nica – round table iv jinr, dubna , september 9-12, 2009
DESCRIPTION
Dileptons in Heavy Ion Collisions. NICA – Round Table IV JINR, Dubna , September 9-12, 2009. Itzhak Tserruya. Outline. Introduction Low energies: DLS and HADES SPS results CERES - Low-mass region NA60 - Low and intermediate mass region RHIC: first results from PHENIX - PowerPoint PPT PresentationTRANSCRIPT
NICA – Round Table IVNICA – Round Table IVJINR, Dubna, September 9-12, 2009JINR, Dubna, September 9-12, 2009
Itzhak TserruyaItzhak Tserruya
Dileptons in Heavy Ion Dileptons in Heavy Ion CollisionsCollisions
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009 22
OutlineOutline IntroductionIntroduction
Low energies: DLS and HADESLow energies: DLS and HADES
SPS resultsSPS results CERES - Low-mass regionCERES - Low-mass region NA60 - Low and intermediate mass regionNA60 - Low and intermediate mass region
RHIC: first results from PHENIXRHIC: first results from PHENIX
Elementary collisions: search for cold nuclear Elementary collisions: search for cold nuclear matter effectsmatter effects
Summary and outlookSummary and outlook
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009 33
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 e+e-) and subsequently a dense HG (+- e+e-)
What have we learned in about 20 years of dilepton measurements? What is the NICA potential?
44
Low-mass dilepton experimentsLow-mass dilepton experiments
Nuclear CollisionsNuclear CollisionsCERESCERESDLS DLS HADESHADESHELIOSHELIOSNA38/50NA38/50NA60NA60PHENIXPHENIX
Elementary ReactionsElementary ReactionsCLASCLASCBELSA/TAPSCBELSA/TAPSKEK E235KEK E235TAGXTAGX
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
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Dileptons in A+A at a Glance: Dileptons in A+A at a Glance:
Time ScaleTime Scale
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
CERES
DLS
NA60
HADES
NICA
90 95 1000 0585
PHENIX
CBM
1
Energy Scale
DLS
HADES
PHENIX
10
158 [A GeV]
17 [GeV]√sNN200
// // //
// // //
CERES
NA60
2nd generation
1st generation
NICA CBM
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009 66
Low-energies:Low-energies:
DLS and HADESDLS and HADES
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DLS “puzzle”DLS “puzzle”
Strong enhancement over hadronic cocktail with “free” spectral function
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
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
HADES confirms the DLS results HADES confirms the DLS results
88Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Mass distribution pT distribution
Good agreement between DLS and HADES in di-electron mass and pT distributions
C+C 1 AGeV
Putting the puzzle together (I) Putting the puzzle together (I)
99Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Spectra normalized to 0 measured in C+C and NN
C+C @ 1 AGeV: <M>/Apart = 0.06 ± 0.07
N+N @ 1.25 GeV (using pp and pd measurements)<M
NN>/Apart = 1/4(pp+2pn+nn)/2 = 1/2(pp+pn) = 0.0760.015
Dielectron spectrum from C+C consistent with superposition of NN collisions!
No compelling evidence for in-medium effects in C+C
C+C @ 1 AGeV – pp & pd @ 1.25 GeV
Putting the puzzle together (II) Putting the puzzle together (II)
1010Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
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.
SPSSPS
Low-massesLow-masses
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CERES Pioneering Results (I) CERES Pioneering Results (I) Strong enhancement of low-mass e+e- pairs
(wrt to expected yield from known sources)
Enhancement factor (0.2 <m < 1.1 GeV/c2 ):
2.45 ± 0.21 (stat) ± 0.35 (syst) ± 0.58 (decays)
No enhancement in pp
nor in pA
Last CERES result (2000 Pb run PLB 666(2008) 425)
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
CERES Pioneering Results (II) CERES Pioneering Results (II)
Strong enhancement of low-mass e+e- pairs in all A-A
systems studied
First CERES result PRL 75, (1995) 1272
Last CERES result PLB 666 (2008) 425
Eur. Phys J. C41 (2005) 475 PRL 91 (2003) 042301
Better tracking and better mass resolution (m/m = 3.8%) due to: Doublet of silicon drift chambers close to the vertex Radial TPC upgrade downstream of the double RICH spectrometer
Itzhak TserruyaItzhak Tserruya 1313NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
ppTT and Multiplicity Dependencies and Multiplicity Dependencies
Enhancement is at low pT
Increases faster than linearly with multiplicity
Dropping Mass or Broadening (I) ?Dropping Mass or Broadening (I) ? Interpretations invoke:
* +- * e+e-
thermal radiation from HG
* in-medium modifications of :
broadening spectral shape (Rapp and Wambach)
dropping meson mass (Brown et al)
CERES Pb-Au 158 A GeV 2000 dataCERES Pb-Au 158 A GeV 2000 data
* vacuum ρ not enough to reproduce data
Data favor the broadening scenario.
Itzhak TserruyaItzhak Tserruya 1515NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
NA60 Low-mass dimuons in In-In at 158 AGeV
, and even peaks clearly visible in dimuon channel
S/B = 1/7
Mass resolution:23 MeV at the position
Real
data !
Superb data!
1616Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Dimuon Excess Dimuon Excess
1717Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Dimuon excess isolated by subtracting the hadron cocktail (without the )
Eur.Phys.J.C 49 (2007) 235
Excess centered at the nominal ρ pole
confirms & consistent with, CERES results
Excess rises and broadens with centrality
More pronounced at low pT
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NA60 low mass: comparison with modelsNA60 low mass: comparison with models
• All calculations normalized to data at m < 0.9 GeV performed by Rapp et al., for <dNch/d> = 140
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?
Subtract the cocktail from the data (without the )
PRL 96 (2006) 162302
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NA60 low mass: comparison with modelsNA60 low mass: comparison with models
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?
Subtract the cocktail from the data (without the ) NP A806, 339 (2008)
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
RHICRHIC
Itzhak TserruyaItzhak Tserruya 2020NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Dileptons in PHENIX: p+p collisions Dileptons in PHENIX: p+p collisions
2121Itzhak TserruyaItzhak Tserruya
Mass spectrum measured from m=0 up to m=8 GeV/c2
Very well understood in terms of: hadron cocktail at low masses heavy flavor + DY at high masses
NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Dileptons in PHENIX: Au+Au collisionsDileptons in PHENIX: Au+Au collisions
Strong enhancement of e+e- 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
Itzhak TserruyaItzhak Tserruya 2222NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Comparison to theoretical model (Au+Au)PHENIX
All models and groups that successfully described the SPS data fail in describing the PHENIX results
Dileptons in PHENIX: Au+Au collisionsDileptons in PHENIX: Au+Au collisions
All pairsCombinatorial BGSignal • BG determined by event mixing technique,
normalized to like sign yield
• Green band: systematic error w/o error on CB
Integral:180,000 above 0:15,000
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
Min bias Au+Au √sNN = 200 GeVarXiv: 0706.3034 [nucl-ex]
Matching resolution in z and
HBD HBD Installed and fully operational in Run-9
Single vs double e separation
Hadron blindnessh in F and R bias e-h separation h rejection
LVM in Elementary CollisionsLVM in Elementary Collisions
Itzhak TserruyaItzhak Tserruya 2626NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
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KEK E235KEK E235p+C, Cu @ E=12 GeVp+C, Cu @ E=12 GeV
Cold nuclear matterCold nuclear matterExcellent mass resolution:
m = 8.9+-0.2 MeV/c2 @ mФ=1017 MeV/c2
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
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Raw spectra
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Raw spectra fitted with known sources.Raw spectra fitted with known sources.
Hadronic sources: , , Ф -> e+e-, -> e+e- , η -> e+e-
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
Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Dropping Dropping and and masses masses
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%
KEK E325
PRL 96, 092301 (2006)
and masses drop by 9.2% at normal nuclear matter density
CBELSA / TAPS CBELSA / TAPS
3131Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
Similar effect seen in 0 photo-produced on Nb and LH2 targets:
At low momenta, clear excess in the low-mass side of the meson for the Nb target
No effect at high momenta
k = 13%
CLAS CLAS
3232Itzhak TserruyaItzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009
No effect seen in e+e- 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 %
Summary and outlook Summary and outlook DLS puzzle solved in C+C. Dilepton spectrum understood as mere DLS puzzle solved in C+C. Dilepton spectrum understood as mere superposition of NN collisions. Is that so also for heavier system? Onset of superposition of NN collisions. Is that so also for heavier system? Onset of low-mass pair enhancement?low-mass pair enhancement?
Consistent picture from the Consistent picture from the SPS:SPS: Low-mass pair enhancement: thermal radiation from the HGLow-mass pair enhancement: thermal radiation from the HG Approach to CSR proceeds through broadening (melting) of the resonances Approach to CSR proceeds through broadening (melting) of the resonances Enhancement persists down to 40 AGeVEnhancement persists down to 40 AGeV
RHIC results very intriguing: RHIC results very intriguing: Strong enhancement of low-mass pairs down to very low massesStrong enhancement of low-mass pairs down to very low masses No enhancement in the IMRNo enhancement in the IMR Challenge for theoretical modelsChallenge for theoretical models Looking forward to more precise results with the HBDLooking forward to more precise results with the HBD
Elementary collisions: no coherent picture and no compelling evidence of in-Elementary collisions: no coherent picture and no compelling evidence of in-medium modification effects of LVM in cold nuclear mattermedium modification effects of LVM in cold nuclear matter
NICA’s energy range very well suited to fill an important niche:NICA’s energy range very well suited to fill an important niche: unveil the onset of the low-mass pair enhancement.unveil the onset of the low-mass pair enhancement. Systematic studies of pA colliisionsSystematic studies of pA colliisions Study pair enhancement under highest baryon density conditionsStudy pair enhancement under highest baryon density conditions