What can we extract from Thermal EM Radiation in Heavy-Ion Collisions?

1. What can we extract fromThermal EM Radiation in Heavy-Ion Collisions? Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA International School on Nuclear Physics, 38th Course Erice (Sicily, Italy), 16.-24.09.16

2. 1.) Intro: EM Spectral Function to Probe Fireball • Thermal Dilepton Rate ImΠem(M,q;mB,T) e+e-→ hadrons r Im Pem(M) / M2 e+ e- e+ e- M [GeV] • Hadronic Resonances - change in degrees of freedom? - restoration of chiral symmetry? • Continuum - temperature? • Low-q0,q limit: transport coefficient (charge)? • Total yields: fireball lifetime?

3. 1.2 30 Years of Dileptons in Heavy-Ion Collisions <Nch>=120 • Robust understanding across QCD phase diagram: QGP + hadronic radiation with meltingr resonance

4. Outline 1.) Introduction 2.) Degrees of Freedom of the Medium  Quark-to-Hadron Transition 3.) Chiral Symmery Restoration  QCD +Weinberg Sum Rules  Other Multiplets 4.) Transport Properties  Electric Conductivity 5.) Phenomenological Tool  Fireball Lifetime + Temperature (Excitation Fct.)  Fireball in pA? 6.) Conclusions

5. 2.) Dilepton Rates and Degrees of FreedomdRee /dM2 ~ ∫d3q f B(q0;T) ImPem • r-meson resonance “melts” • spectral function merges into QGP description • Direct evidence for transition hadrons → quarks + gluons - [qq→ee] [HTL] dRee/d4q 1.4Tc (quenched) q=0 [Ding et al ’10] [RR,Wambach et al ’99]

6. Outline 1.) Introduction 2.) Degrees of Freedom of the Medium  Quark-to-Hadron Transition 3.) Chiral Symmery Restoration  QCD +Weinberg Sum Rules  Other Multiplets 4.) Transport Properties  Electric Conductivity 5.) Phenomenological Tool  Fireball Lifetime + Temperature (Excitation Fct.)  Fireball in pA? 6.) Conclusions

7. 3.1 QCD + Weinberg Sum Rules [Hatsuda+Lee’91, Asakawa+Ko ’93, Leupold et al ’98, …] r a1 [Weinberg ’67, Das et al ’67; Kapusta+Shuryak ‘94] Dr = rV -rA • accurately satisfied in vacuum • In Medium: condensates from hadron resonance gas, constrained by lattice-QCD T [GeV]

8. 3.1.2 QCD + Weinberg Sum Rules in Medium → Search for solution for axial-vector spectral function [Hohler +RR ‘13] • quantitatively compatible with (approach to) chiral restoration • strong constraints by combining SRs • Chiral mass splitting “burns off”, resonances melt

9. 3.2 Lattice-QCD Results for N(940)-N*(1535) Euclidean Correlator Ratios Exponential Mass Extraction “N*(1535)” “Nucleon” R=∫(G+-G-)/(G++G-) [Aarts et al ‘15] • also indicates MN*(T) → MN (T) ≈ MNvac • see also talk by R.-A. Tripolt

10. Outline 1.) Introduction 2.) Degrees of Freedom of the Medium  Quark-to-Hadron Transition 3.) Chiral Symmery Restoration  QCD +Weinberg Sum Rules  Other Multiplets 4.) Transport Properties  Electric Conductivity 5.) Phenomenological Tool  Fireball Lifetime + Temperature (Excitation Fct.)  Fireball in pA? 6.) Conclusions

11. 4.) Electric Conductivity • Similar behavior for different transport? h/s ~ (2pT) DsHF ~ sEM/T • Probes soft limit of EM spectral function sEM(T) = - e2 limq0→0 [ ∂/∂q0 Im PEM(q0,q=0;T) ] • Need density-squared contributions • Non-trivial for vertex corrections (usually evaluated with vacuum propagators) • Start out with pion gas: dress pions in r-cloud + vertex corrections [Atchison+RR in prog.]

12. 4.2 Low-Energy Limit of Spectral Function Pion Gas Perturbative QGP T=150MeV ar = 0.7 ar = 1.2 ar = 2.7 0 2 4 6 8 10 q0 [MeV] [Moore+Robert ‘06] • conductivity peak strongly smeared out • suggestive for strongly coupled system

13. 4.3 Conductivity: Comparison to other Approaches in-med.pgas → [Greif et al ‘16] • in-medium pion gas well above SYM limit • interactions with anti-/baryons likely to reduce it

14. Outline 1.) Introduction 2.) Degrees of Freedom of the Medium  Quark-to-Hadron Transition 3.) Chiral Symmery Restoration  QCD +Weinberg Sum Rules  Other Mutiplets 4.) Transport Properties  Electric Conductivity 5.) Phenomenological Tool  Fireball Lifetime + Temperature (Excitation Fct.)  Fireball in pA? 6.) Conclusions

15. 5.1 Fireball Lifetime Excitation Function of Low-Mass Dilepton Excess Yield 1000 [STAR ‘15] [RR+vanHees ‘14] • Low-mass excess tracks lifetime well (medium effects!) • Tool for critical point search?

16. 5.2 Fireball Temperature Slope of Intermediate-Mass Excess Dileptons • unique ``early” temperature measurement (no blue-shift!) • Ts approaches Ti toward lower energies • first-order “plateau” at BES-II/CBM?

17. 5.3 Low-Mass Dileptons in p-Pb (5.02GeV) • Thermal radiation at ~10% of cocktail • follows excess-lifetime systematics • photons [Shen et al ‘15]

18. 6.) Conclusions • Explicit evidence for parton-hadron transition (rmelting) • Progress in understanding mechanisms of chiral restoration - evaporation of chiral mass r-a1 splitting (sum rules, MYM) • Electric conductivity suggests strongly coupled medium • Dilepton radiation as a precision tool to measure - fireball lifetime (low mass), including pA - early temperature (intermediate mass; no blue-shift)

19. 4.3 Comparison to Data: RHIC Ideal Hydro Viscous Hydro [van Hees et al, ‘11, ’14] [Paquet et al ’16] • same rates + intial flow  similar results from various evolution models

20. 3.2 Massive Yang-Mills in Hot Pion Gas Temperature progression of vector + axialvector spectral functions • supports “burning” of chiral-mass splitting as mechanism for chiral restoration [as found in sum rule analysis]

21. 4.1 Initial Flow + Thermal Photon-v2 Bulk-Flow Evolution Direct-Photon v2 Ideal Hydro 0-20% Au-Au • initial radial flow: - accelerates bulk v2 - harder radiation spectra (pheno.: coalescence, multi-strange f.o.) • much enhances thermal-photon v2 [He et al ’14]

22. 4.2 Thermal Photon Rates • ``Cocktail” of hadronic sources (available in parameterized form) • Sizable new hadronic sources: pr → gw , pw → gr , rw → gp [Heffernan et al ‘15] [Holt,Hohler+RR in prep] • Hadronic emission rate close to QGP-AMY • semi-QGP much more suppressed [Pisarski et al ‘14]

23. 3.2 Massive Yang-Mills Approach in Vaccum • Gauge r + a1 into chiral pion lagrangian: • problems with vacuum phenomenology → global gauge? • Recent progress: - full rpropagator in a1 selfenergy - vertex corrections to preserve PCAC: [Urban et al ‘02, Rischke et al ‘10] [Hohler +RR ‘14] • enables fit to t-decay data! • local-gauge approach viable • starting point for addressing chiral restoration in medium

24. 4.3.2 Photon Puzzle!? • Tslopeexcess ~240 MeV • blue-shift: Tslope ~ T √(1+b)/(1-b) T ~ 240/1.4 ~170 MeV

25. 4.1.2 Sensitivity to Spectral Function In-Medium r-Meson Width • avg. Gr(T~150MeV)~370MeVGr (T~Tc) ≈ 600 MeV → mr • driven by (anti-) baryons Mmm [GeV]

26. 4.2 Low-Mass Dileptons: Chronometer In-In Nch>30 • first “explicit” measurement of interacting-fireball lifetime: tFB≈ (7±1) fm/c

27. 4.1 Prospects I: Spectral Shape at mB ~ 0 STAR Excess Dileptons [STAR ‘14] • rather different spectral shapes compatible with data • QGP contribution?

28. 4.5 QGP Barometer: Blue Shift vs. Temperature SPS RHIC • QGP-flow driven increase of Teff ~ T + M (bflow)2 at RHIC • high pt: high T wins over high-flow r’s → minimum (opposite to SPS!) • saturates at “true” early temperature T0 (no flow)

29. 2.3 Low-Mass e+e- Excitation Function: 20-200 GeV P. Huck et al. [STAR], QM14 • compatible with predictions from melting r meson • “universal” source around Tpc

30. 3.3.2 Effective Slopes of Thermal Photons Thermal Fireball Viscous Hydro [van Hees,Gale+RR ’11] [S.Chen et al ‘13] • thermal slope can only arise from T ≤ Tc(constrained by • closely confirmed by hydro hadron data) • exotic mechanisms: glasma BE? Magnetic fields+ UA(1)? [Liao at al ’12, Skokov et al ’12, F. Liu ’13,…]

31. 2.2 Transverse-Momentum Dependence pT -Sliced Mass Spectra mT -Slopes x100 • spectral shape as function of pair-pT • entangled with transverse flow (barometer)

32. 3.1.2 Transverse-Momentum Spectra: Baro-meter Effective Slope Parameters RHIC SPS QGP HG [Deng,Wang, Xu+Zhuang ‘11] • qualitative change from SPS to RHIC: flowing QGP • true temperature “shines” at large mT

33. 2.2 Chiral Condensate + r-Meson Broadening > Sp effective hadronic theory > - Sp • h = mq h|qq|h > 0 contains quark core + pion cloud = Shcore + Shcloud ~ ++ • matches spectral medium effects: resonances + pion cloud • resonances + chiral mixing drive r-SF toward chiral restoration r - - qq / qq0

34. 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

35. 4.4 Elliptic Flow of Dileptons at RHIC • maximum structure due to late r decays [He et al ‘12] [Chatterjee et al ‘07, Zhuang et al ‘09]

36. 3.3.2 Fireball vs. Viscous Hydro Evolution [van Hees, Gale+RR ’11] [S.Chen et al ‘13] • very similar!

37. 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 - -

38. 4.1 Nuclear Photoproduction: rMeson in Cold Matter g + A → e+e- X • extracted “in-med” r-width Gr≈ 220 MeV e+ e- Eg≈1.5-3 GeV g r [CLAS+GiBUU ‘08] • Microscopic Approach: + in-med. r spectral fct. product. amplitude full calculation fix density 0.4r0 Fe-Ti r g N [Riek et al ’08, ‘10] M[GeV] • r-broadening reduced at high 3-momentum; need low momentum cut!