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Dileptons in Heavy-Ion Collisions and Chiral Symmetry Hendrik van Hees Justus-Liebig Universit at Gieen September 10, 2010 JUSTUS-LIEBIG- Institut fr UNIVERSITT GIESSEN Theoretische Physik Hendrik van Hees (JLU Gieen) Dileptons

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SLIDE 1

Dileptons in Heavy-Ion Collisions and Chiral Symmetry

Hendrik van Hees

Justus-Liebig Universit¨ at Gießen

September 10, 2010

Institut für Theoretische Physik

JUSTUS-LIEBIG- UNIVERSITÄT GIESSEN

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 1 / 20

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SLIDE 2

Outline

1

Electromagnetic probes in heavy-ion collisions

2

Dileptons at SIS energies

3

Dileptons at SPS and RHIC

4

Conclusions

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 2 / 20

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SLIDE 3

Electromagnetic probes in heavy-ion collisions

γ, ℓ±: no strong interactions reflect whole “history” of collision:

from pre-equilibrium phase from thermalized medium QGP and hot hadron gas from VM decays after thermal freezeout

ρ/ω γ ∗ e− π, ... e+

1 2 3 4 5

M [GeV/c] J/ Drell−Yan DD Low− Intermediate− High−Mass Region

> 10 fm > 1 fm < 0.1 fm

ψ’ π0, η Dalitz decays dN/(dy dm) ρ/ω Φ Ψ

  • Fig. by A. Drees

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 3 / 20

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SLIDE 4

Vector Mesons and electromagnetic Probes

ℓ+ℓ− thermal emission rates ⇔ em. current-correlation function, Πµν

[L. McLerran, T. Toimela 85, H. A. Weldon 90, C. Gale, J.I. Kapusta 91]

dNe+e− d4xd4q = −gµν α2 3q2π3 Im Π(ret)

µν (q)

  • q2=M2

e+e−

fB(q0) vector-meson dominance model:

Πµν =

Gρ γ∗ γ∗ hadronic many-body theory for vector mesons ρ ρ π π B , a , K ,... *

1 1

π,... N, K, ρ ρ elementary processes ⇔ cut self-energy diagrams

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 4 / 20

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SLIDE 5

Relation to the QCD-phase diagram

at high temperature/density: restoration of chiral symmetry Lattice QCD: T χ

c ≃ T deconf c Mass Spectral Function Spectral Function

"a1" "ρ"

  • pert. QCD

Melting Resonances? "a1" "ρ"

  • pert. QCD

Dropping Masses?

Mechanism of chiral restoration? Two main theoretical ideas

“dropping masses”: mhad ∝ ¯ ψψ

  • “melting resonances”: broadening of spectra through medium effects

More theoretical question: Realization of chiral symmetry in nature?

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 5 / 20

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SLIDE 6

Dileptons at SIS energies

dileptons from heavy-ion collisions at DLS at E = 1A GeV

[Porter et al, PRL 79, 1229 (1997)]

large enhancement at low invariant masses unexplained DLS puzzle

[Bratkovskaya et al (1999)] [Ernst et al (1998)] [Fuchs et al (2003)] Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 6 / 20

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SLIDE 7

Experimental solution

DLS measurement confirmed by HADES at GSI

[Sudol et al, EPJC 62, 81 (2009)] Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 7 / 20

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SLIDE 8

Theoretical “HADES Puzzle”

  • ne-boson-exchange model by Shyam/Mosel

Bremsstrahlung in pp and pn collisions

π,ρ,σ,ω

ρ

+ρ −

,

ρ

+ρ −

,

N

∆

N*

γ*

e−

+

e

(a) (b) (c) (d) pp pn

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 8 / 20

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SLIDE 9

Recent update: π em. form factors

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 10

1

10

2

dσ/dM [µb/(GeV/c

2)] ρ

0 decay

π

0 Dalitz decay

Total QM (NEFF) Total QM (FF2) Sum

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 M (GeV/c

2)

10

  • 6

10

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 10

1

10

2

dσ/dM [µb/(GeV/c

2)] η Dalitz decay

p + p 1.25 GeV quasi-free p + n 1.25 GeV

[R. Shyam, U. Mosel, arXiv:1006.3873 [hep-ph] ]

problems with the pn bremsstrahlung contributions importance of π formfactors for radiation from the charged-meson exchange line here form factors from

[ G. E. Brown, M. Rho, and W. Weise, NPA 454, 669 (1986)] Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 9 / 20

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SLIDE 10

GiBUU

Gießen Boltzmann-Uehling-Uhlenbeck transport model describes pp data also here importance of em. form factors ∆-Dalitz decay with form factors from [Q. Wan, F. Iachello, IJMPA 20, 1846 (2005)]

[J. Weil, private communication] Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 10 / 20

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SLIDE 11

Dileptons at SPS and RHIC

radiation from thermal sources: Hadronic many-body theory

[R. Rapp, J. Wambach 99]

baryon effects important nB+n ¯

B relevant quantity (not net-baryon density)!

ρ ρ π π B , a , K ,... *

1 1

π,... N, K, ρ ρ

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 11 / 20

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SLIDE 12

Dilepton rates: Hadron gas ↔ QGP

in-medium hadron gas matches with QGP similar results also for γ rates “quark-hadron duality”!? consistent with chiral-symmetry restoration “resonance melting” rather than “dropping masses”

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 12 / 20

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SLIDE 13

Sources of dilepton emission in heavy-ion collisions

1 initial hard processes: Drell Yan 2 “core” ⇔ emission from thermal source [McLerran, Toimela 1985]

1 qT dN(thermal) dMdqT =

  • d4x
  • dy
  • MdϕdN(thermal)

d4xd4q Acc(M, qT , y) use cylindrical thermal fireball with QGP, mixed and hadronic phase

3 “corona” ⇔ emission from “primordial” mesons (jet-quenching) 4 after thermal freeze-out ⇔ emission from “freeze-out” mesons [Cooper, Frye 1975]

N(fo) = d3q q0

  • qµdσµfB(uµqµ/T)Γmeson→ℓ+ℓ−

Γmeson Acc

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 13 / 20

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SLIDE 14

CERES/NA45 dielectron spectra

good agreement also for dielectron spectra in 158 GeV Pb-Au low-mass tail from baryon effects

0.2 0.4 0.6 0.8 1 1.2

Mee [GeV]

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(dNee/dM) / <Nch> [100 MeV/c

2]

  • 1

CERES ’00 in-med ρ+ω+φ 4π mix QGP DY total total (no bar ρ)

Pb(158 AGeV)+Au

×10

−6

<Nch>=335

pt>0.2GeV 2.1<η<2.65 Θee>35mrad

(7% central) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Mee [GeV]

10

  • 8

10

  • 7

10

  • 6

10

  • 5

(d

2Nee/dηdM) / (dNch/dη) [100MeV]

  • 1

CERES ’95+’96 free ρ in-med ρ+ω+φ+QGP+4π

35% Central Pb(158AGeV)+Au

<Nch>=250

pt>0.2GeV 2.1<η<2.65 Θee>35mrad

QGP Cocktail 4π

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 14 / 20

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SLIDE 15

Hadron spectra

NA60: Extracted hadronic pT spectra from µ+µ− “cocktail” analysis of “cocktail”: hadron-mT spectra comparison to fireball evolution ⇔ fixes radial acceleration “sequential freeze-out” due to different coupling strength

103 104 105 106 0.5 1 1.5 2 2.5 (1/mT) dN/dmT (a.u.) mT-M (GeV) ρ (semicentral) (0.6 GeV<M<0.9 GeV) Tc=Tch=175 MeV, aT=0.1 c2/fm prim+fo fo prim NA60 ρ

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 15 / 20

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SLIDE 16

Hadron spectra

NA60: Extracted hadronic pT spectra from µ+µ− “cocktail” analysis of “cocktail”: hadron-mT spectra comparison to fireball evolution ⇔ fixes radial acceleration “sequential freeze-out” due to different coupling strength

103 104 105 106 107 0.5 1 1.5 2 2.5 (1/mT) dN/dmT (a.u.) mT-M (GeV) ω (semicentral) Tc=Tch=175 MeV aT=0.1 c2/fm, tfo=5 fm prim+fo fo prim NA60 ω

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 15 / 20

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SLIDE 17

Hadron spectra

NA60: Extracted hadronic pT spectra from µ+µ− “cocktail” analysis of “cocktail”: hadron-mT spectra comparison to fireball evolution ⇔ fixes radial acceleration “sequential freeze-out” due to different coupling strength

103 104 105 106 107 0.5 1 1.5 2 2.5 (1/mT) dN/dmT (a.u.) mT-M (GeV) φ (semicentral) Tc=Tch=175 MeV aT=0.1 c2/fm, tfo=4 fm prim+fo fo prim NA60 φ

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 15 / 20

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SLIDE 18

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) all qT Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 19

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 0<qt<0.2 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 20

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 0.2 GeV<qt<0.4 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 21

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 0.4 GeV<qt<0.6 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 22

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 0.6 GeV<qt<0.8 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 23

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 0.8 GeV<qt<1.0 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 24

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 1.0 GeV<qt<1.2 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 25

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 2.0 GeV<qt<2.2 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

slide-26
SLIDE 26

M spectra (in pT slices)

norm corrected by ∼3% due to centrality correction (min-bias data: Nch = 120, calculation Nch = 140)

10-9 10-8 10-7 10-6 10-5 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) 2.2 GeV<qt<2.4 GeV Tc=Tch=175 MeV, at=0.1 c2/fm +ω-t ex total ω-t ex ω φ DY 4π mix FO ρ prim ρ QGP in-med ρ NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 16 / 20

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SLIDE 27

Importance of baryon effects

baryonic interactions important! in-medium broadening low-mass tail!

10-9 10-8 10-7 10-6 0.2 0.4 0.6 0.8 1 1.2 1.4 (1/Nch) d2Nµµ/(dM dη) (20 MeV)-1 M (GeV) all qT Tc=Tch=175 MeV, at=0.1 c2/fm full model no baryons NA60

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 17 / 20

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SLIDE 28

IMR: QGP vs. multi-pion radiation

50 100 150 200 0.9 1 1.1 1.2 1.3 1.4 dNµµ/dM (counts) M (GeV) semicentral In-In all pT QGP (EoS-A) QGP (EoS-B) QGP (EoS-C) 4π (EoS-A) 4π (EoS-B) 4π (EoS-C)

different critical and freeze-out temperatures Tc = 160 . . . 190 MeV, Tchem = 160 . . . 175 MeV M- and pT spectra comparably well described! reason: T vs. volume ⇒ maximal l+l− emission for T = Tmax = M/5.5 hadronic and partonic radiation “dual” for T ∼ Tc compatible with chiral-symmetry restoration! inconclusive whether hadronic or partonic emission in IMR!

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 18 / 20

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SLIDE 29

Dileptons@RHIC: (Another) new Puzzle?

huge enhancement in the LMR unexplained yet!

)

2

(GeV/c

ee

m 0.2 0.4 0.6 0.8 1 1.2 /GeV) IN PHENIX ACCEPTANCE

2

(c

ee

dN/dm

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10

= 200 GeV

NN

s

  • min. bias Au+Au

R.Rapp & H.vanHees cocktail vacuum ρ sum w/ broadening ρ sum w/ dropping) ρ sum/ partonic yield (PY) ee (PYTHIA) → c c ee (random correlation) → c c

DATA |y| < 0.35 > 0.2 GeV/c

e T

p

model: Rapp, HvH

[A. Adare et al (PHENIX), arXiv:0912.0244 [nucl-ex]]

)

2

(GeV/c

ee

m 0.2 0.4 0.6 0.8 1 1.2 /GeV) IN PHENIX ACCEPTANCE

2

(c

ee

dN/dm

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10

= 200 GeV

NN

s

  • min. bias Au+Au

HSD cocktail broadening ρ sum w/

  • broad. + drop.

ρ sum w/ ee (PYTHIA) → c c ee (random correlation) → c c

DATA |y| < 0.35 > 0.2 GeV/c

e T

p

model: HSD Bratkovskaya, Cassing

[A. Adare et al (PHENIX), arXiv:0912.0244 [nucl-ex]] Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 19 / 20

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SLIDE 30

Conclusions and Outlook

dilepton spectra ⇔ in-medium em. current correlator SIS energies

dominated by bremsstrahlung and Dalitz decays pp and pn bremsstrahlung importance of em. form factors

SPS and RHIC energies

excess yield dominated by radiation from thermal sources baryons essential for in-medium properties of vector mesons melting vector mesons with little mass shift IMR well described by scenarios with radiation dominated either by QGP or multi-pion processes (depending on EoS) “quark-hadron duality” of ℓ+ℓ− rates around Tc compatible with chiral symmetry restoration! new puzzle @ RHIC?!? recent review:

  • R. Rapp, J. Wambach, HvH, Landolt-B¨
  • rnstein, 1-23A

arXiv: 0901.3289 [hep-ph]

Hendrik van Hees (JLU Gießen) Dileptons in heavy-ion collisions September 10, 2010 20 / 20