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

)T,q(fMqdxd

dN Bee023

2em

44 π

α−= Im Πem(M,q;µB,T)

Im Π

em(M

) / M

2

e+e- → hadrons

1.) Intro: EM Spectral Function to Probe Fireball

e+ e-

ρ

•  Hadronic Resonances - change in degrees of freedom? - restoration of chiral symmetry?

•  Continuum - temperature?

M [GeV]

•  Thermal Dilepton Rate

e+ e-

•  Total yields: fireball lifetime?

1.2 30 Years of Dileptons in Heavy-Ion Collisions

•  Robust understanding across QCD phase diagram: QGP + hadronic radiation with melting ρ resonance

<Nch>=120

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

Outline

[RR+van Hees ‘14]

Excitation Function of Low-Mass Excess-Dilepton Yield

•  Low-mass excess tracks lifetime well (medium effects!) •  Tool for critical point search?

2.1 Fireball Lifetime

[STAR ‘15]

•  unique ``early” temperature measurement (no blue-shift!) •  Ts approaches Ti toward lower energies •  first-order plateau at BES-II/CBM?

2.2 Fireball Temperature Slope of Intermediate-Mass Excess-Dileptons

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]

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

Outline

3.1 QCD + Weinberg Sum Rules

T [GeV]

[Hatsuda+Lee’91, Asakawa+Ko ’93, Leupold et al ’98, …]

21πρρ

πf)(s

dsAV =−∫

[Weinberg ’67, Das et al ’67; Kapusta+Shuryak ‘94]

qqm)(dsqAV −=−∫ ρρ

π2)qq(c)(sds

sAV αρρπ

=−∫

•  accurately satisfied in vacuum

•  In Medium: condensates from hadron resonance gas, constrained by lattice-QCD

ρ a1

3.1.2 QCD + Weinberg Sum Rules in Medium

•  quantitatively compatible with (approach to) chiral restoration •  strong constraints by combining SRs •  Chiral mass splitting “burns off”, resonances melt

[Hohler +RR ‘13]

→ Search for solution for axialvector spectral function

3.2 Massive Yang-Mills Approach in Vaccum •  Gauge ρ + a1 into chiral pion lagrangian: •  problems with vacuum phenomenology → global gauge?

•  Recent progress: - full ρ propagator in a1 selfenergy - vertex corrections to preserve PCAC:

•  enables fit to τ-decay data! •  local-gauge approach viable •  starting point for addressing chiral restoration in medium

[Urban et al ‘02, Rischke et al ‘10]

[Hohler +RR ‘14]

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]

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

Outline

4.) Thermal Photons

•  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

•  Evolve rates over fireball: qd

dRqxd

qddN

qthermtherm fo

304

30

0

γτ

τ

γ∫=

[van Hees et al ’11, ‘14] [Shen et al 13]

PHENIX ‘14

4.1 Initial Flow + Thermal Photon-v2

•  initial radial flow: - accelerates bulk v2 - harder radiation spectra (pheno.: coalescence, multi-strange f.o.) •  much enhances thermal-photon v2

Bulk-Flow Evolution Direct-Photon v2

[He et al ’14]

Ideal Hydro 0-20% Au-Au

[Heffernan et al ‘15]

4.2 Thermal Photon Rates

•  ``Cocktail” of hadronic sources (available in parameterized form)

[Holt,Hohler+RR in prep]

•  Sizable new hadronic sources: πρ → γω , πω → γρ , ρω → γπ

•  Hadronic emission rate close to QGP-AMY •  semi-QGP much more suppressed [Pisarski et al ‘14]

4.3 Comparison to Data: RHIC

[van Hees et al, ‘11, ’14]

•  same rates + intial flow ⇒ similar results from various evolution models [Paquet et al ’15]

Fireball Ideal Hydro Viscous Hydro

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-a1 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!?

•  Tslopeexcess ~240 MeV

•  blue-shift: Tslope ~ T √(1+β)/(1-β) ⇒ T ~ 240/1.4 ~ 170 MeV

Mµµ [GeV]

4.1.2 Sensitivity to Spectral Function

•  avg. Γρ (T~150MeV) ~ 370 MeV ⇒ Γρ (T~Tc) ≈ 600 MeV → mρ

•  driven by (anti-) baryons

In-Medium ρ-Meson Width

4.2 Low-Mass Dileptons: Chronometer

•  first “explicit” measurement of interacting-fireball lifetime: τFB ≈ (7±1) fm/c

In-In Nch>30

3.2 Vector Correlator in Thermal Lattice QCD

]T/q[)]T/(q[)T;q,q(dq)T;q,( i ii i 2s inh

21cosh2 0

00

em

0

0em −= ∫∞

τρ

πτΠ•  Analyticity:

)T,(G)T,(G

V

Vτ

τfree

Spectral Function Euclidean Correlator Ratio

•  correlator enhancement comparable to lattice QCD •  indicates transition from hadronic to partonic degrees of freedom

[Ding et al ‘10]

[RR ‘02]

•  rather different spectral shapes compatible with data •  QGP contribution?

4.1 Prospects I: Spectral Shape at µB ~ 0

STAR Excess Dileptons [STAR ‘14]

4.5 QGP Barometer: Blue Shift vs. Temperature

•  QGP-flow driven increase of Teff ~ T + M (βflow)2 at RHIC •  high pt: high T wins over high-flow ρ’s → minimum (opposite to SPS!) •  saturates at “true” early temperature T0 (no flow)

SPS RHIC

•  compatible with predictions from melting ρ meson •  “universal” source around Tpc

P. Huck et al. [STAR], QM14

2.3 Low-Mass e+e- Excitation Function: 20-200 GeV

3.3.2 Effective Slopes of Thermal Photons

•  thermal slope can only arise from T ≤ Tc (constrained by •  closely confirmed by hydro hadron data) •  exotic mechanisms: glasma BE? Magnetic fields+ UA(1)?

[van Hees,Gale+RR ’11]

[Liao at al ’12, Skokov et al ’12, F. Liu ’13,…]

[S.Chen et al ‘13]

Thermal Fireball Viscous Hydro

2.2 Transverse-Momentum Dependence pT -Sliced Mass Spectra

mT -Slopes

x 100

•  spectral shape as function of pair-pT •  entangled with transverse flow (barometer)

3.1.2 Transverse-Momentum Spectra: Baro-meter

SPS Effective Slope Parameters

•  qualitative change from SPS to RHIC: flowing QGP •  true temperature “shines” at large mT

RHIC

[Deng,Wang, Xu+Zhuang ‘11]

QGP HG

2.2 Chiral Condensate + ρ-Meson Broadening 〈〈

qq〉〉

/ 〈q

q〉0

-

-

effective hadronic theory

•  Σh = mq 〈h|qq|h〉 > 0 contains quark core + pion cloud

= Σhcore + Σh

cloud ~ + + •  matches spectral medium effects: resonances + pion cloud •  resonances + chiral mixing drive ρ-SF toward chiral restoration

Σπ

Σπ

>

>

- 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-) pt cuts

4.4 Elliptic Flow of Dileptons at RHIC

•  maximum structure due to late ρ decays

[Chatterjee et al ‘07, Zhuang et al ‘09]

[He et al ‘12]

3.3.2 Fireball vs. Viscous Hydro Evolution

•  very similar!

[van Hees, Gale+RR ’11]

[S.Chen et al ‘13]

2.3 Dilepton Rates vs. Exp.: NA60 “Spectrometer”

•  invariant-mass spectrum directly reflects thermal emission rate!

Mµµ [GeV]

•  Evolve rates over fireball expansion:

[van Hees+RR ’08]

qddR

qqdM)(VddM

dN therm

FB

therm fo

40

3

0

µµτ

τ

µµ ττ ∫∫=

In-In(17.3GeV) [NA60 ‘09]

Acc.-corrected µ+µ- Excess Spectra

2.2 Dilepton Rates: Hadronic - Lattice - Perturbative

dRee /dM2 ~ ∫d3q f B(q0;T) Im Πem •  continuous rate through Tpc •  3-fold “degeneracy” toward ~Tpc

[qq→ee] - [HTL]

[RR,Wambach et al ’99]

[Ding et al ’10]

dRee/d4q 1.4Tc (quenched) q=0

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

•  multi-strange hadrons at “Tc” •  v2

bulk fully built up at hadronization •  chemical potentials for π, K, …

•  Hadron - QGP continuity! •  conservative estimates…

Emission Rates Fireball Evolution

[van Hees et al ’11]

[Turbide et al ’04]

4.7.2 Light Vector Mesons at RHIC + LHC

•  baryon effects important even at ρB,tot= 0 : sensitive to ρBtot= ρΒ + ρB (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

[CLAS+GiBUU ‘08]

Eγ≈1.5-3 GeV

ρ

e+ e-

γ

•  extracted “in-med” ρ-width Γρ ≈ 220 MeV

•  Microscopic Approach:

Fe - Ti

γ

N

ρ

product. amplitude in-med. ρ spectral fct. +

M [GeV] [Riek et al ’08, ‘10]

full calculation fix density 0.4ρ0

•  ρ-broadening reduced at high 3-momentum; need low momentum cut!