NICA

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NICA Round Table IVJINR, Dubna, September
9-12, 2009
Dileptons in Heavy Ion Collisions

• Itzhak Tserruya

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Outline

• Introduction
• Low energies DLS and HADES
• SPS results
• CERES - Low-mass region
• NA60 - Low and intermediate mass region
• RHIC first results from PHENIX
• Elementary collisions search for cold nuclear
  matter effects
• Summary and outlook

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Introduction

• Electromagnetic probes (real or virtual photons)
  are sensitive probes of the two fundamental
  properties of the QGP
• Deconfinement
• Chiral symmetry restoration
• Lepton pairs are unique probes of CSR
• The therrmal radiation emitted in the form of
  dileptons (virtual photons) provides a direct
  fingerprint of the matter created first the QGP
  (qqbar ? ee-) and subsequently a dense HG
  (??- ? ee-)
• What have we learned in about 20 years of
  dilepton measurements? What is the NICA
  potential?

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Low-mass dilepton experiments

• Nuclear Collisions
• CERES
• DLS
• HADES
• HELIOS
• NA38/50
• NA60
• PHENIX

• Elementary Reactions
• CLAS
• CBELSA/TAPS
• KEK E235
• TAGX

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Dileptons in AA at a Glance
Time Scale
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• Low-energies
• DLS and HADES

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DLS puzzle
DLS data Porter et al., PRL 79, 1229 (1997)
Calculations Bratkovskaya et al., NP A634, 168
(1998)

• Enhancement not described by in-medium ?
  spectral function
• All other attempts to reproduce the DLS results
  failed
• Main motivation for the HADES experiment

Strong enhancement over hadronic cocktail with
free ? spectral function
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HADES confirms the DLS results
CC 1 AGeV
Mass distribution
pT distribution
Good agreement between DLS and HADES in
di-electron mass and pT distributions
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Putting the puzzle together (I)
CC _at_ 1 AGeV pp pd _at_ 1.25 GeV

• Spectra normalized to ?0 measured in CC and NN
• CC _at_ 1 AGeV
• ltM?gt/Apart 0.06 0.07
• NN _at_ 1.25 GeV (using pp and pd measurements)
• ltM?NNgt/Apart 1/4(pp2pnnn)/2
• 1/2(pppn) 0.076?0.015

Dielectron spectrum from CC consistent with
superposition of NN collisions! No compelling
evidence for in-medium effects in CC
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Putting the puzzle together (II)
Recent transport calculations enhanced NN
bremsstrahlung , in line with recent OBE
calculations) HSD Bratkovskaya et al. NPA
807214 (2008)
The DLS puzzle seems to be reduced to an
understanding of the elementary contributions to
NN reactions.
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• SPS
• Low-masses

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CERES Pioneering Results (I)
Strong enhancement of low-mass ee- pairs (wrt
to expected yield from known sources)
Last CERES result (2000 Pb run PLB 666(2008) 425)
Enhancement factor (0.2 ltm lt 1.1 GeV/c2 ) 2.45
0.21 (stat) 0.35 (syst) 0.58 (decays)
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CERES Pioneering Results (II)
First CERES result PRL 75, (1995) 1272
Last CERES result PLB 666 (2008) 425
Strong enhancement of low-mass ee- pairs in all
A-A systems studied
Eur. Phys J. C41 (2005) 475
PRL 91 (2003) 042301
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pT and Multiplicity Dependencies
Enhancement is at low pT
Increases faster than linearly with multiplicity
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Dropping Mass or Broadening (I) ?
Interpretations invoke ??- ???
? ? ee- thermal radiation from HG
CERES Pb-Au 158 A GeV 2000 data
vacuum ? not enough to reproduce data

• in-medium modifications of ?
• broadening ? spectral shape
• (Rapp and Wambach)
• dropping ? meson mass
• (Brown et al)

Data favor the broadening scenario.
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NA60 Low-mass dimuons in In-In at 158 AGeV
Real data !
Superb data!

• Mass resolution23 MeV at the ? position

• S/B 1/7

• ?, ? and even ? peaks clearly visible in
  dimuon channel

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Dimuon Excess
Dimuon excess isolated by subtracting the hadron
cocktail (without the ?)

• Excess centered at the nominal ? pole

• Excess rises and broadens with centrality
• More pronounced at low pT

confirms consistent with, CERES results
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NA60 low mass comparison with models
PRL 96 (2006) 162302

• Subtract the cocktail from the data (without the
  ?)

• Excess shape consistent with broadening of the ?
  
• (Rapp-Wambach)

• Mass shift of the ? (Brown-Rho)
• is ruled out

• Is this telling us something
• about CSR?

• All calculations normalized to data at m?? lt 0.9
  GeV performed by Rapp et al., for ltdNch/d?gt 140

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NA60 low mass comparison with models

• Subtract the cocktail from the data (without the
  ?)

NP A806, 339 (2008)

• Excess shape consistent with broadening of the ?
  
• (Rapp-Wambach)

• Mass shift of the ? (Brown-Rho)
• is ruled out

• Is this telling us something
• about CSR?

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• RHIC

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Dileptons in PHENIX pp collisions

• Mass spectrum measured from m0 up to m8
  GeV/c2
• Very well understood in terms of
• hadron cocktail at low masses
• heavy flavor DY at high masses

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Dileptons in PHENIX AuAu collisions

• Strong enhancement of ee- pairs at low masses
• m 0.2 0.7 GeV/c2.
• Very different from SPS
• Enhancement down to very low masses
• Enhancement concentrated at central collisions
• No enhancement in the IMR

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Comparison to theoretical model (AuAu)
PHENIX
All models and groups that successfully described
the SPS data fail in describing the PHENIX results
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Dileptons in PHENIX AuAu collisions
Min bias AuAu vsNN 200 GeV arXiv 0706.3034
nucl-ex
Integral180,000 above p015,000
All pairs Combinatorial BG Signal

• BG determined by event mixing technique,
  normalized to like sign yield

• Green band systematic error w/o error on CB

PHENIX has mastered the event mixing technique to
unprecedented precision (0.25). But with a S/B
1/200 the statistical significance is
largely reduced and the systematic errors
are large
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HBD
Single vs double e separation
Installed and fully operational in Run-9
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• LVM in Elementary Collisions

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KEK E235 pC, Cu _at_ E12 GeV Cold nuclear matter
Excellent mass resolution ?m 8.9-0.2 MeV/c2 _at_
m?1017 MeV/c2
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Raw spectra
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Raw spectra fitted with known sources.

• Hadronic sources ?, ?, ? -gt ee-, ? -gt? ee- , ?
  -gt ? ee-
• Width Breit-Wigner shape convoluted
  with experimental resolution.
• Position PDG values
• Relative abundances determined by fit
  
• Combinatorial background event mixing method

Cannot fit the ? with m and ? from PDG ? yield
consistent with zero
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Dropping ? and ? masses
KEK E325 PRL 96, 092301 (2006)
Model ? and ? produced at nuclear surface,
decay with modified mass if decay point is
inside nucleus mV(?) / mV(0) 1 k(?/?0)
common k parameter for C and Cu target and for ?
and ?. k 9.2
? and ? masses drop by 9.2 at normal nuclear
matter density
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CBELSA / TAPS
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CLAS
No effect seen in ? ee- photo-produced
on H2, C, Fe and Ti targets Mass spectra look
very similar to those measured by KEK . However,
CLAS results can be very well reproduced by a
transport model using the vacuum mass values of
?, ? and ?.
k 2 ? 2
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Summary and outlook

• DLS puzzle solved in CC. Dilepton spectrum
  understood as mere superposition of NN
  collisions. Is that so also for heavier system?
  Onset of low-mass pair enhancement?
• Consistent picture from the SPS
• Low-mass pair enhancement thermal radiation from
  the HG
• Approach to CSR proceeds through broadening
  (melting) of the resonances
• Enhancement persists down to 40 AGeV
• RHIC results very intriguing
• Strong enhancement of low-mass pairs down to very
  low masses
• No enhancement in the IMR
• Challenge for theoretical models
• Looking forward to more precise results with the
  HBD
• Elementary collisions no coherent picture and no
  compelling evidence of in-medium modification
  effects of LVM in cold nuclear matter
• NICAs energy range very well suited to fill an
  important niche
• unveil the onset of the low-mass pair
  enhancement.
• Systematic studies of pA colliisions
• Study pair enhancement under highest baryon
  density conditions