Electromagnetic Probes of
Medium Effects in Heavy-Ion Collisions
Ralf RappCyclotron Institute
+ Physics Department Texas A&M University
College Station, USA
International on Workshop“Probing QCD with Heavy Ions”
Hirschegg, 21.01.05
1. Introduction
2. Chiral Symmetry in QCD 3. E.M. Emission and in-Medium Effects E.M. Correlation Function + Chiral Symmetry Vector Mesons in Medium Photon and Dilepton Rates
4. Exp. Puzzles and Theoretical Attempts Low-pt “Anomalies” at SPS (WA98, CERES/NA45)
5. Perspectives for RHIC
6. Conclusions
Outline
1.) Four Pillars of Thermal E.M. Radiation
low-mass eein-med →eeChiral Symmetry
restoration?
low-energy hadron decays/scatt.a1→, →Medium effects?
high-energy continuum emissionHG vs. QGP , O(s)
QGP radiation? )M(ImTdM
dRV
dMd
dNem
T/qeeFB
ee
0
3
1 e
q0≈0.5GeV Tmax≈0.17GeV , q0≈1.5GeV Tmax=0.5GeV
Thermal rate:
int-mass eecontinuum emission a1→ ee , qq→eeQGP radiation?
2.) Chiral Symmetry in QCD: Vacuum
2
4
1aq Gq)m̂Agi(q QCDL SU(2)L × SU(2)R
invariant (mu,d ≈ 0)
Spontaneous Breaking: strong qq attraction Bose Condensate fills QCD vacuum!
0 LRRL qqqqqq >
>
>
>qLqR
qL-qR
-[cf. Superconductor: ‹ee›≠0 Magnet ‹M›≠0 , … ]
-
Profound Consequences:• energy gap: ↔ mass generation!
• massless Goldstone bosons 0,±
• “chiral partners” split, M≈0.5GeV:
qqm*qqq 2
JP=0± 1± 1/2±
3.) Electromagnetic Emission Rates
Tiqx jxjexdiqΠ )0()()( emem
4em E.M. Correlation Function:
e+
e-
γ
)T(fqd
dR Bee 24
)T(fqd
dRq B
30
Im Πem(M,q)
Im Πem(q0=q)
= O(1)= O(1)
= O(= O(ααs s ))
also: e.m susceptibility (charge fluct.): χ = Πem(q0=0,q→0)
In URHICs:• source strength: dependence on T, B, , medium effects, …• system evolution: V(), T(), B(), transverse expansion, …• nonthermal sources: Drell-Yan, open-charm, hadron decays, … • consistency!
3.1 E.M. Correlator in Vacuum: (e+e-→hadrons)
)s(Im em)s(DIm
g
mV
,, V
V
22
s,d,u
Sqc
)s()e(N
s
1
122
e+
e-
h1
h2…
s ≥ (1.5GeV)2 : pQCD continuum
s < (1.5GeV)2 :V-meson spectral functs.
q
q_
qq_
3.2 Low-Mass Dileptons + Chiral Symmetry
Im Πem(M) dominated by -meson → chiral partner: a1(1260)
Vacuum At Tc: Chiral Restoration
pQCD cont.
or: long chiral partner of ≡ “Vector Manifestation” [Harada+Yamawaki ’01]
)Im(Im2AVs
dsf
3.3 Electromagnetic Probes at SPS: Anno ~2002Low-Mass Dileptons
MediumEffects!
10% QGP
BaryonDensity!
[RR+ Shuryak ’99]
Intermediate-Mass Dileptons
30%QGP
[Turbide,RR+Gale’04]
Direct Photons
>
>
B*,a1,K1...
N,,K…
Constraints:- B,M→N, - N,A,N→N- QCDSRs, lattice
3.4. Vector Mesons in Medium
D(M,q:B,T)=[M2-m2--B-M ]-1
(a) Hadronic Many-Body Theory
medmed DvD 2
]ff[vD MMMM,B
2
Propagator:
[Chanfray etal, Herrmann etal, RR etal, Koch etal, Weise etal, Post etal, Eletsky etal, Oset etal, …]
(b) Effective Field TheoryHLS with L≡(“VM”); vacuum: loop exp. OO(p/, m/, g)
In-Med.: T-dep. of bare m(0), g via matching to OPE, match<
+ RG-running to on-shell dropping -mass
[Harada, Yamawaki, Sasaki etal]
[RR+Gale ’99]
–Meson Spectral Functions
• -meson “melts” in hot and dense matter
• baryon densityB more important than temperature
B/0 0 0.1 0.7 2.6
Hot+Dense Matter Hot Meson Gas
[RR+Wambach ’99]
[Eletsky etal ’01]
Model Comparison
[RR+Wambach ’99]
Dilepton Emission Rates : dRee /dM2 ~ f B Imem
• HTL much enhanced over Born rate
• “matching” of HG and QGP automatic!
• Quark-Hadron Duality ?!
[Braaten,Pisarski +Yuan ’90]
[qq→ee][qq+O(s)-HTL]
--
3.5 Thermal Photons
Quark-Gluon Plasma
q
gq
But: other contributions in OO(αs)
collinear enhanced Dg=(t-mD2)-1~1/αs
[Aurenche etal ’00, Arnold,Moore+Yaffe ’01]
Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM)
“Naïve” LO: q + q (g) → g (q) + γ
[Kapusta,Lichard+Seibert ’91, … , Turbide,RR+Gale’04]
Hot and Dense Hadron Gas
γ
a1,
Im Πem(q0=q) ~ Im Dvec(q0=q)
Low energy: vector dominance
High energy: meson exchange
Emission Rates
In-med QGP ≈ total HG ! to be understood…
4.1 Direct Photon Spectra: WA98 at SPSHydrodynamics: QGP + HG
[Huovinen,Ruuskanen+Räsänen ’02]
• T0≈260MeV, QGP-dominated
• still true if pp→X included
[Turbide,RR+Gale’04]Expanding Fireball + pQCD
• pQCD+Cronin at qt >1.5GeV T0=205MeV suff., HG-dom.
4.1.2 WA98 “Low-qt Anomaly”
[Turbide,RR+Gale’04]
Expanding Fireball Model
• current HG rate much below• 30% longer FB 30% increase
Include→ S-wave
• slight improvement• in-medium “” or ?!
Lower pt-cut
• enhancement increases (well) above theory
4.2 Low-Mass Dileptons Again: New CERES Data
“Standard” pt-cut
• theory: 30%-central scaled by Nch ~ 375/250
• 40% longer lifetime insufficient
4.2.2 Attempts at the Low-pt Anomaly II:Source Parameterization
→ LO-pQCD (QGP) emission rates (“quark-hadron duality”), space-time volume free parameter [Gallmeister+Kämpfer ’05]
• 2 sources required (shape!): T1≈170MeV + T2≈120MeV
• shapes ok, but: very large VFB∙ FB (unrealistic …)
CERES/NA45 ’00 prelim
4.2.3 Attempts III:Disoriented Chiral Condensate
→ at Tc, condense in “misaligned” vacuum: ‹› ≠ 0 , annihilate thermal ’s on DCC [Kluger,Koch,Randrup+Wang ’98]
CERES/NA45acceptance with pt>60MeV
• DCC contribution ≈ Dalitz decay (much too small)
4.2.4 Attempts IV: QGP-like Emission→ employ QGP-HTL rate in hadronic phase (just. at high mass)
• enhancement above almost ok ↔ NA50• low mass?! Theoretically not justified …
4.3 Low-Mass Dileptons at SIS / BEVALAC
Transport Calculation [Shekhter,Fuchs etal ’03]
• Extended VDM pp→pp• “standard” calculation factor 2-3 below data• improvement due to decoherence in-medium • “optimal” values for in-med. coll. broadening
MeVcollcoll 200
consistent with [Bratkovskaya etal ’98]
[RR ’01]
• low mass: thermal dominant• int. mass: cc e+X , rescatt.? e-X
-
[Averbeck ‘01]
5.) Perspectives for RHIC I: Dileptons
• Medium effects sensitive to B,tot=B+Bbar !
RHIC II: Bound States in the sQGP
→ based on finite-T lattice potentials approach to “zero-binding line” ~ stable-mass-resonance
[Shuryak,Zahed, Brown, …]
• factor 2 enhancement over pQCD rate!?
Dilepton Radiationratio to pert. qq rate
Mee/mq
_
[Casalderrey+Shuryak’04]
6.) Conclusions
• Thermal E.M. Radiation in QCD: em(q0,q,B,T)
- low mass: importance of baryon effects - chiral restoration ↔ -a1 degeneracy ( a1
± → ± - thermal photons • extrapolations into phase transition region in-med HG and QGP shine equally bright lattice calculations? deeper reason?
• phenomenology for URHIC’s ok until 2002; ’03/’04: low-pt anomalies in central Pb-Pb, new medium effects??
• much excitement ahead: NA60, HADES, PHENIX, ALICE,…
… and theory!
Trento ECT* Workshop on
Electromagnetic Probes in Heavy-Ion Collisions
June 2-12, 2005
P. Braun-Munzinger, C. Gale+ R. Rapp (coordinator)
Additional Slides
(ii) Vector Mesons at RHIC
baryon effects important even at B,net=0 :sensitive to B,tot=+B , more robust ↔ OZI -
e+e- Emission Rates: dRee/dM ~ f B Imem
Quark-Hadron Duality ?!
in-med HG ≈ in-med QGP !
[qq→ee][qq+O(s)]
--
4.3 Lattice Studies of Medium Effects
)2/sinh(
))2/1(cosh(),(Im),(
0
00
00 Tq
TqTqdqT
calculatedon lattice
more stable than below Tc?! (but: quenched)
MEM
1-
0-
extracted
[Laermann, Karsch ’04]
4.3.2 Comparison of Hadronic Models to LGT
)2/sinh(
))2/1(cosh(),(Im),(
0
00
00 Tq
TqTqdqT
calculate
integrate
More direct!
Proof of principle, not yet meaningful (need unquenched)
2.3.3 Baryonic Contributions
• use in-medium–spectral funct:
• constrained by nucl. -absorption:
)qq(DImg
mIm med 02
4
em
>
>
B*,a1,K1...
N,,K…
)qq(ImqA
)q(
N
absA 0
0
0 4em
N → N,
N →
NA
-ex
[Urban,Buballa,RR+Wambach ’98]
2.3.4 HG Emission Rates: Summary
B=220MeV
[Turbide,RR+Gale ’04]
• t-channel (very) important at high energy
• formfactor suppression (2-4)
• strangeness significant
• baryons at low energy
5.1 Towards a Chiral + Resonance SchemeOptions for resonance implementation:(i) generate dynamically from pion cloud [Lutz et al ‘03, …]
(ii) genuine resonances on quark level → representations of chiral group [DeTar+Kunihiro ‘89, Jido etal ’00 ,…]
e.g.
N+
N(1535)-
a1 N(1520)-
N(1900)+ (1700)-
(?) (1920)+
S
P
S
S SS
P SS (a1)S
Importance of baryon spectroscopyto identify relevant decay modes!
2
3S
2
1S
5.2 Current Status of a1(1260)
>
> >
>
N(1520)…
,N(1900)…
a1 + + . . .
Exp: - HADES (A): a1→(+-) - URHICs (AA): a1→
)DImg
mDIm
g
m(
s
dsf a
a
a1
1
1
2
4
2
42
7.2 Perspectives on Photon Data at RHIC
• large “pre-equilibrium” yield from parton cascade (no LPM)• thermal yields ~ consistent• QGP undersat. small effect
Predictions for Central Au-Au PHENIX Data
• consistent with pQCD only• disfavors parton cascade• not sensitive to thermal yet
2.2.4 In-Medium Baryons: (1232)
long history in nuclear physics ! ( A , A )
e.g. nuclear photoabsorption: M, up by 20MeV
little attention at finite temperature
-Propagator at finite B and T [van Hees + RR ’04]
in-medium vertex corrections incl. g’-cloud, (“induced interaction”)(1+ f - f N) thermal -gas
→N(1440), N(1520), (1600)
+ + + + ...
>
>>
> >>
>> NN-1 N-1
Rho Spectral Function at Future GSI
• high-density effects most prominent at low mass
2.1 Light Hadrons: Vacuum
Tiqx jxjexdiq )0()()( 4
Correlation Function:Timelike (q2>0) : Im q0,q) → physical excitations
)Im(Im2AVs
dsf
)(Im)()(Im 22
sDg
ms
=1± (qq)
Chiral breaking: Q2 < (1.5-2 GeV)2 , J± < 5/2 (?!)
(qqq)
