EMMI Workshop and XXVI Max Born Symposium Wroclaw, July 9-11, 2009 Itzhak Tserruya Dileptons in...

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EMMI Workshop and XXVI Max Born EMMI Workshop and XXVI Max Born SymposiumSymposium

Wroclaw, July 9-11, 2009 Wroclaw, July 9-11, 2009

Itzhak TserruyaItzhak Tserruya

Dileptons in Heavy Ion Dileptons in Heavy Ion CollisionsCollisions

Itzhak TserruyaItzhak TserruyaEMMI Workshop, Wroclaw, July 9-11, EMMI Workshop, Wroclaw, July 9-11,

20092009 22

OutlineOutline IntroductionIntroduction

SPS resultsSPS results Low-mass regionLow-mass region Intermediate mass regionIntermediate mass region

RHIC: first results from PHENIXRHIC: first results from PHENIX

Low energies: DLS and HADESLow energies: DLS and HADES

mesonmeson

Elementary collisions: search for cold nuclear Elementary collisions: search for cold nuclear matter effectsmatter effects

Summary and outlookSummary and outlook

Itzhak TserruyaItzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 2009 33

Introduction Electromagnetic probes (real or virtual photons) are

sensitive probes of the two fundamental properties of the QGP:

Chiral symmetry restoration Deconfinement

Lepton pairs are unique probes of CSR.

Thermal radiation emitted in the form of dileptons (virtual photons) provide a direct fingerprint of the matter formed: QGP and dense HG

What have we learned in about 20 years of dilepton measurements?

Low-mass dilepton experimentsLow-mass dilepton experiments

Nuclear CollisionsNuclear CollisionsCBMCBMCERESCERESDLS DLS HADESHADESHELIOSHELIOSNA38/50NA38/50NA60NA60PHENIXPHENIX

Elementary ReactionsElementary ReactionsCLASCLASCBELSA/TAPSCBELSA/TAPSKEK E235KEK E235TAGXTAGX

Itzhak TserruyaItzhak TserruyaEMMI Workshop, Wroclaw, July 9-11, EMMI Workshop, Wroclaw, July 9-11,

20092009

Dileptons in A+A at a Glance: Dileptons in A+A at a Glance:

CBM MPD

90 95 1000 0585

PHENIX

Time ScaleTime Scale

= Period of data taking

Energy Scale

PHENIX

10 158 [A GeV]

17 [GeV]√sNN200

// // //

CBMMPD

Itzhak TserruyaItzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 2009

SPSSPS

Low-massesLow-masses

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

1111Itzhak TserruyaItzhak Tserruya

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

data !

Superb data!

Dimuon Excess Dimuon Excess

1616Itzhak TserruyaItzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 2009

PRL 96 (2006) 162302 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

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

SPSSPS

Intermediate massesIntermediate masses

NA50 IMR Results NA50 IMR Results

p-A is well described by the sum of Drell-Yan and Open Charm contributions (obtained

from Pythia)

The yield observed in heavy-ion collisions exceeds the sum of DY and OC decays,

extrapolated from the p-A data.

The excess has mass and pT shapes similar to the contribution of the Open Charm (DY +

3.6OC nicely reproduces the data).

Drell Yan + Open charm

Drell Yan + 3.6 x Open charm

charm enhancement?

NA60: IMR excess in agreement with NA50

IMR yield in In-In collisions enhanced compared to expected yield from DY and OC

Can be fitted with fixed DY (within 10%) and OC enhanced by a factor of ~3 Fit range

4000 A, 2 <1.5

2.90.14

4000 A,2 <1.5

2.750.14

4000 A, 2 <1.5

1.120.17

Free prompt and open charm scaling factors

Full agreement with NA50

… But the offset distribution is not compatible with this assumption

Fixed prompt and free open charm

NA60: IMR excess is a prompt source

Origin of the IMR Excess Origin of the IMR Excess

Hees/Rapp, PRL 97, 102301 (2006) Renk/Ruppert, PRL 100,162301 (2008)

Dominant process in mass region m > 1 GeV/c2:

EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 2009

hadronic processes, 4 … partonic processes, qq annihilation

Quark-Hadron duality?

ppTT distributions distributions

Low-mass region Intermediate mass region

Fit in 0.5<PT<2 GeV/c(as in LMR analysis)

The mT spectra are exponential, the inverse slopes do not depend on mass.

The mT spectra are exponential, the inverse slopes depend on mass.

Radial Flow

Partonic radiation?

RHICRHIC

Dileptons in PHENIX: p+p collisions Dileptons in PHENIX: p+p collisions

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

EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 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: [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

Itzhak TserruyaItzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009EMMI Workshop, Wroclaw, July 9-11, 2009 3333

Low-energies:Low-energies:

DLS and HADESDLS and HADES

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

Mass distribution pT distribution

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

C+C @ 1 AGeV – pp & pd @ 1.25 GeV

Dielectron spectrum from C+C consistent with superposition of NN collisions!

No compelling evidence for in-medium effects in C+C

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 understanting of the elementary contributions to NN reactions.

The The meson meson ll++ll- - and and K K++KK--

Inconclusive results Inconclusive results

PHENIX

Uncertainties in the e+e- channel too large for a conclusive statement. Waiting for HBD improved results

The reanalyzed NA50 results in and the CERES results in the ee are compatible within 1-2σ and within errors there is room for some effect.

LVM in Elementary CollisionsLVM in Elementary Collisions

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

Raw spectra

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

Consistent and coherent picture from the Consistent and coherent 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

IMR enhancement: thermal radiation from partonic phaseIMR enhancement: thermal radiation from partonic phase

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?

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

meson results inconclusivemeson results inconclusive

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

EMMI Workshop and XXVI Max Born Symposium Wroclaw, July 9-11, 2009 Itzhak Tserruya Dileptons in...

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