Search for the Quark-Gluon Plasma

1
Search for the Quark-Gluon Plasma in Heavy Ion
Collision
V. Greco
2
Outline II

• Probes of QGP in HIC
• What we have find till now!
• strangeness enhancement
• jet quenching
• coalescence
• J/Y suppression

• What we have learned
• ?

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Probes of QGP

• Strangeness enhancement
• J/? suppression
• Jet quenching
• Thermal QGP radiation
• Dilepton enhancement
• Quark recombination
• Enhancement of fluctuations

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Strangeness Enhancement

• Basic Idea
• Production threshold is lowered by the chiral
  restoration

In the QGP

• Equilibration timescale ? How much time do you
  have?

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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 give t5-10 fm/c
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How one calculates the Equilibration Time
Similarly in hadronic case but more channels
Reaction dominate by gg
6 fm/c

• (pQCD) Equilibration time in QGP teq 10 fm/c gt
  tQGP
• Hadronic matter teq 20-30 fm/c

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Experimental results
Strangeness enh. 1
Strangeness enhancement 2
Schwinger mechanism
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Present Knoweldge

• AGS (6GeV) explained by hadronic models
• Enhancement at SPS and RHIC (8.8 GeV-200 GeV)
• - not explained by hadronic models
• - unless chiral symmetry effects are modelled
  
• Enh. Agrees with statistical models in
  grand-canonical ensemble
• - no canonical suppression

Present Unknoweldge

• What means the absence of canonical suppression?
• - multiparticle dynamics in QGP
• - higher cross section respect to pQCD
• Enh. is more a signal of chiral
• restoration already in dense hadronic matter?
• Why Enh. Larger at SPS than at RHIC?

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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 ?
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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
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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)
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High pT Particle Production in AA
Known from pp and pA
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Energy Loss

Gluon multiple scattering
Static scattering centers assumed Gauge
invariance O(1/E2)
Transport coefficient
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Medium Induced Radiation
Clearly similar Recursion Method is needed
to go toward a large number of scatterings!
Ivan Vitev, LANL
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Large radiative energy loss in a QGP medium
L/l opacity
DE/E 0.5
Non abelian energy loss
weak pT dependence of quenching
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Energy Loss and expanding QGP
Probe the density
In the transverse plane
Quenching is angle dependent
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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
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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.

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Is the plasma a QCD-QGP?

• Consistent with L2 non-abelian plasma behavior
• Consistent with e 10 GeV (similar to hydro)

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pions
PHENIX, nucl-ex/0304022
p0 suppression evidence of jet quenching
before fragmentation
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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
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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

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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
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Distribution Function
Hadron from coalescence may follow jet structure
(away suppr.)
REALITY one spectrum with correlation kept also
at pT lt 2 GeV
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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 suppression hidden
• by coalescence!

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

Fragmentation not included for L
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(No Transcript)
28

K, L, p v2 not affected by resonances!
p coal. moved towards p data
nucl-th/0402020
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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 with s-h with away side
suppressed, but same side is reduced if no
futher correlation
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What was not emphasized
IAA 1 peak like in pp IAA gt 1 against the

• how explain p/p ratio,
• v2B/v2M ?
• at lower pT correlation
• increase !?

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J/Y suppression
m m- 6
cc bound state, MY 3.1 GeV
e e- 6
Charm Thermalize in the plasma
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J/Y suppression

• In a QGP enviroment
• Color charge is subject to screening of the
  medium
• -gt qq interaction is weakened
• Linear string term vanish in the confined phase
• s(T) -gt 0 deconfinement

T 4 Tc
T Tc
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Suppression respect to extrapolation from pp
J/Y Initial production
Dissociation In the plasma
Recombine with light quarks

• 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

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• NUCLEAR ABSORBTION
• pre-equilibrium cc formation time and
• absorption by co-moving hadrons
• HADRONIC ABSORBTION
• re-scattering after QGP formation
• DYNAMICAL SUPPRESSION
• (time scale, gJ/Y -gt cc,)

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

Regeneration in QGP / at Tc J/y g c
c X
-
?
?
Grandchamp Rapp 03
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Charmonia in URHICs
RHIC
SPS
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Does Charm quark thermalize?

• pT Spectra and Yield of D and/or J/Y

• v2 of D meson (single e)

From hard pp collision
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D meson spectra
Single electron does not resolve the two
scenarios
Elliptic flow better probe of interaction
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V2 of electrons
VGCMKRR, PLB595 (04) 202
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AMPT, L.W. Chen, C.M. Ko, nucl-th 0409058
Similar to the cross section needed in the light
sector !
42
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

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Result for V channel (J/y)
A(w) w2r (w)
J/y (p 0) disappears between 1.62Tc and 1.70Tc
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Result for PS channel (hc)
A(w) w2r (w)

hc (p 0) also disappears between 1.62Tc and
1.70Tc
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Sketch of Strong QGP
The elementary excitation are not free gluons and
quarks, but hadronic excitations with strongly
modified in-medium properties and with chirally
restored phase

• Loosely bound states crucial for particle
• scattering
• -gt large cross section (Breit-Wigner )
• One has also to reproduce lattice EOS

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In Conclusion

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

Needed - Thermal spectrum -
Dilepton enhancement
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Big Bang

• e. m. decouple (T 1eV , 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 t lt 3 .105 ys
(hidden behind the curtain of the cosmic
microwave background)
HIC can do it!
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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 supressed !