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Update on Search for Chiral Symmetry Restoration in Heavy-Ion Collisions

Update on Search for Chiral Symmetry Restoration in Heavy-Ion Collisions. Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA Heavy-Ion Forum Seminar CERN (Geneva, Switzerland), 03.05.12.

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Update on Search for Chiral Symmetry Restoration in Heavy-Ion Collisions

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  1. Update on Search for Chiral Symmetry Restoration in Heavy-Ion Collisions Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA Heavy-Ion Forum Seminar CERN (Geneva, Switzerland), 03.05.12

  2. 1.) Intro-I:Probing Strongly Interacting Matter • Bulk Properties: • Equation of State • Microscopic Properties: • -Degrees of Freedom • - Spectral Functions • Phase Transitions: • (Pseudo-) Order Parameters • Would like to extract from Observables: • temperature + transport properties • in-medium spectral functions • signatures of deconfinement + chiral symmetry restoration

  3. e+ e- γ 1.2 Electromagnetic Probes in HICs • Thermal Emission Rate • micro-physics: • → Electro-Magnetic spectral function • Space-Time Evolution of Fireball • macro-physics (equation of state: e(P)) • → temperature + flow fields • Emission Characteristics • interplay temperature – 3-volume: • high T, small V → large mass (M>1GeV) • lower T, large V → low mass (M<1GeV)

  4. 1.3 EM Spectral Function + Fate of Resonances Im Pem(M) in Vacuum Im Πem(M,q;mB,T) • Electromagn. spectral function • -√s < 2 GeV: non-perturbative • -√s > 2 GeV: perturbative (“dual”) • Vector resonances “prototypes” • - representative for bulk hadrons: • neither Goldstone nor heavy flavor • Modifications of resonances • ↔ phase structure: • - hadron gas → Quark-Gluon Plasma • - realization of transition? e+e-→ hadrons √s = M

  5. 1.4 Phase Transition(s) in Lattice QCD - - ≈ qq / qq “Tcchiral”~150MeV “Tcconf” ~170MeV [Fodor et al ’10] • different “transition” temperatures?! • smooth transitions! (smooth e+e- rate) • chiral restoration in “hadronic phase”?! • (low-mass dileptons!) • hadron resonance gas

  6. Outline 2.) Chiral Symmetry Breaking in Vacuum  “Higgs Mechanism”, Condensates + Mass Gap in QCD  Hadron Spectrum, Chiral Partners + Sum Rules 3.) EM Spectral Function in Medium  Hadronic Theory  QGP + Lattice QCD 4.) Highlights of EM Probes in Heavy-Ion Collisions  Spectro-, Thermo-, Chrono- + Baro-meter  Thermal Photons 5.) Low-Mass Dileptons at LHC  Mass Spectra + Collectivity 6.) Conclusions

  7. Q2≤ 2GeV2 → transition to “strong” QCD: • effective d.o.f. = hadrons (Confinement) • massive “constituent quarks” • mq* ≈ 350 MeV ≈ ⅓ Mp (Chiral Symmetry • ~ ‹0|qq|0› condensate! Breaking) ↕⅔fm 2.1 Nonperturbative QCD • well tested at high energies, Q2>2GeV2: • perturbation theory (as = g2/4π<< 1) • degrees of freedom = quarks + gluons (mu ≈ md ≈ 5 MeV ) _

  8. qR qL > > > > - - qL qR 2.2 Chiral Symmetry + QCD Vacuum : flavor + “chiral” (left/right) invariant • “Higgs” Mechanism in Strong Interactions: • qqattraction  condensate fills QCD vacuum! • Spontaneous Chiral Symmetry Breaking - • Profound Consequences: • effective quark mass: • ↔ mass generation! • near-massless Goldstone bosons p0,± • “chiral partners” split: DM ≈ 0.5GeV JP=0±1± 1/2±

  9. 2.3 Mass Gap and Chiral Partners Axial-/Vector Correlators Constituent Quark Mass “Data”: lattice [Bowman et al ‘02] Theory: Instanton Model [Diakonov+Petrov; Shuryak ‘85] pQCD cont. • Spectral shape matters for • chiral symmetry breaking! ● Chiral breaking:|q2| ≤ 2 GeV2

  10. 2.4 Chiral (Weinberg) Sum Rules • Quantify chiral symmetry breaking via observable spectral functions • Vector (r) - Axialvector (a1) spectral splitting [Weinberg ’67, Das et al ’67] t→(2n+1)p t→(2n)p [ALEPH ‘98, OPAL ‘99] pQCD pQCD • Key features of updated “fit”: [Hohler+RR ‘12] • r+a1 resonance, excited states (r’+a1’), universal continuum (pQCD!)

  11. 2.4.2 Evaluation of Chiral Sum Rules in Vacuum • pion decay • constants • chiral quark • condensates • vector-axialvector splitting (one of the) cleanest observable of • spontaneous chiral symmetry breaking • promising (best?) starting point to search for chiral restoration

  12. 2.5 QCD Sum Rules: r and a1 in Vacuum • dispersion relation: [Shifman,Vainshtein+Zakharov ’79] • lhs:hadronic spectral fct. • rhs:operator product expansion • 4-quark + gluon condensate dominant

  13. Outline 2.) Chiral Symmetry Breaking in Vacuum  “Higgs Mechanism”, Condensates + Mass Gap in QCD  Hadron Spectrum, Chiral Partners + Sum Rules 3.) EM Spectral Function in Medium  Hadronic Theory  QGP + Lattice QCD 4.) Highlights of EM Probes in Heavy-Ion Collisions  Spectro-, Thermo-, Chrono- + Baro-meter  Thermal Photons 5.) Low-Mass Dileptons at LHC  Mass Spectra + Collectivity 6.) Conclusions

  14. 3.1 Vector Mesons in Hadronic Matter > rB /r0 0 0.1 0.7 2.6 > [Chanfray et al, Herrmann et al, Asakawa et al, RR et al, Koch et al, Klingl et al, Post et al, Eletsky et al, Harada et al …] Dr (M,q;mB ,T) = [M 2 - mr2 -Srpp -SrB -SrM ] -1 r-Propagator: B*,a1,K1... r Sp r SrB,rM= Selfenergies: Srpp= N,p,K… Sp Constraints:decays:B,M→ rN, rp, ... ; scattering:pN→rN, gA, … SPS RHIC/LHC

  15. 3.2 QCD Sum Rules: r(770) in Hot Matter [Hatsuda+Lee’91, Asakawa+Ko ’93, Klingl et al ’97, Leupold et al ’98, Kämpfer et al ‘03, Ruppert et al ’05] • dispersion relation: • medium effects largely driven by 4-quark condensate • similar for axialvector

  16. p Sp Sp Sp r Sr Sr Sr 3.3 Axialvector in Medium: Dynamical a1(1260) p a1 resonance + + . . . = Vacuum: r In Medium: + + . . . [Cabrera,Jido, Roca+RR ’09] • in-medium p + r propagators • broadening of p-r scatt. Amplitude • pion decay constant in medium:

  17. 3.4 Vector Correlator in Thermal Lattice QCD • Euclidean Correlation fct. Lattice (quenched) [Ding et al ‘10] Hadronic Many-Body [RR ‘02] • “Parton-Hadron Duality” of lattice and in-medium hadronic?!

  18. 3.4.2 Back to Spectral Function -Im Pem /(C T q0) • suggests approach to chiral restoration + deconfinement

  19. 3.5 Dilepton Rates: Hadronic - Lattice - Perturbative dRee /dM2 ~ ∫d3q f B(q0;T) Im PV • Hadronic, pert. + lattice QCD • tend to “degenerate” toward~Tc • Quark-Hadron Duality at all M?! • ( degenerate axialvector SF!) dRee/d4q 1.4Tc (quenched) q=0 - [qq→ee] [HTL] [Ding et al ’10] [RR,Wambach et al ’99]

  20. 3.6 Summary: Criteria for Chiral Restoration • Vector (r) – Axialvector (a1) degenerate [Weinberg ’67, Das et al ’67] pQCD • QCD sum rules: • medium modifications ↔ vanishing of condensates • Agreement with thermal lattice-QCD • Approach to perturbative rate (QGP)

  21. Outline 2.) Chiral Symmetry Breaking in Vacuum  “Higgs Mechanism”, Condensates + Mass Gap in QCD  Hadron Spectrum, Chiral Partners + Sum Rules 3.) EM Spectral Function in Medium  Hadronic Theory  QGP + Lattice QCD 4.) Highlights of EM Probes in Heavy-Ion Collisions  Spectro-, Thermo-, Chrono- + Baro-meter  Thermal Photons 5.) Low-Mass Dileptons at LHC  Mass Spectra + Collectivity 6.) Conclusions

  22. 4.1 Dilepton Rates vs. Exp.: NA60 “Spectrometer” • Evolve rates over fireball expansion: Thermal m+m- Emission Rate Acc.-correctedm+m- Excess Spectra In-In(158AGeV) [NA60 ‘09] Mmm [GeV] [van Hees+RR ’08] • invariant-mass spectrum directly • reflects thermal emission rate!

  23. 4.1.2 Sensitivity of NA60 to Spectral Function Emp. scatt. ampl. + T-r approximation Hadronic many-body Chiral virial expansion Thermometer [CERN Courier Nov. 2009] • Significant differences in low-mass region • Overall slope T~150-200MeV (true T, no blue shift!)

  24. 4.1.3 Sensitivity to Spectral Function II In-Medium r-Meson Width • avg. Gr(T~150MeV)~370MeVGr (T~Tc) ≈ 600 MeV → mr • driven by baryons Mmm [GeV]

  25. 4.2 Low-Mass Dileptons: Chronometer In-In Nch>30 • first “explicit” measurement of interacting-fireball lifetime: • tFB≈ (7±1) fm/c

  26. 4.3 Dimuon pt-Spectra and Slopes: Barometer Effective Slopes Teff • theo. slopes originally too soft • increase fireball acceleration, • e.g. a┴ = 0.085/fm → 0.1/fm • insensitive to Tc=160-190MeV

  27. 4.4 Low-Mass e+e- at RHIC: PHENIX vs. STAR • “large” enhancement not accounted • for by theory • cannot be filled by QGP radiation… • (very) low-mass region • overpredicted… (SPS?!)

  28. 4.5 Direct Photons at RHIC Spectra Elliptic Flow ← excess radiation • Teffexcess = (220±25) MeV • QGP radiation? • radial flow? • v2g,dir comparable to pions! • under-predicted by ealry QGP • emission [Holopainen et al ’11,…]

  29. 4.5.2 Revisit Ingredients Emission Rates Fireball Evolution • multi-strange hadrons at “Tc” • v2bulkfully built up at hadronization • chemical potentials for p, K, … • Hadron - QGP continuity! [Turbide et al ’04] [van Hees et al ’11]

  30. 4.5.3 Thermal Photon Spectra + v2: PHENIX thermal + prim. g • both spectral slope and v2 point at • blue-shifted hadronic source… • to be tested in full hydro [van Hees,Gale+RR ’11]

  31. Outline 2.) Chiral Symmetry Breaking in Vacuum  “Higgs Mechanism”, Condensates + Mass Gap in QCD  Hadron Spectrum, Chiral Partners + Sum Rules 3.) EM Spectral Function in Medium  Hadronic Theory  QGP + Lattice QCD 4.) Highlights of EM Probes in Heavy-Ion Collisions  Spectro-, Thermo-, Chrono- + Baro-meter  Thermal Photons 5.) Low-Mass Dileptons at LHC  Mass Spectra + Collectivity 6.) Conclusions

  32. 5.1 Thermal Dileptons at LHC • charm comparable, accurate (in-medium) measurement critical • low-mass spectral shape in chiral restoration window

  33. 5.2 Chiral Restoration Window at LHC • low-mass spectral shape in chiral restoration window: • ~60% of thermal low-mass yield in “chiral transition region” • (T=125-180MeV) • enrich with (low-) pt cuts

  34. 5.3 QGP Barometer: Blue Shift vs. Temperature RHIC SPS • QGP-flow driven increase of Teff ~ T + M (bflow)2 at RHIC • temperature overcomes flowing late r’s → minimum (opposite to SPS!) • expect to be more pronounced at LHC

  35. 5.4 Elliptic Flow Diagnostics (RHIC) • maximum structure due to late r decays

  36. 6.) Conclusions:Potential of Thermal EM Radiation at LHC • Spectrometer: “prototype” in-med. spectral function, • hadron-to-quark transition • Chiral Restoration: Weinberg + QCD sum rules, • lattice QCD (mB~0!) + perturbative limit • Thermometer: int.-mass dileptons (heavy-flavor subtracted) • Chronometer: fireball lifetimes • Barometer: pt spectra + elliptic flow in M-slices (0…2GeV) • Quality control: rates (constraints, continuity), medium evolution, • consistency with SPS+RHIC, … • Needed: high-quality data to determine spectral shape, then use • with flow diagnostics • (Non-) Redundancy of RHIC and LHC critical

  37. 2.3.2 NA60 Mass Spectra: pt Dependence Mmm [GeV] • more involved at pT>1.5GeV: Drell-Yan, primordial/freezeout r , …

  38. 2.2 EM Probes at SPS • all calculated with the same e.m. spectral function! • thermal source: Ti≈210MeV, HG-dominated, r-meson melting!

  39. 2.) Transport: Electric Conductivity • hadronic theories (T~150MeV): • - chiral pert. theory (pion gas): sem / T ~ 0.11 e2 • - hadronic many-body theory: sem / T ~ 0.09 e2 [Fernandez-Fraile+ Gomez-Nicola ’07] • lattice QCD (T ~ (1.5-3) Tc ): • sem /T ~ (0.26±0.02) e2 [Gupta ’04, Aarts et al ’07, Ding et al. ‘11] • soft-photon limit of • thermal emission rate • EM Susceptibility ( → charge fluctuations): • Q2 -Q 2 =χem = Πem(q0=0,q→0)

  40. 5.2 Intermediate-Mass Dileptons: Thermometer • use invariant continuum radiation (M>1GeV): no blue shift, Tslope = T ! Thermometer • independent of partition HG vs QGP (dilepton rate continuous/dual) • initial temperature Ti ~ 190-220 MeV at CERN-SPS

  41. Add pp→ppgBremsstrahlung [Liu+RR’06] 2.2 Soft Photons at SPS: WA98 Thermal Radiation + pQCD [Turbide,RR +Gale’04]

  42. 4.1.2 Mass-Temperature Emission Correlation • generic space-time argument: •  • Tmax ≈ M / 5.5 • (forImPem =const) • thermal photons: • Tmax≈ (q0/5) * (T/Teff)2 • → reduced by flow blue-shift! • Teff ~ T * √(1+b)/(1-b)

  43. 4.7.2 Light Vector Mesons at RHIC + LHC • baryon effects important even at rB,tot= 0 : • sensitive to rBtot= rB + rB (r-N and r-N interactions identical) • w also melts, f more robust ↔ OZI - -

  44. = = 5.3 Intermediate Mass Emission: “Chiral Mixing” [Dey, Eletsky +Ioffe ’90] • low-energy pion interactions fixed by chiral symmetry 0 0 0 0 • mixing parameter • degeneracy with perturbative • spectral fct. down to M~1GeV • physical processes at M≥ 1GeV: • pa1→ e+e- etc. (“4p annihilation”)

  45. 3.2 Dimuon pt-Spectra and Slopes: Barometer pions: Tch=160MeV a┴ =0.1/fm pions: Tch=175MeV a┴ =0.085/fm • modify fireball evolution: • e.g. a┴ = 0.085/fm → 0.1/fm • both large and small Tccompatible • with excess dilepton slopes

  46. 2.3.2 Acceptance-Corrected NA60 Spectra Mmm [GeV] Mmm [GeV] • more involved at pT>1.5GeV: Drell-Yan, primordial/freezeout r , …

  47. 4.4.3 Origin of the Low-Mass Excess in PHENIX? • QGP radiation insufficient: • space-time , lattice QGP rate + • resum. pert. rates too small • must be of long-lived hadronic origin • Disoriented Chiral Condensate (DCC)? • Lumps of self-bound pion liquid? • Challenge: consistency with hadronic data, NA60 spectra! [Bjorken et al ’93, Rajagopal+Wilczek ’93] - “baked Alaska” ↔ small T - rapid quench+large domains ↔ central A-A - ptherm + pDCC → e+ e- ↔ M~0.3GeV, small pt [Z.Huang+X.N.Wang ’96 Kluger,Koch,Randrup ‘98]

  48. 2.3.3 Spectrometer III: Before Acceptance Correction emp. ampl. + “hard” fireball hadr. many-body + fireball schem. broad./drop. + HSD transport chiral virial + hydro • Discrimination power much reduced • can compensate spectral “deficit” by larger flow: lift pairs into acceptance

  49. 4.2 Improved Low-Mass QGP Emission • LO pQCD spectral function: rV(q0,q) = 6∕9 3M2/2p [1+QHTL(q0)] • 3-momentum augmented lattice-QCD rate (finite g rate)

  50. 4.4.1 Variations in QGP Radiation • improvements in QGP rate insufficient

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