dileptons at rhic ralf rapp cyclotron inst. + physics dept. texas a&m university college...
TRANSCRIPT
Dileptons at RHIC
Ralf RappCyclotron Inst. + Physics Dept.
Texas A&M UniversityCollege Station, USA
International CCAST Workshop“QCD and RHIC Physics”
Beijing, 10.08.04
… and the Quest for Chiral Symmetry Restoration
1. Introduction
2. Chiral Symmetry in QCD 3. E.M. Correlation Function + Thermal Radiation
4. Low-Mass Dileptons 4.1 Axial-/Vector Correlators
4.2 Medium Effects and Excitation Function
4.3 Lattice QCD
5. Intermediate-Mass Dileptons: QGP Radiation?
6. Perspectives for RHIC
7. Conclusions
Outline
1.) Introduction: Towards QGP Discovery
• So far: RHIC observables ↔ bulk properties of the produced matter:
- energy density ≈20GeVfm-3 ↔ jet quenching (high-pt) - thermalization + EoS ↔ hydrodynamics (v0,v2)
- partonic degrees of freedom ↔ coalescence (p/, v2-scal)
• Future: need to understand microscopic properties (phase transition, “QGP” !?):
- Deconfinement ↔ quarkonia (J/, Y, …) - Chiral Symmetry Restoration ↔ dileptons ( - temperature ↔ photons )
2.) Chiral Symmetry in QCD: Vacuum
2
4
1aq Gq)m̂Aigi(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±
2.2 “Melting” the Chiral Condensate
How?
Excite vacuum (hot+dense matter)
• quarks “percolate” / liberated Deconfinement • ‹qq› condensate “melts”, iral Symm. chiral partners degenerate Restoration(-, -a1, … medium effects → precursor!)
0 0.05 0.3 0.75 [GeVfm-3] 120, 0.50 150-160, 20 175, 50 T[MeV], had
PT many-body degrees of freedom? QGP (2 ↔ 2) (3-body,...) (resonances?) consistent extrapolate pQCD
-
1.0 T/Tc
m‹qq›-lattice QCD
2.3 Dilepton Data at CERN-SPS
Low Mass: CERES/NA45 Intermediate Mass: NA50
Central Pb-Pb 158 AGeV
opencharm
Drell- Yan
Mee [GeV] M [GeV]
• strong excess around M≈0.5GeV• little excess in region
• factor ~2 excess• open charm? thermal? …
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!
3.2 Two Regimes of Thermal Dilepton 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:
4.) Low-Mass Dileptons + Chiral Symmetry
Im Πem(M) ~ Im D(M) vector-meson spectral functions
dominated by -meson → chiral partner: a1(1260)
)Im(Im2AVs
dsf
Chiral breaking: Q2 < 3GeV2
Vacuum At Tc: Chiral Restoration
pQCD cont.
+>
>
B*,a1,K1...
N,,K…
Constraints:- branching ratios B,M→N, - N,Aabsorpt.,N→N- QCD sum rules, lattice
4.2 Vector Mesons in Medium: Many-Body Theory
• -meson “melts” in hot and dense matter
• baryon densityB more important than temperature
(i) SPS ConditionsB/0 0 0.1 0.7 2.6
D(M,q:B,T)=[M2-m2--B-M]-1
(ii) Vector Mesons at RHIC
baryon effects important even at Bnet=0 :sensitive to Btot=+B , more robust ↔ OZI -
Dilepton Emission Rates
Quark-Hadron Duality ?!
in-med HG ≈ in-med QGP !
[qq→ee][qq+O(s)]
--
Lower SPS Energy
• enhancement increases!• precision test by NA60!?
4.3 Low-Mass Dileptons in URHICs
Top SPS Energy
• baryon effects important!
BEVALAC/SIS Energy
DLS
• enhancement increases still: DLS puzzle → HADES!?
4.4 Current Status of a1(1260)
>
> >
>
N(1520)…
,N(1900)…
a1 + + . . .
Exp: - HADES (A): a1→(+-) - URHICs (A-A) : a1→
)DImg
mDIm
g
m(
s
dsf a
a
a1
1
1
2
4
2
42
4.5 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)
Ti≈300MeV, QGP-dominated
Hydrodynamics (chem-eq)
[Kvasnikowa,Gale+Srivastava ’02]
5.) Intermediate-Mass Dileptons: NA50 (SPS)e.m. corr. continuum-like: Im Πem~ M2 (1+s/+…)
Ti≈210MeV , HG-dominated
Thermal Fireball (chem-off-eq)
[RR+Shuryak ’99]
QGP + HG!
• low mass: thermal dominant• int. mass: cc e+X , rescatt.? e-X
[RR ’01]
-
[R. Averbeck, PHENIX]
6.) Dilepton Spectrum at RHIC
MinBias Au-Au (200AGeV)
run-4 results eagerly awaited …
thermal
8.) Conclusions
• Thermal Dileptons in QCD: em(q0,q,B,T)
- low mass: , chiral restoration ↔ -a1 degeneracy
- intermediate mass: QGP radiation (open charm?!) ( - thermal photons )
• extrapolations into phase transition region in-med HG and QGP shine equally bright lattice calculations? deeper reason?
• phenomenology for URHIC’s promising; precision data+theory needed for definite conclusions
• much excitement ahead: PHENIX, NA60, HADES, ALICE,… and theory!
7.) 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
Total HG ≈ in-med QGP ! to be understood…
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
4.2 Comparison to Data I: WA98 at SPS
Hydrodynamics: 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.
4.2 Comp. to Data II: 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 ?!
2. Thermal Photon Radiation2.1 Generalities
),( 0230 Tqfqd
dRq B
Emission Rate per 4-volume and 3-momentum
γ
Im Πem(q0=q)T
transverse photon selfenergymany-bodylanguage:
kinetictheory:
γ
2
)](1[)()(
(...))2(8
321
)4(9
3,2,1
3,2,13
30
EfEfEf
E
pdN
qd
dRq
|M||M|22
in-medium effects,resummations, …
cut
γ
γ
a1
a1
• Photon-producing reactions:
mostly at dominant (q0>0.5GeV) gauge invariance! q0<0.5GeV a1-strength problematic
[Song ’93, Halasz etal ’98,…]
2.3.1 Hot Hadronic Matter: --a1 Gas
),(][][2
1)]2()[(
422
022
2
'LL ATrmFTrUUMUDTr
f
Chiral Lagrangian + Axial/Vector-mesons, e.g. HLS or MYM:
• (g0,m0,,) fit to ma1 , ,a1
D/S and a1→γ) not optimal HLSMYMKap.’91 (no a1)
• quantitative analysis: account for finite hadron size• improves a1 phenomenology
• t-channel exchange: gauge invariance nontrivial [Kapusta etal ’91]
simplified approach: [Turbide,Gale+RR ’04]
2.3.1.b Hadronic Formfactors
2
2
2
2
2
tΛ
ΛF(t) with
,...a,xmqt x 102
Factor 3-4 suppressionAt intermediate andHigh photon energies
2.3.2 Further Meson Gas Sources
(i) Strangeness Contributions: SU(3)F MYM
(iii) Higher Resonances
Ax-Vec: a1,h1→, Vec: ,’,’’→ other: (1300)→ f1→ , K1→K K*→K a2(1320)→
γ
KK
γ
K* K~25% of
→~40% of→
(ii) t-Channel γ
G large!
potentially important …[Turbide,Gale +RR ’04]
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]
2.3.3(b) Photon Rates from Spectral Function:
Baryons + Meson-Resonances
• baryonic contributions dominant for q0<1GeV (CERES enhancement!)
• also true at RHIC+LHC: at T=180MeV, B=0
B=220MeV031 .BB
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
2.3.5 In-Medium Effects
• many-body approach: encoded in vector-spectral function, relevant below M , q0 ~ 1-1.5 GeV
• “dropping masses”: large enhancement due to increased phase space [Song+Fai ’98, Alam etal ’03]
unless: vector coupling decreases towards Tc (HLS, a→1) [Harada+Yamawaki ’01, Halasz etal ’98]
3.2 Thermal Evolution: QGP→ Mix→ HG
QGP: initial conditions [SPS]
• 0=1fm/c → 0=0.5fm/c: ~2-3• s=CdQGT3; dQG=40 → 32: ~2• pre-equilibrium?!
HG: chemistry [LHC]
T
[GeV
]• conserved BB use entropy• build-up of >0 (N=const)• accelerated cooling
HG: chemistry and trans. flow
• R~exp(3) for → , …• yield up at low qt , down above • large blue shift from coll. flow
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
4.3 Perspectives on Data III: 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 initial only• disfavors parton cascade• not sensitive to thermal yet