NICA – Round Table IV JINR, Dubna , September 9-12, 2009

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


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?

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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


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


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


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)


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)


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

Itzhak TserruyaItzhak Tserruya 1111NICA - RT, JINR, Sept. 9-12, 2009NICA - RT, JINR, Sept. 9-12, 2009

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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)


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


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.


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!


Dimuon Excess Dimuon Excess


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


1919

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)


RHICRHIC


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


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


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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


2828

Raw spectra


2929

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


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


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


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

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NICA – Round Table IV JINR, Dubna , September 9-12, 2009