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Electromagnetic Probes of Medium Effects in Heavy-Ion Collisions. Ralf Rapp Cyclotron Institute + Physics Department Texas A&M University College Station, USA International on Workshop “Probing QCD with Heavy Ions” Hirschegg, 21.01.05. Outline. 1. Introduction
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Electromagnetic Probes of Medium Effects in Heavy-Ion Collisions Ralf Rapp Cyclotron Institute + Physics Department Texas A&M University College Station, USA International on Workshop “Probing QCD with Heavy Ions” Hirschegg, 21.01.05
Outline 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
low-massee in-medr,w,f →ee Chiral Symmetry restoration? int-massee continuum emission p a1→ ee , qq→ee QGP radiation? qq low-energy g hadron decays/scatt. a1→pg, pr→pg Medium effects? high-energy g continuum emission HG vs. QGP , O(as) QGP radiation? 1.) Four Pillars of Thermal E.M. Radiation Thermal rate: q0≈0.5GeV Tmax≈0.17GeV , q0≈1.5GeV Tmax=0.5GeV
qR qL • Profound Consequences: • energy gap: • ↔ mass generation! • massless Goldstone bosonsp0,± • “chiral partners” split,DM≈0.5GeV: > > > > - - qR qL JP=0±1± 1/2± 2.) Chiral Symmetry in QCD:Vacuum SU(2)L × SU(2)R invariant (mu,d ≈ 0) - Spontaneous Breaking:strongqqattraction Bose Condensate fillsQCD vacuum! [cf. Superconductor: ‹ee›≠0 Magnet ‹M›≠0 , … ]
= O(1) = O(αs ) e+ e- γ 3.) Electromagnetic Emission Rates E.M. Correlation Function: Im Πem(M,q) Im Πem(q0=q) also: e.m susceptibility (charge fluct.):χ = Πem(q0=0,q→0) • In URHICs: • source strength:dependence onT, mB, mp , medium effects, … • system evolution:V(t), T(t), mB(t), transverse expansion, … • nonthermal sources: Drell-Yan, open-charm, hadron decays, … • consistency!
rI =1 r+w+f _ qq 3.1 E.M. Correlator in Vacuum: s(e+e-→hadrons) h1 h2 e+ e- q q … _ s ≥ (1.5GeV)2 : pQCD continuum s < (1.5GeV)2 : V-meson spectral functs.
At Tc: Chiral Restoration or:rlongchiral partner ofp≡ “Vector Manifestation” [Harada+ Yamawaki ’01] 3.2 Low-Mass Dileptons + Chiral Symmetry Im Πem(M)dominated by r-meson → chiral partner: a1(1260) Vacuum pQCD cont.
Intermediate-Mass Dileptons 30% QGP [RR+ Shuryak ’99] Direct Photons [Turbide,RR+Gale’04] 3.3 Electromagnetic Probes at SPS: Anno ~2002 Low-Mass Dileptons Medium Effects! 10% QGP Baryon Density!
3.4. Vector Mesons in Medium r Sp > Sp > (b) Effective Field Theory HLS with rL≡p (“VM”); vacuum: loop exp.O(p/Lc , mr /Lc , g) In-Med.:T-dep. of bare mr(0), gr via matching to OPE, Lmatch<Lc + RG-running to on-shell dropping r-mass [Harada, Yamawaki, Sasaki etal] [Chanfray etal, Herrmann etal, RR etal, Koch etal, Weise etal, Post etal, Eletsky etal, Oset etal, …] (a) Hadronic Many-Body Theory Dr(M,q:mB,T)=[M2-mr2-Srpp-SrB-SrM ]-1 Propagator: Constraints: -B,M→rN,rp -gN,gA,pN→rN - QCDSRs, lattice B*,a1,K1... N,p,K…
rB/r0 0 0.1 0.7 2.6 Model Comparison [Eletsky etal ’01] [RR+Wambach ’99] r –Meson Spectral Functions Hot+Dense Matter Hot Meson Gas [RR+Gale ’99] [RR+Wambach ’99] • r-meson “melts” in hot and dense matter • baryon density rB more important than temperature
- - [qq→ee] [qq+O(as)-HTL] Dilepton Emission Rates :dRee /dM2 ~ fB ImPem [Braaten,Pisarski +Yuan ’90] • HTL much • enhanced over • Born rate • “matching” of • HG and QGP • automatic! • Quark-Hadron • Duality ?!
r Sp Hot and Dense Hadron Gas Emission Rates Low energy: vector dominance Sp q Im Πem(q0=q) ~ Im Dvec(q0=q) g q p γ p,a1,w High energy: meson exchange r p In-med QGP ≈ total HG ! to be understood… [Kapusta,Lichard+Seibert ’91, … , Turbide,RR+Gale’04] 3.5 Thermal Photons Quark-Gluon Plasma “Naïve” LO: q + q (g) → g (q) +γ But:other contributions inO(αs) collinear enhanced Dg=(t-mD2)-1~1/αs Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM) [Aurenche etal ’00, Arnold,Moore+Yaffe ’01]
Expanding Fireball + pQCD [Turbide,RR+Gale’04] • pQCD+Cronin at qt >1.5GeV • T0=205MeV suff., HG-dom. 4.1 Direct Photon Spectra: WA98 at SPS Hydrodynamics: QGP + HG [Huovinen,Ruuskanen+Räsänen ’02] • T0≈260MeV, QGP-dominated • still true if pp→gX included
4.1.2 WA98 “Low-qt Anomaly” Expanding Fireball Model Includepp→ppgS-wave [Turbide,RR+Gale’04] • current HG rate much below • 30% longer tFB 30% increase • slight improvement • in-medium “s” or D ?!
Lower pt-cut • enhancement increases (well) • above theory • 40% longer lifetime insufficient 4.2 Low-Mass Dileptons Again: New CERES Data “Standard” pt-cut • theory: 30%-central scaled • by Nch ~ 375/250
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] CERES/NA45 ’00 prelim • 2 sources required (shape!): T1≈170MeV + T2≈120MeV • shapes ok, but: very largeVFB∙ tFB(unrealistic …)
4.2.3 Attempts III:Disoriented Chiral Condensate → at Tc, condense in “misaligned” vacuum: ‹p› ≠ 0 , annihilate thermal p’s on DCC[Kluger,Koch,Randrup+Wang ’98] CERES/NA45 acceptance with pt>60MeV • DCC contribution ≈h 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 f almost ok ↔ NA50 • low mass?! Theoretically not justified …
consistent with [Bratkovskaya etal ’98] 4.3 Low-Mass Dileptons at SIS / BEVALAC • Transport Calculation • [Shekhter,Fuchs etal ’03] • Extended VDM pp→ppw • “standard” calculation • factor 2-3 below data • improvement due to • decoherence in-medium • “optimal” values for • in-med. coll. broadening
low mass: thermal dominant • int. mass:cc e+X , rescatt.? • e-X - 5.) Perspectives for RHIC I: Dileptons [Averbeck ‘01] [RR ’01] • Medium effects sensitive • to rB,tot=rB+rBbar !
Dilepton Radiation ratio to pert. qq rate _ [Casalderrey+Shuryak’04] Mee/mq RHIC II: Bound States in the sQGP → based on finite-T lattice potentials approach to “zero-binding line” ~ stable-massr-resonance [Shuryak,Zahed, Brown, …] • factor 2 enhancement • over pQCD rate!?
6.) Conclusions • Thermal E.M. Radiation in QCD: Pem(q0,q,mB,T) • - low mass: r,w,f,importance of baryon effects • - chiral restoration ↔r-a1degeneracy ( a1± → p± g ?!) • - thermal photons • extrapolations into phase transition region • in-med HG and QGP shine equallybright • 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)
e+e- Emission Rates: dRee/dM ~ f B ImPem - - [qq→ee] [qq+O(as)] baryon effects important even at rB,net=0: sensitive to rB,tot=rB+rB , f more robust ↔ OZI in-med HG ≈ in-med QGP ! - Quark-Hadron Duality ?! (ii) Vector Mesons at RHIC
1- MEM 0- extracted [Laermann, Karsch ’04] 4.3 Lattice Studies of Medium Effects calculated on lattice p more stable than r below Tc?! (but: quenched)
calculate integrate More direct! Proof of principle, not yet meaningful (need unquenched) 4.3.2 Comparison of Hadronic Models to LGT
r Sp > Sp > g N → p N,D gN gA g N → B* p-ex [Urban,Buballa,RR+Wambach ’98] 2.3.3 Baryonic Contributions • use in-medium r –spectral funct: • constrained by nucl. g-absorption: B*,a1,K1... N,p,K…
2.3.4 HG Emission Rates: Summary • wt-channel (very) important • at high energy • formfactor suppression (2-4) • strangeness significant • baryons at low energy mB=220MeV [Turbide,RR+Gale ’04]
pS pS pS pS pS pP pP 5.1 Towards a Chiral + Resonance Scheme Options 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. p s N+ N(1535)- r a1D+ N(1520)- N(1900)+ D(1700)-(?) D(1920)+ rS (a1)S rS Importance of baryon spectroscopy to identify relevant decay modes!
> D,N(1900)… Sp a1 Sp + + . . . > N(1520)… Sr > > Exp: - HADES(pA): a1→(p+p-)p - URHICs (AA): a1→pg 5.2 Current Status of a1(1260)
7.2 Perspectives on Photon Data at RHIC Predictions for Central Au-Au PHENIXData • large “pre-equilibrium” yield • from parton cascade (no LPM) • thermal yields ~ consistent • QGP undersat. small effect • consistent with pQCD only • disfavors parton cascade • not sensitive to thermal yet
NN-1DN-1 Sp D + + + + ... > > > > > pD→N(1440), N(1520), D(1600) > in-medium vertex corrections incl. g’ p-cloud, (“induced interaction”) (1+ f p - f N) thermal p-gas > > 2.2.4 In-Medium Baryons: D(1232) long history in nuclear physics ! (pA , gA ) e.g. nuclear photoabsorption:MD, GDup by 20MeV little attention at finite temperature D-Propagator at finite rB and T[van Hees + RR ’04]
Rho Spectral Function at Future GSI • high-density effects most prominent at low mass
a=1±(qq) (qqq) Chiral breaking: Q2 < (1.5-2 GeV)2 , J± < 5/2 (?!) 2.1 Light Hadrons: Vacuum Correlation Function: Timelike (q2>0) : ImPa(q0,q) → physical excitations