1 / 36

Dileptons at RHIC

Dileptons at RHIC. … and the Quest for Chiral Symmetry Restoration. Ralf Rapp Cyclotron Inst. + Physics Dept. Texas A&M University College Station, USA International CCAST Workshop “QCD and RHIC Physics” Beijing, 10.08.04. Outline. 1. Introduction 2. Chiral Symmetry in QCD

ron
Download Presentation

Dileptons at RHIC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dileptons at RHIC … and the Quest for Chiral Symmetry Restoration Ralf Rapp Cyclotron Inst. + Physics Dept. Texas A&M University College Station, USA International CCAST Workshop “QCD and RHIC Physics” Beijing, 10.08.04

  2. Outline 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

  3. 1.) Introduction:Towards QGP Discovery • So far: RHIC observables • ↔bulk properties of the produced matter: • - energy densitye≈20GeVfm-3↔ jet quenching(high-pt) • - thermalization + EoS↔ hydrodynamics (v0,v2) • - partonic degrees of freedom↔ coalescence (p/p,v2-scal) • Future: need to understand • microscopic properties (phasetransition, “QGP” !?): • - Deconfinement ↔quarkonia (J/y,Y, …) • - Chiral Symmetry Restoration↔ dileptons • ( - temperature ↔ photons )

  4. 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 , … ]

  5. lattice QCD - cm ‹qq› 1.0 T/Tc cPTmany-bodydegrees of freedom?QGP (2 ↔ 2)(3-body,...) (resonances?) consistentextrapolatepQCD 0 0.05 0.3 0.75 e[GeVfm-3] 120, 0.5r0 150-160, 2r0 175, 5r0 T[MeV], rhad 2.2 “Melting” the Chiral Condensate • Excite vacuum (hot+dense matter) • quarks “percolate” / liberated •  Deconfinement • ‹qq›condensate “melts”, ciral Symm. • chiral partners degenerate Restoration • (p-s, r-a1, … medium effects → precursor!) - How?

  6. Central Pb-Pb 158 AGeV open charm Drell- Yan 2.3 Dilepton Data at CERN-SPS Low Mass:CERES/NA45Intermediate Mass:NA50 Mee [GeV] Mmm [GeV] • factor ~2 excess • open charm? thermal? … • strong excess around M≈0.5GeV • little excess in r,w,f region

  7. = 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!

  8. qq 3.2 Two Regimes of Thermal Dilepton Radiation Thermal rate: q0≈0.5GeV  Tmax≈0.17GeV , q0≈1.5GeV  Tmax=0.5GeV

  9. At Tc: Chiral Restoration 4.) Low-Mass Dileptons + Chiral Symmetry Im Πem(M) ~ Im Dr+w+f (M) vector-meson spectral functions dominated by r-meson → chiral partner: a1(1260) Vacuum pQCD cont. Chiral breaking:Q2 < 3GeV2

  10. 4.2 Vector Mesons in Medium: Many-Body Theory r Sp > Sp > rB/r0 0 0.1 0.7 2.6 (i) SPS Conditions • r-meson “melts” • in hot and dense matter • baryon density rB more • important than temperature Constraints: - branching ratiosB,M→rN,rp -gN,gAabsorpt.,pN→rN - QCD sum rules, lattice B*,a1,K1... + N,p,K… Dr(M,q:mB,T)=[M2-mr2-Srpp-SrB-SrM]-1

  11. Dilepton Emission Rates - - [qq→ee] [qq+O(as)] baryon effects important even at rBnet=0: sensitive to rBtot=rB+rB , f more robust ↔ OZI in-med HG ≈ in-med QGP ! - Quark-Hadron Duality ?! (ii) Vector Mesons at RHIC

  12. Lower SPS Energy BEVALAC/SIS Energy DLS • enhancement increases! • enhancement increases still: • DLS puzzle → HADES!? • precision test by NA60!? 4.3 Low-Mass Dileptons in URHICs Top SPS Energy • baryon effects important!

  13. > D,N(1900)… Sp a1 Sp + + . . . > N(1520)… Sr > > Exp: - HADES(pA): a1→(p+p-)p - URHICs (A-A) : a1→pg 4.4 Current Status of a1(1260)

  14. calculate integrate More direct! Proof of principle, not yet meaningful (need unquenched) 4.5 Comparison of Hadronic Models to LGT

  15. Hydrodynamics (chem-eq) Thermal Fireball (chem-off-eq) Ti≈300MeV, QGP-dominated Ti≈210MeV, HG-dominated [RR+Shuryak ’99] [Kvasnikowa,Gale+Srivastava ’02] 5.) Intermediate-Mass Dileptons: NA50 (SPS) e.m. corr. continuum-like: Im Πem~ M2 (1+as/p+…) QGP + HG!

  16. MinBias Au-Au (200AGeV) [R. Averbeck, PHENIX] [RR ’01] thermal run-4 results eagerly awaited … • low mass: thermal dominant • int. mass:cc e+X , rescatt.? • e-X - 6.) Dilepton Spectrum at RHIC

  17. 8.) Conclusions • Thermal Dileptons in QCD: Pem(q0,q,mB,T) • - low mass: r,w,f,chiral restoration ↔r-a1degeneracy • - intermediate mass: QGP radiation (open charm?!) • (- thermal photons ) • extrapolations into phase transition region •  in-med HG and QGP shine equallybright • 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!

  18. Additional Slides

  19. r Sp Emission Rates Hot and Dense Hadron Gas 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 Total HG ≈ in-med QGP ! to be understood… [Kapusta,Lichard+Seibert ’91, … , Turbide,RR+Gale’04] 7.) 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]

  20. 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

  21. Expanding Fireball + pQCD [Turbide,RR+Gale’04] • pQCD+Cronin at qt >1.5GeV •  T0=205MeV suff., HG dom. 4.2 Comparison to Data I: WA98 at SPS Hydrodynamics: QGP + HG [Huovinen,Ruuskanen+Räsänen ’02] • T0≈260MeV, QGP-dominated • still true if pp→gX included

  22. Includepp→ppgS-wave • slight improvement • in-medium “s” or D ?! 4.2 Comp. to Data II: WA98 “Low-qt Anomaly” Expanding Fireball Model [Turbide,RR+Gale’04] • current HG rate much below • 30% longer tFB 30% increase

  23. T Im Πem(q0=q) p γ r cut 2 p γ kinetic theory: r p |M|2 2. Thermal Photon Radiation 2.1 Generalities Emission Rate per 4-volume and 3-momentum transverse photon selfenergy many-body language: in-medium effects, resummations, …

  24. HLS MYM Kap.’91 (no a1) p p γ γ p,a1 r p r p p,a1 2.3.1 Hot Hadronic Matter: p-r-a1 Gas Chiral Lagrangian + Axial/Vector-mesons, e.g. HLS or MYM: • (g0,m0,s,x)fit tomr,a1 ,Gr,a1 • D/SandG(a1→pγ)not optimal [Song ’93, Halasz etal ’98,…] • Photon-producing reactions: mostly at dominant (q0>0.5GeV) gauge invariance! q0<0.5GeV a1-strength problematic

  25. Factor 3-4 suppression At intermediate and High photon energies 2.3.1.b Hadronic Formfactors • quantitative analysis: account for finite hadron size • improves a1phenomenology • t-channel exchange: gauge invariance nontrivial [Kapusta etal ’91] • simplified approach: [Turbide,Gale+RR ’04] with

  26. p γ p γ p K K* K K* (ii) wt-Channel p γ Gwrplarge! potentially important … w [Turbide,Gale +RR ’04] r p 2.3.2 Further Meson Gas Sources (i) Strangeness Contributions: SU(3)F MYM ~25%of pp→ργ ~40%of pr→pγ! (iii) Higher Resonances Ax-Vec:a1,h1→pg,Vec:w,w’,w’’→pgother:p(1300)→pg f1→rg,K1→KgK*→Kg a2(1320)→pg

  27. 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…

  28. 2.3.3(b) Photon Rates from r Spectral Function:Baryons + Meson-Resonances • baryonic contributions • dominant forq0<1GeV • (CERES enhancement!) • also true at RHIC+LHC: • atT=180MeV, mB=0 mB=220MeV

  29. 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]

  30. 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]

  31. HG: chemistry and trans. flow HG: chemistry [LHC] T [GeV] • R~exp(3mp) for pr→pg , … • yield up at low qt , down above • large blue shift from coll. flow • conserved BB use entropy • build-up of mp>0 (Np=const) • accelerated cooling 3.2 Thermal Evolution:QGP→ Mix→ HG QGP: initial conditions [SPS] • t0=1fm/c → t0=0.5fm/c: ~2-3 • s=CdQGT3; dQG=40 → 32: ~2 • pre-equilibrium?!

  32. Photon Properties in Colorsuperconductors

  33. 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]

  34. 3.3 Dilepton Spectrum at RHIC

  35. 4.3 Perspectives on Data III: RHIC Predictions for Central Au-Au PHENIX Data • large “pre-equilibrium” yield • from parton cascade (no LPM) • thermal yields ~ consistent • QGP undersat. small effect • consistent with initial only • disfavors parton cascade • not sensitive to thermal yet

More Related