Thermal Dileptons from High to Low Energies Ralf

Thermal Dileptons from High to Low Energies Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA ISF Research Workshop on Study of High-Density Matter with Hadron Beams Weizmann Institute (Rehovot, Israel), 28. -31. 03. 17

1. ) Intro: EM Spectral Function to Probe Fireball Im Πem(M, q; m. B, T) • Thermal Dilepton Rate r e+ e- Im Pem(M) / M 2 e+e- → hadrons e+ e- M [Ge. V] • Hadronic Resonances - change in degrees of freedom - restoration of chiral symmetry • Continuum - temperature • Low-q 0, q limit: transport coefficient (EM conductivity) • Total yields: fireball lifetime

1. 2 30 Years of Dileptons in Heavy-Ion Collisions =120 • Robust understanding across QCD phase diagram: QGP + hadronic radiation with melting r resonance

Outline 1. ) Introduction 2. ) Degrees of Freedom of the Medium Quark-to-Hadron Transition 3. ) Chiral Symmetry Restoration QCD +Weinberg Sum Rules Other Multiplets 4. ) Phenomenological Tool Fireball Temperature Fireball Lifetime (p. A? ) 5. ) Conclusions

2. 1 In-Medium r-Meson Spectral Functions Dr (M, q; m. B, T) = 1 / [M 2 – (mr(0))2 - Srpp - Sr. B - Sr. M ] Hot + Dense Matter Hot Meson Gas r. B/r 0 0 0. 1 0. 7 2. 6 m. B =330 Me. V [RR+Wambach ’ 99] [RR+Gale ’ 99] • r-meson “melts” in hot/dense matter • baryon density r. B more important than temperature

2. 2 Dilepton Rates and Degrees of Freedom d. Ree /d. M 2 ~ ∫d 3 q/q 0 f B(q 0; T) Im Pem /M 2 [qq→ee] [HTL] • r-meson resonance “melts” • spectral function merges into QGP description Þ Direct evidence for transition hadrons → quarks + gluons d. Ree/d 4 q 1. 4 Tc (quenched) q=0 [Ding et al ’ 10] [RR, Wambach et al ’ 99]

Outline 1. ) Introduction 2. ) Degrees of Freedom of the Medium Quark-to-Hadron Transition 3. ) Chiral Symmetry Restoration QCD +Weinberg Sum Rules Other Multiplets 4. ) Phenomenological Tool Fireball Temperature Fireball Lifetime (p. A? ) 5. ) Conclusions

3. 1 QCD + Weinberg Sum Rules [Hatsuda+Lee’ 91, Asakawa+Ko ’ 93, Leupold et al ’ 98, …] r a 1 D r = r V -r A [Weinberg ’ 67, Das et al ’ 67; Kapusta+Shuryak ‘ 94] • accurately satisfied in vacuum • In Medium: condensates from hadron resonance gas, constrained by lattice-QCD T [Ge. V]

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

3. 2 Lattice-QCD Results for N(940)-N*(1535) Euclidean Correlator Ratios “Nucleon” Exponential Mass Extraction “N*(1535)” R = ∫(G+-G-)/(G++G-) [Aarts et al ‘ 15] • also indicates MN*(T) → MN (T) ≈ MNvac

Outline 1. ) Introduction 2. ) Degrees of Freedom of the Medium Quark-to-Hadron Transition 3. ) Chiral Symmetry Restoration QCD +Weinberg Sum Rules Other Multiplets 4. ) Phenomenological Tool: Excitation Functions Fireball Temperature Fireball Lifetime (p. A? ) 5. ) Conclusions

4. 1 NA 60 Dimuons at SPS (√s=17. 3 Ge. V) • Integrates over thermal fireball: Radiation Spectrum q q Low mass: r-meson melting, fireball lifetime e+ er High mass: QGP thermometer Tavg ~ 200 Me. V

4. 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/NICA?

4. 3 Fireball Lifetime Excitation Function of Low-Mass Dilepton Excess Yield 1000 [RR+van Hees ‘ 14] [STAR ‘ 15] • Low-mass excess tracks lifetime well (medium effects!) • Tool for critical point search?

4. 4 Dileptons at HADES: Coarse Graining • Coarse-graining of hadronic transport to → extract thermodynamic variables → convolute with thermal dilepton rate Temperature + Baryon Density [Huovinen et al. ’ 02, Endres et al ’ 15, Galatyuk et al ‘ 16] Dilepton Yield vs. Nucleon Flow Au-Au (1. 23 AGe. V) • build-up of collectivity EM radiation “thermal” fireball • fireball lifetime “only” tfb ~ 13 fm/c

4. 4. 2 Dileptons at HADES: Spectra + Lifetimes Coarse-Graining Results with in-Medium Spectral Functions • Fair consistency with mass spectra and lifetime systematics

4. 5 Lifetime from Dileptons vs. HBT Radii • Rlong qualitatively consistent, quantitatively much smaller

5. ) Conclusions • Explicit evidence for parton-hadron transition: r melting • Progress in understanding mechanisms of chiral restoration - evaporation of chiral mass r-a 1 splitting (sum rules, MYM) • Dilepton radiation as a precision tool to measure - fireball lifetime (low mass), including p. A - early temperature (intermediate mass; no blue-shift) • Coarse-graining enables to extend thermal-radiation tool to lower energies

5. 3 Low-Mass Dileptons in p-Pb (5. 02 Ge. V) • Thermal radiation at ~10% of cocktail • follows excess-lifetime systematics

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 p. A? 6. ) Conclusions

4. 1 Nuclear Photoproduction: r Meson in Cold Matter g + A → e +e - X e+ g r • extracted “in-med” r-width Gr ≈ 220 Me. V Eg≈1. 5 -3 Ge. V e- [CLAS+Gi. BUU ‘ 08] • Microscopic Approach: product. amplitude g + in-med. r spectral fct. r Fe - Ti full calculation fix density 0. 4 r 0 N [Riek et al ’ 08, ‘ 10] M [Ge. V] • r-broadening reduced at high 3 -momentum; need low momentum cut!

2. 1 In-Medium Vector Mesons at RHIC + LHC • Anti-/baryon effects melt the r meson • w also melts, f more robust ↔ OZI

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

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]

4. 1 Initial Flow + Thermal Photon-v 2 Bulk-Flow Evolution Direct-Photon v 2 Ideal Hydro 0 -20% Au-Au [He et al ’ 14] • initial radial flow: - accelerates bulk v 2 - harder radiation spectra (pheno. : coalescence, multi-strange f. o. ) • much enhances thermal-photon v 2

4. 2 Thermal Photon Rates • ``Cocktail” of hadronic sources (available in parameterized form) [Heffernan et al ‘ 15] • Sizable new hadronic sources: pr → gw , pw → gr , rw → gp [Holt, Hohler+RR in prep] • Hadronic emission rate close to QGP-AMY • semi-QGP much more suppressed [Pisarski et al ‘ 14]

3. 2 Massive Yang-Mills Approach in Vaccum • Gauge r + a 1 into chiral pion lagrangian: • problems with vacuum phenomenology → global gauge? [Urban etetalal‘ 02, Rischke ‘ 10] • Recent progress: - full r propagator in a 1 selfenergy - vertex corrections to preserve PCAC: • enables fit to t-decay data! • local-gauge approach viable • starting point for addressing chiral restoration in medium [Hohler +RR ‘ 14]

4. 3. 2 Photon Puzzle!? • Tslopeexcess ~240 Me. V • blue-shift: Tslope ~ T √(1+b)/(1 -b) T ~ 240/1. 4 ~ 170 Me. V

2. 2 Transverse-Momentum Dependence p. T -Sliced Mass Spectra m. T -Slopes x 100 • spectral shape as function of pair-p. T • entangled with transverse flow (barometer)

4. 1. 2 Sensitivity to Spectral Function In-Medium r-Meson Width Mmm [Ge. V] • avg. Gr (T~150 Me. V) ~ 370 Me. V Gr (T~Tc) ≈ 600 Me. V → mr • driven by (anti-) baryons

4. 2 Low-Mass Dileptons: Chronometer In-In Nch>30 • first “explicit” measurement of interacting-fireball lifetime: t. FB ≈ (7± 1) fm/c

4. 1 Prospects I: Spectral Shape at m. B ~ 0 STAR Excess Dileptons [STAR ‘ 14] • rather different spectral shapes compatible with data • QGP contribution?

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 T 0 (no flow)

2. 3 Low-Mass e+e- Excitation Function: 20 -200 Ge. V P. Huck et al. [STAR], QM 14 • compatible with predictions from melting r meson • “universal” source around Tpc

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, …]

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 m. T

- qq / - 0 2. 2 Chiral Condensate + r-Meson Broadening effective hadronic theory • h = mq h|qq|h > 0 contains quark core + pion cloud + > = hcore + hcloud ~ > + p p • matches spectral medium effects: resonances + pion cloud • resonances + chiral mixing drive r-SF toward chiral restoration r

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 -180 Me. V) • enrich with (low-) pt cuts

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]

4. ) Electric Conductivity • Similar behavior for different transport? h/s ~ (2 p. T) Ds. HF ~ s. EM/T • Probes soft limit of EM spectral function s. EM(T) = - e 2 limq 0→ 0 [ ∂/∂q 0 Im PEM(q 0, 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. ]

4. 2 Low-Energy Limit of Spectral Function Pion Gas Perturbative QGP T=150 Me. V ar = 0. 7 ar = 1. 2 ar = 2. 7 0 2 4 6 q 0 [Me. V] 8 10 • conductivity peak strongly smeared out • suggestive for strongly coupled system [Moore+Robert ‘ 06]

4. 3 Conductivity: Comparison to other Approaches in-med. p gas → [Greif et al ‘ 16] • in-medium pion gas well above SYM limit • interactions with anti-/baryons likely to reduce it

3. 3. 2 Fireball vs. Viscous Hydro Evolution [van Hees, Gale+RR ’ 11] [S. Chen et al ‘ 13] • very similar!