Thermal EM Radiation in Heavy-Ion Collisions Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA Symposium on Jet and Electromagnetic Tomography of Dense Matter McGill University (Montreal, Canada), 26.+27.06.15
1.) Intro: EM Spectral Function to Probe Fireball Thermal Dilepton Rate d dn 4 x d ee 4 2 α B = em f ( q0,t ) Im Π 3 2 em (M,q;µ B,T) q π M e + e - hadrons ρ e + e - Im Π em (M) / M 2 Hadronic Resonances - change in degrees of freedom? - restoration of chiral symmetry? M [GeV] Continuum - temperature? e + e - Total yields: fireball lifetime?
1.2 30 Years of Dileptons in Heavy-Ion Collisions <N ch >=120 Robust understanding across QCD phase diagram: QGP + hadronic radiation with melting ρ resonance
Outline 1.) Introduction 2.) Dileptons as Phenomenological Tool Excitation Function: Fireball Lifetime + Temperature Fireball in pa? 3.) Dileptons as Theoretical Tool Chiral Restoration: - QCD +Weinberg Sum Rules - Massive Yang-Mills Revisited 4.) Thermal Photon Puzzle(?) Space-Time Evolution Emission Rates 5.) Conclusions
2.1 Fireball Lifetime Excitation Function of Low-Mass Excess-Dilepton Yield [RR+van Hees 14] [STAR 15] Low-mass excess tracks lifetime well (medium effects!) Tool for critical point search?
2.2 Fireball Temperature Slope of Intermediate-Mass Excess-Dileptons unique ``early temperature measurement (no blue-shift!) T s approaches T i toward lower energies first-order plateau at BES-II/CBM?
2.3 Low-Mass Dileptons in p-pb (5.02GeV) Thermal radiation at ~10% of cocktail follows excess-lifetime systematics photons [Shen et al 15]
Outline 1.) Introduction 2.) Dileptons as Phenomenological Tool Excitation Function: Fireball Lifetime + Temperature Fireball in pa? 3.) Dileptons as Theoretical Tool Chiral Restoration: - QCD +Weinberg Sum Rules - Massive Yang-Mills Revisited 4.) Thermal Photon Puzzle(?) Space-Time Evolution Emission Rates 5.) Conclusions
3.1 QCD + Weinberg Sum Rules [Hatsuda+Lee 91, Asakawa+Ko 93, Leupold et al 98, ] ds π ds π ds π 1 ( ρ ρ ) s V A = ( ρ ρ ) = V s ( ρ V ρ A A ) = f 2 π m cα s q q q ( q q ) 2 ρ a 1 [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 [GeV]
3.1.2 QCD + Weinberg Sum Rules in Medium Search for solution for axialvector 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 Massive Yang-Mills Approach in Vaccum Gauge ρ + a 1 into chiral pion lagrangian: problems with vacuum phenomenology global gauge? [Urban et al 02, Rischke et al 10] Recent progress: - full ρ propagator in a 1 selfenergy - vertex corrections to preserve PCAC: [Hohler +RR 14] enables fit to τ-decay data! local-gauge approach viable starting point for addressing chiral restoration in medium
3.2.2 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]
Outline 1.) Introduction 2.) Dileptons as Phenomenological Tool Excitation Function: Fireball Lifetime + Temperature Fireball in pa? 3.) Dileptons as Theoretical Tool Chiral Restoration: - QCD +Weinberg Sum Rules - Massive Yang-Mills Revisited 4.) Thermal Photon Puzzle(?) Space-Time Evolution Emission Rates 5.) Conclusions
4.) Thermal Photons Evolve rates over fireball: q 0 dn d therm γ 3 q = τ τ fo 0 d 4 x q 0 dr d therm γ 3 q Sensitive to flow profile (contrary to dilepton invariant-mass spectra) 3 challenges from experiment: suggestive for: spectral yield - large large rates, large radial flow spectral slope - small later emission elliptic flow - large later emission, rapid build-up PHENIX 14 [van Hees et al 11, 14] [Shen et al 13]
4.1 Initial Flow + Thermal Photon-v 2 Bulk-Flow Evolution Direct-Photon v 2 Ideal Hydro 0-20% Au-Au initial radial flow: - accelerates bulk v 2 - harder radiation spectra (pheno.: coalescence, multi-strange f.o.) [He et al 14] 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: πρ γω, πω γρ, ρω γπ [Holt,Hohler+RR in prep] Hadronic emission rate close to QGP-AMY semi-qgp much more suppressed [Pisarski et al 14]
4.3 Comparison to Data: RHIC Fireball Ideal Hydro Viscous Hydro [van Hees et al, 11, 14] [Paquet et al 15] same rates + intial flow similar results from various evolution models
5.) Conclusions Dilepton radiation as a precision tool to measure - fireball lifetime (low mass) - early temperature (intermed. mass; no blue-shift) Progress in understanding mechanisms of chiral restoration - evaporation of chiral mass ρ-a 1 splitting (sum rules, MYM) Direct photons - identify key components of space-time evolution + rates (e.g., initial flow, ``realistic QGP + hadronic rates) - mild discrepancies between data and theory
4.3.2 Photon Puzzle!? T slope excess ~240 MeV blue-shift: T slope ~ T (1+β)/(1-β) T ~ 240/1.4 ~ 170 MeV
4.1.2 Sensitivity to Spectral Function In-Medium ρ-meson Width M µµ [GeV] avg. Γ ρ (T~150MeV) ~ 370 MeV Γ ρ (T~T c ) 600 MeV m ρ driven by (anti-) baryons
4.2 Low-Mass Dileptons: Chronometer In-In N ch >30 first explicit measurement of interacting-fireball lifetime: τ FB (7±1) fm/c
3.2 Vector Correlator in Thermal Lattice QCD Analyticity: Π dq ( τ,q ;T ) = 0 ρ π ii cosh[ q ( / T ) 0 τ 1 2 sinh[ q / T ] em em ii ( q,q ;T 2 0 0 2 0 )] Euclidean Correlator Ratio Spectral Function [Ding et al 10] [RR 02] G G V free V ( τ,t ) ( τ,t ) correlator enhancement comparable to lattice QCD indicates transition from hadronic to partonic degrees of freedom
4.1 Prospects I: Spectral Shape at µ 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 T eff ~ T + M (β flow ) 2 at RHIC high p t : high T wins over high-flow ρ s minimum (opposite to SPS!) saturates at true early temperature T 0 (no flow)
2.3 Low-Mass e + e - Excitation Function: 20-200 GeV compatible with predictions from melting ρ meson universal source around T pc P. Huck et al. [STAR], QM14
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 T c (constrained by closely confirmed by hydro hadron data) exotic mechanisms: glasma BE? Magnetic fields+ U A (1)? [Liao at al 12, Skokov et al 12, F. Liu 13, ]
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)
3.1.2 Transverse-Momentum Spectra: Baro-meter SPS Effective Slope Parameters RHIC HG QGP [Deng,Wang, Xu+Zhuang 11] qualitative change from SPS to RHIC: flowing QGP true temperature shines at large m T
2.2 Chiral Condensate + ρ-meson Broadening qq - / qq - 0 effective hadronic theory Σ h = m q h qq h - > 0 contains quark core + pion cloud = Σ h core + Σ h cloud ~ + + matches spectral medium effects: resonances + pion cloud resonances + chiral mixing drive ρ-sf toward chiral restoration > > Σ π Σ π ρ
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-) p t cuts
4.4 Elliptic Flow of Dileptons at RHIC maximum structure due to late ρ decays [He et al 12] [Chatterjee et al 07, Zhuang et al 09]
3.3.2 Fireball vs. Viscous Hydro Evolution [van Hees, Gale+RR 11] [S.Chen et al 13] very similar!
2.3 Dilepton Rates vs. Exp.: NA60 Spectrometer Evolve rates over fireball expansion: therm µµ dn dm dτ V Acc.-corrected µ + µ - Excess Spectra = τ τ fo 0 FB ( τ ) M d q 0 3 q dr therm µµ 4 d q In-In(17.3GeV) [NA60 09] [van Hees+RR 08] M µµ [GeV] invariant-mass spectrum directly reflects thermal emission rate!
2.2 Dilepton Rates: Hadronic - Lattice - Perturbative dr ee /dm 2 ~ d 3 q f B (q 0 ;T) Im Π em [qq ee] - [HTL] continuous rate through T pc 3-fold degeneracy toward ~T pc dr ee /d 4 q 1.4T c (quenched) q=0 [Ding et al 10] [RR,Wambach et al 99]
4.2 Low-Mass e + e - at RHIC: PHENIX vs. STAR PHENIX enhancement (central!) not accounted for by theory STAR data ok with theory (charm?!)
4.3.2 Revisit Ingredients Emission Rates Fireball Evolution Hadron - QGP continuity! conservative estimates [Turbide et al 04] multi-strange hadrons at T c v bulk 2 fully built up at hadronization chemical potentials for π, K, [van Hees et al 11]
4.7.2 Light Vector Mesons at RHIC + LHC baryon effects important even at ρ B,tot = 0 : - sensitive to ρ Btot = ρ Β + ρ- B (ρ-n and ρ-n interactions identical) ω also melts, φ more robust OZI
4.1 Nuclear Photoproduction: ρ Meson in Cold Matter γ + A e + e - X γ ρ e + e - E γ 1.5-3 GeV extracted in-med ρ-width Γ ρ 220 MeV [CLAS+GiBUU 08] Microscopic Approach: product. amplitude in-med. ρ spectral fct. γ ρ + Fe - Ti full calculation fix density 0.4ρ 0 N [Riek et al 08, 10] M [GeV] ρ-broadening reduced at high 3-momentum; need low momentum cut!