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Dileptons and Medium Effects in Heavy-Ion Collisions

Dileptons and Medium Effects in Heavy-Ion Collisions. Ralf Rapp Cyclotron Institute + Physics Department Texas A&M University College Station, USA Perspectives in Hadronic Physics Conference 2006 ICTP Trieste, 24.05.06. Spectral Functions. Chiral Condensate. c m. -.

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Dileptons and Medium Effects in Heavy-Ion Collisions

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  1. Dileptons and Medium Effects in Heavy-Ion Collisions Ralf Rapp Cyclotron Institute + Physics Department Texas A&M University College Station, USA Perspectives in Hadronic Physics Conference 2006 ICTP Trieste, 24.05.06

  2. Spectral Functions Chiral Condensate cm - [Bielefeld] T=1.4Tc ‹qq› [Tokyo] 1.0 T/Tc 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

  3. 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

  4. e+ e- q q _ e+ e- γ • Radiation Sources:Relevance: • Quark-Gluon Plasma: high mass + temp. • qq → e+e-, …M > 1.5GeV, T >Tc • Hot + Dense Hadron Gas: M ≤ 1 GeV • p +p - → e+e-, … T ≤ Tc - e+ e- p- p+ r 2.) EM Emission Rates and Chiral Symmetry E.M. Correlation Function: Im Πem(M,q;mB,T) Im Πem(q0=q; mB,T)

  5. at Tc: Chiral Restoration • Low-Mass Dilepton Rate: r -meson dominated! ImPem ~ [ImDr+ImDw /10+ImDf /5] • Axialvector Channel:p±ginvariant mass-spectra~ Im Da1(M) ?! 2.2 Chiral Symmetry Breaking and Restoration Splitting of “chiral partners”r - a1(1260) Chiral Symmetry Breaking Axial-/Vector in Vacuum pQCD cont.

  6. explicit link: • V - A spectral fcts. (models) ↔ order parameters (lattice QCD) • extended to finite temperature [Kapusta+ Shuryak ‘93] 2.3 Chiral Sum Rules and the a1(1260) • Energy-weighted moments of difference vector – axialvector: [Das etal ’67] [Weinberg ’67]

  7. 3.1 Medium Effects I: Hadronic Many-Body Theory > > Constraints: - vacuum decays: B,M→ rN, rp - scattering data: gN ,gA , pN→rN Nucleon Nuclei rN=0.8r0 rN → 0 gN gA p-ex [Urban et al. ’98] [Chanfray etal, Herrmann etal, RR etal, Weise etal, Post etal, Eletsky etal, Oset etal, …] Dr (M,q;mB ,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

  8. 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] rN=0.5r0 rN=r0 rN=r0 p N →r NPWA Constraints: g N ,g A • good agreement: strong broadening + small mass-shift up • constraints from (vacuum) data important quantitatively

  9. 0.2% 1% [Leupold ’98, Ruppert etal ’05] 4-quark condensate! 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 (s>0):

  10. rB /r0 0 0.1 0.7 2.6 Model Comparison [Eletsky etal ’01] [RR+Wambach ’99] 3.1.4r-Meson Spectral Functions at SPS Hot+Dense Matter Hot Meson Gas [RR+Wambach ’99] [RR+Gale ’99] • r-meson “melts” in hot and dense matter • baryon densityrB more important than temperature • reasonable agreement between models

  11. - - [qq→ee] [qq + HTL] 3.2 Dilepton Emission Rate: Hadron Gas vs. QGP [Braaten,Pisarski+Yuan ’90] • Hard-Thermal-Loop QGP rate • enhanced over Born rate • “matching” of HG and QGP • in vicinity of Tc • “Quark-Hadron Duality” ?!

  12. 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 )= (z0+vzt ) p (R┴+ 0.5a┴t 2)2

  13. 4.1 Pb-Au Collisions at SPS: CERES/NA45 • QGP contribution small • medium effects on r-meson! • dropping mass or broadening?!

  14. quantitative theory? 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!

  15. 4.2.2 In-In at SPS: Theory vs. NA60 • predictions based r-spectral function of [RR+Wambach ’99] • uncertainty in fireball lifetime (±25% norm.); or: infer tFB≈7fm/c ! • relative strength of thermal sources fix • good agreement with r melting, including pt dependence [van Hees +RR ‘06]

  16. [van Hees+RR ‘06] 4.2.3 Intermediate-Mass Region • “4p“ states dominate in the vacuum • e.m. correlator above M ≈ 1.1GeV • lower estimate: • use vacuum4p correlator • upper estimate: • O(T2) medium effect → • “chiral V-A mixing”: • with 4p 2p [Eletsky+Ioffe ‘90]

  17. dropping mass as used for CERES • disfavored (free r decays?) 4.2.4 NA60 Data: Other r-Spectral Functions • switch off medium modifications • T-, rB- dependence of bare parameters: dropping mass [Brown+Rho ’91, Hatsuda+Lee ‘92] • free spectral function ruled out • meson gas insufficient either

  18. 4.2.5 (Some) Open Issues • Heavy-Ion Collisions[NA60] • - centrality dependence, free r’s (surface vs. volume) • - sensitivity to fireball evolution • - quantitative w and f • - thermal radiation at intermediate mass (M=1.5-3 GeV) • - chiral restoration: ▪ “duality” (hadron liquid → sQGP) • ▪ chiral sum rules • ▪ chiral mixing in the M=1-1.5GeV region • Cold Nuclei [CB/TAPS, KEK-E325] • - dropping w-mass + broadening • - dropping r-mass without broadening ?!

  19. 5.) Conclusions • Strong medium effects in l+l- 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 …

  20. 3.3 Medium Effects II: Dropping Mass [Brown+Rho ’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]

  21. 3.) Medium Effects and Thermal Dileptons 3.1 Lattice QCD (QGP) Dilepton Rate ~ ImP(w,q=0)/w2 EM Correlator ImP(w,q)/w2 T=1.5Tc [Bielefeld Group ’02, ‘05] • lQCD << pQCD at low mass (finite volume?) • currently no thermal photons from lQCD • vanishing electric conductivity!? but: [Gavai ’04]

  22. p g ~(a-2) ~a p e.m. spectral function? matching HG-QGP: massless mesons? 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), Lmatch<Lc , • 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]

  23. 4.2 Recent Advances at SPS: Power of Precision drop. mass (norm.) drop. mass (norm.) drop. mass (norm.) NA60Data vs. Model Predictions [RR+Wambach ’99; RR’03] • r-meson “melting” supported (baryons!) • dropping mass (as used to explain CERES data) ruled out • open issues: • (1) M > 0.9GeV (4p→m+m-!?) • (2) normalization: 0.6 (pt <0.5GeV), 0.8 (all pt ),~2 (pt >1GeV) • (3) other models (vector manifestation, chiral virial approach, …)

  24. [van Hees+ RR in prep.] [courtesy S.Damjanovic] • reasonable agreement with absolute normalization, but … • too little yield at high pt; “free r”? w? check central … 4.2.2 Modified Fireball and Absolute Normalization • r-spectral function unchanged since [RR+Wambach ’99] • expanding fireball, fixed S (↔Nch): VFB(t)=(z0+vzt ) p (R┴0+ 0.5a┴t 2)2 • Increasea┴ reduced lifetime (t =9→6fm/c), increased v┴=0.4→0.5c

  25. Revival Attempts for Dropping r-Mass E.g., [Skokov+Toneev ‘05] Fireball Evolution e>1GeVfm-3 ≈ ec for Dt=8 fm/c?! Bjorken regime: tFB=0.5fm/c?! • Not compatible with gauge • invariance (no mr* in VDM) • acceptance?

  26. M ≥ 1GeV in NA60 [H. van Hess + RR, in prep.] • combination of 4-p + QGP + charm?! • (beware: schematic acceptance)

  27. 4.2.5 Chiral Virial Approach vs. NA60 (central) [Steele,Yamagishi +Zahed ’99] [implementation van Hees+RR ’05]

  28. WA98 “Low-qt Anomaly” • addt’l meson-Bremsstrahlung • pp→ ppgpK→pKg • substantial at low qt [Liu+ RR’05] 5.) Electromagnetic Probes 5.1.1 Thermal Photons I : SPS Expanding Fireball + pQCD • pQCD+Cronin at qt >1.6GeV •  T0=205MeV suff., HG dom. [Turbide,RR+Gale’04]

  29. 5.1.2 Thermal Photons II: RHIC • thermal radiation qt<3GeV ?! • QGP window 1.5<qt<3GeV ?! • also: g-radiation off jets • shrinks QGP window qt<2GeV ?! [Gale,Fries,Turbide,Srivastava ’04]

  30. Dilepton Emission Rates - - [qq→ee] [qq+O(as)] in-med HG ≈ in-med QGP ! Quark-Hadron Duality ?! 5.3.1 RHIC: Vector Mesons in Medium Hadronic Many-Body Theory • baryon effects important even at rB,net=0: • sensitive to rB,tot=rB+rB , most pronounced at low M • f more robust ↔ OZI -

  31. 5.3.2 Dileptons II: RHIC [R. Averbeck, PHENIX] [RR ’01] QGP • low mass: thermal! (mostly in-medium r) • connection to Chiral Restoration: a1 (1260)→ pg ,3p • int. mass:QGP (resonances?)vs.cc → e+e-X (softening?) -

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