Dileptons and Medium Effects in Heavy-Ion Collisions

1
Dileptons and Medium Effects in Heavy-Ion
Collisions
Ralf Rapp Cyclotron Institute Physics
Department Texas AM University College
Station, USA Perspectives in Hadronic Physics
Conference 2006 ICTP Trieste, 24.05.06
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Introduction EM-Probes -- Basic Questions
QCD Phase Diagram

• Thermalization ? study the phase diagram
• (highest) temperature of the matter
• chiral symmetry restoration (mass generation!)
• in-medium spectral properties below above Tc

Inevitable consequences of QGP, link to lattice
QCD
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Outline
2.) Electromagnetic Emission and Chiral
Symmetry ? EM Thermal Rates ?
Axial-/Vector Correlators and Chiral Sum
Rules 3.) Medium Effects and Thermal Dileptons
? Vector Mesons in Medium Hadronic Many-Body
Theory ? Experimental and Theoretical
Constraints ? Dilepton Rates Hadronic vs.
QGP 4.) Dileptons at SPS ? CERES and NA60
Data ? Interpretation Open Issues 5.)
Conclusions
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2.) EM Emission Rates and Chiral Symmetry
E.M. Correlation Function
Im ?em(M,qmB,T)
Im ?em(q0q mB,T)
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2.2 Chiral Symmetry Breaking and Restoration
Splitting of chiral partners r - a1(1260) ?
Chiral Symmetry Breaking
Axial-/Vector in Vacuum
pQCD cont.
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2.3 Chiral Sum Rules and the a1(1260)

• Energy-weighted moments of difference vector
  axialvector

Das etal 67
Weinberg 67
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3.1 Medium Effects I Hadronic Many-Body Theory
Chanfray etal, Herrmann etal, RR etal, Weise
etal, Post etal, Eletsky etal, Oset etal,
Dr (M,qmB ,T) M 2- mr2 Srpp SrB -SrM -1
r-Propagator
r
B,a1,K1...
r
Sp
SrB,M
Srpp
r-Selfenergies
N,p,K
Sp
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3.1.2 r(770) Spectral Function in Nuclear Matter
In-med p-cloud r -N ? B resonances
Relativist. r -N ? B (low-density approx)
In-med p-cloud r -N ? N(1520)
Urban etal 98
Post etal 02
Cabrera etal 02
rN0.5r0
rNr0
rNr0
Constraints g N , g A
p N ?r N PWA

• good agreement strong broadening small
  mass-shift up
• constraints from (vacuum) data important
  quantitatively

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3.1.3 QCD Sum Rules r(770) in Nuclear Matter
dispersion relation for correlator
Shifman,Vainshtein Zakharov 79

• lhs OPE (spacelike Q2)

• rhs hadronic model (sgt0)

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3.1.4 r-Meson Spectral Functions at SPS
HotDense Matter
Hot Meson Gas
RRWambach 99
RRGale 99

• r-meson melts in hot and dense matter
• baryon density rB more important than
  temperature
• reasonable agreement between models

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3.2 Dilepton Emission Rate Hadron Gas vs. QGP
Braaten,PisarskiYuan 90

• Hard-Thermal-Loop QGP rate
• enhanced over Born rate
• matching of HG and QGP
• in vicinity of Tc
• Quark-Hadron Duality ?!

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4.) Dilepton Spectra in Heavy-Ion Collisions
Thermal Emission
Pb-Pb Collisions Trajectories in the Phase
Diagram
mN GeV t fm/c

• based on entropy (baryon-number) conservation
• volume expansion VFB(t ) (z0vzt ) p (R-
  0.5a-t 2)2

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4.1 Pb-Au Collisions at SPS CERES/NA45

• QGP contribution small
• medium effects on r-meson!
• dropping mass or broadening?!

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4.2 In-In at SPS Dimuons from NA60
Damjanovic et al. PRL 06

• excellent mass resolution and statistics
• for the first time, dilepton excess spectra
  could be extracted!

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4.2.2 In-In at SPS Theory vs. NA60

• predictions based r-spectral function of
  RRWambach 99
• uncertainty in fireball lifetime (25 norm.)
  or infer tFB7fm/c !
• relative strength of thermal sources fix

• good agreement with r melting, including pt
  dependence

van Hees RR 06
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4.2.3 Intermediate-Mass Region

• 4p states dominate in the vacuum
• e.m. correlator above M 1.1GeV
• lower estimate
• use vacuum 4p correlator
• upper estimate
• O(T2) medium effect ?
• chiral V-A mixing
• with

4p
2p
EletskyIoffe 90
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4.2.4 NA60 Data Other r-Spectral Functions

• switch off medium modifications
• T-, rB- dependence of bare parameters dropping
  mass

BrownRho 91, HatsudaLee 92

• free spectral function ruled out
• meson gas insufficient either

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4.2.5 (Some) Open Issues

• Heavy-Ion Collisions NA60
• - centrality dependence, free rs (surface vs.
  volume)
• - sensitivity to fireball evolution
• - quantitative w and f
• - thermal radiation at intermediate mass
  (M1.5-3 GeV)
• - chiral restoration ? duality (hadron
  liquid ? sQGP)
• ? chiral sum
  rules
• ? chiral
  mixing in the M1-1.5GeV region

• Cold Nuclei CB/TAPS, KEK-E325
• - dropping w-mass broadening
• - dropping r-mass without broadening ?!

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5.) Conclusions

• Strong medium effects in ll - spectra
• new level of precision in NA60 ? model
  discrimination
• r-melting at Tc, no apparent mass shift
• alternative models? (quality control)

• Chiral Restoration
• - direct (exp.) measure axialvector
• - indirect (theo.) (1) effective model
  (constraints)
• (2) chiral sum
  rules (V-A moments) vs. lQCD
• (3)
  compatibility with dilepton/photon data
• HADES, RHIC, LHC, SPS-09, CBM, , elementary
  reactions

In-medium V-meson spectroscopy has begun
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3.3 Medium Effects II Dropping Mass
BrownRho 91, 02
Scale Invariance of LQCD ? bare parameters
change!?

• density dependence
• QCD sum rules C 0.15
• temperature dependence a
• quark condensate from chiral
• perturbation theory
• vector dominance coupling
• (gauge invariance!)

Hatsuda Lee 92
Pelaez 03
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3.) Medium Effects and Thermal Dileptons
3.1 Lattice QCD (QGP)
Dilepton Rate ImP(w,q0)/w2
EM Correlator ImP(w,q)/w2
T1.5Tc
Bielefeld Group 02, 05

• lQCD ltlt pQCD at low mass (finite volume?)
• currently no thermal photons from lQCD
• vanishing electric conductivity!? but Gavai
  04

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3.4 In-Medium IV Vector Manifestation of Chiral
Symmetry

• Hidden Local Symmetry r-meson introduced as
  gauge boson,
• 
  Higgs mechanism generates r-mass
• Vacuum rL?p, good phenomenology (loop exp.
  O(p/Lc , mr /Lc , g))
• In-Medium T-dep. mr(0), gr matched to OPE
  (spacelike), LmatchltLc ,
• Renormalization Group
  running ? on-shell
• ? - dropping r-mass ? 0 (RG fixed point at
  Tc) ,
• - violation of vector dominance a
  2 ? 1

Harada, Yamawaki etal, 01
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4.2 Recent Advances at SPS Power of Precision
NA60 Data vs. Model Predictions
RRWambach 99 RR03

• r-meson melting supported (baryons!)
• dropping mass (as used to explain CERES data)
  ruled out
• open issues
• (1) M gt 0.9GeV (4p?mm- !?)
• (2) normalization 0.6 (pt lt0.5GeV), 0.8 (all
  pt ), 2 (pt gt1GeV)
• (3) other models (vector manifestation, chiral
  virial approach, )

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4.2.2 Modified Fireball and Absolute Normalization

• r-spectral function unchanged since RRWambach
  99
• expanding fireball, fixed S (?Nch)
  VFB(t)(z0vzt ) p (R-0 0.5a-t 2)2
• Increase a- ? reduced lifetime (t 9?6fm/c),
  increased v-0.4?0.5c

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Revival Attempts for Dropping r-Mass
E.g., SkokovToneev 05
Fireball Evolution
egt1GeVfm-3 ec for Dt8 fm/c?!
Bjorken regime tFB0.5 fm/c?!

• Not compatible with gauge
• invariance (no mr in VDM)
• acceptance?

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M 1GeV in NA60
H. van Hess RR, in prep.

• combination of 4-p QGP charm?!
• (beware schematic acceptance)

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4.2.5 Chiral Virial Approach vs. NA60 (central)
Steele,Yamagishi Zahed 99
implementation van HeesRR 05
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5.) Electromagnetic Probes
5.1.1 Thermal Photons I SPS
Expanding Fireball pQCD

• pQCDCronin at qt gt1.6GeV
• ? T0205MeV suff., HG dom.

Turbide,RRGale04
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5.1.2 Thermal Photons II RHIC

• thermal radiation qtlt3GeV ?!
• QGP window 1.5ltqtlt3GeV ?!

• also g -radiation off jets
• shrinks QGP window qtlt2GeV ?!

Gale,Fries,Turbide,Srivastava 04
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5.3.1 RHIC Vector Mesons in Medium
Hadronic Many-Body Theory

• baryon effects important even at rB,net0
• sensitive to rB,totrBrB , most pronounced at
  low M
• f more robust ? OZI

-
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5.3.2 Dileptons II RHIC
RR 01
QGP

• low mass thermal! (mostly in-medium r)
• connection to Chiral Restoration a1 (1260)? pg
  , 3p
• int. mass QGP (resonances?) vs. cc ? ee-X
  (softening?)

-