lD ,
    dissociation is more effective
  • but of course also recombination
  • Associated suppression of charmonium resonances
    Y, cc ,

as a thermometer, like spectral lines for
stellar interiors
  • B quark in similar condition at RHIC as
    Charmonium at SPS

32
  • NUCLEAR ABSORBTION
  • pre-equilibrium cc formation time and
  • absorbtion by comoving hadrons
  • HADRONIC ABSORBTION
  • rescattering after QGP formation
  • DYNAMICAL SUPPRESSION
  • (time scale, gJ/Y -gt cc,)

pA (models) sabs 6 mb
33
Fireball dynamical evolution
gluon-dissociation, inefficient for my 2
mc quasifree dissoc. Grandchamp 01
34
  • RHIC central Ncc10-20,
  • QCD lattice J/ys to 2Tc

Regeneration in QGP / at Tc J/y g c
c X
-
?
?
Grandchamp Rapp 03
35
Charmonia in URHICs
RHIC
SPS
36
Pion interferometry
open without Coulomb solid with Coulomb
STAR

AuAu _at_ 130 GeV
STAR AuAu _at_ 130 AGeV
Ro/Rs1 smaller than expected 1.5
37
Source radii from hydrodynamic model
Fails to explain the extracted source sizes
38
Two-Pion Correlation Functions and source radii
from AMPT
Lin, Ko Pal, PRL 89, 152301 (2002)
AuAu _at_ 130 AGeV
Need string melting and large parton scattering
cross section which may be due to quasi bound
states in QGP and/or multiparton dynamics
(gg?ggg)
39
Emission Function from AMPT
  • Shift in out direction (ltxoutgt gt 0)
  • Strong positive correlation between out position
    and emission time
  • Large halo due to resonance (?) decay and
    explosion
  • ? non-Gaussian source

40
Jet quenching
Decrease of mini-jet hadrons (pTgt 2 GeV)
yield, because of in medium radiation.
Ok, what is a mini-jet? why it is quenched ?
41
High pT Particle Production
Jet A localized collection of hadrons which come
from a fragmenting parton
c
a
Parton Distribution Functions Hard-scattering
cross-section Fragmentation Function
b
d
phad z pc , z lt1 energy needed to create
quarks from vacuum
Collinear factorization
42
Jet Fragmentation-factorization
p, K, p ...
c
a
b
A
B
d
ph z pc , z lt1 energy needed to create quarks
from vacuum
AB pp (ee-)
a,b,c,d g,u,d,s.
Parton distribution after pp collision
p/p lt 0.2
B.A. Kniehl et al., NPB 582 (00) 514
( phenomenological kT smearing due to vacuum
radiation)
43
High pT Particle Production in AA
Known from pp and pA
44
Energy Loss

Gluon multiple scattering
Static scattering centers assumed
Transport coefficient
45

Medium Induced Radiation
Clearly similar Recursion Method is needed
to go toward a large number of scatterings!
Ivan Vitev, LANL
46
Large radiative energy loss in a QGP medium
L/l opacity
DE/E 0.5
Non abelian energy loss
weak pT dependence of quenching
47
Energy Loss and expanding QGP
Probe the density
In the transverse plane
Quenching is angle dependent
48
How to measure the quenching
Self-Analyzing (High pT) Probes of the Matter at
RHIC
Nuclear Modification Factor
nucleon-nucleon cross section
ltNcollgt
AA
If R 1 here, nothing new going on
49
Centrality Dependence
Au Au Experiment
d Au Control
  • Dramatically different and opposite centrality
    evolution of AuAu experiment from dAu control.
  • Jet suppression is clearly a final state effect.

50
Is the plasma a QCD-QGP?
  • Consistent with L2 non-abelian plasma behavior
  • Consistent with e 10 GeV (similar to hydro)

51
pions
protons
PHENIX,nucl-ex/0212014
  • Fragmentation p/p 0.1-0.2
  • Jet quenching should affect both

PHENIX, nucl-ex/0304022
Fragmentation is not the dominant mechanism of
hadronization at pT 1-5 GeV !?
p0 suppression evidence of jet quenching
before fragmentation
52
Parton spectrum
Coalescence
  • partons are already there
  • to be close in phase space
  • ph n pT ,, n 2 , 3
  • baryons from lower momenta

B M
Even if eventually Fragm. takes over
53
npQCD
Mqq-gtm2 depends only on the phase space
weighted by wave function (npQCD also encoded in
the quark masses , mq0.3 GeV, ms0.475 GeV)
  • Energy not conserved
  • No confinement constraint

54
fq invariant parton distribution function thermal
(mq0.3 GeV, ms0.47 GeV) with radial flow
(b0.5) quenched minijets (L/l3.5)
fH hadron Wigner function
Dx 1/Dp coalescence radius
In the rest frame
55
Distribution Function
Hadron from coalescence may have jet structure
(away suppr.)
REALITY one spectrum with correlation kept also
at pT lt 2 GeV
56
AuAu _at_200AGeV (central)
V. Greco et al., PRL90 (03)202302
PRC68(03) 034904 R. Fries et al.,
PRL90(03)202303
PRC68(03)44902 R. C. Hwa et al., PRC66(02)025205
  • Proton enhancement
  • due to coalescence!

57
  • Resonance decays (r -gt p p)
  • Shrinking of baryon phase
  • space

Fragmentation not included for L
58
Momentum-space coalescence model
Kolb, Chen, Greco, Ko, PRC 69 (2004) 051901
Including 4th order quark flow

Meson flow
Baryon flow
59
(No Transcript)
60

K, L, p v2 not affected by resonances!
p coal. moved towards p data
nucl-th/0402020
61
Higher-order anisotropic flows
Data can be described by a multiphase transport
(AMPT) model
Parton cascade
Data
62
Back-to-Back Correlation
quenched
Trigger is a particle at 4 GeV lt pTrig lt 6 GeV
Away Side quenching has di-jet structure Same
Side Indep. Fragm. equal (?!) to pp
Associated is a particle at 2 GeV lt pT lt pTrig
Coalescence from s-h leads to away side
suppression, While same side is reduced if no
further correlation
63
Unexpected Appreciable charm flow
64
Does Charm quark thermalize?
  • v2 of D meson (single e)
  • coalescence/fragmentation?
  • energy loss?
  • pT Spectra and Yield of J/Y

From hard pp collision
65
D meson spectra
Single electron does not resolve the two
scenarios
Elliptic flow better probe of interaction
66
V2 of electrons
VGCMKRR, PLB595 (04) 202
67
Quark gluon plasma was predicted to be a weakly
interacting gas of quarks and gluons
  • The matter created is not a firework of multiple
    minijets
  • Strong Collective phenomena

68
Summary
  • Most proposed QGP signatures are observed at
    RHIC.
  • Strangeness production is enhanced and is
    consistent with
  • formation of hadronic matter at Tc.
  • Large elliptic flow requires large parton cross
    sections in transport
  • model or earlier equilibration in hydrodynamic
    model.
  • HBT correlation is consistent with formation of
    strongly interacting
  • partonic matter.
  • Jet quenching due to radiation requires initial
    matter with energy density
  • order of magnitude higher than that of QCD at
    Tc.
  • Quark number scaling of elliptic flow of
    identified hadrons is consistent
  • with hadronization via quark coalescence or
    recombination.
  • Studies are needed for electromagnetic probes
    and heavy flavor hadrons.
  • Theoretical models have played and will continue
    to play essential roles
  • in understanding RHIC physics.

69
Conclusions
  • Matter with energy density too high for simple
    hadronic
  • phase ( e gt ec from lattice)
  • Matter is approximately thermalized (T gtTc )
  • Jet quenching consistent with a hot and dense
    medium
  • described by the hydrodymic approach
  • Hadrons seem to have typical features of
    recombination
  • Strangeness enhancement consistent with grand
    canonical
  • ensemble
  • J/y ...

Needed - Thermal spectrum -
Dilepton enhancement
70
A Lot of work to do
  • Lattice QCD
  • Effective field theory
  • Transport theory (quantum, field condensate,)
  • pQCD
  • Understanding of Non-Abelian Interaction !
  • Scientific approach to an important part
  • of the evolution of the primordial plasma
  • can be achieved
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