Dielectron Spectroscopy at SIS-18 Energies

Joachim StrothNuclear Physics Institute, Goethe-University Frankfurt

and GSI

International School of Nuclear PhysicsInternational School of Nuclear PhysicsErice 16-24 September 2008Erice 16-24 September 2008

MotivationMotivation

• At 1-2 GeV/u (Au-Au) long lifetime of the dense system:

– fm/c >> Rho

• Dilepton decays of vector mesons extremely rare probe:

– Pee<10-7/collisions

? What is the microscopic structure of this matter

– In-medium spectral functions

? What are the appropriate models to relate the microscopic properties to observables

SIS-18

Chemical Freeze-out: A. Andronic, PBM, J. Stachel et al.; Cleymans, K. Redlich et al.

<qqbar>

B.J. Schäfer, J. Wambachpriv. communication

AGS

SPSRHIC

QGP

hadrons

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SIS-100SIS-300

Dielectron Spectroscopy at the BevalacDielectron Spectroscopy at the Bevalac

The puzzle !

Calculation: E.L.Bratkovskaya et al.Phys. Lett. B445 (1999) 265

Calculation: Ernst et al.Phys. Rev. C58 (1998) 447

1 AGeV

Calculation: C. Fuchs et al. Phys. Rev. C68 (2003) 014904

Observed excess remained unexplained over years

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Spectrometer conceptSpectrometer concept

Spectrometer with large acceptance and high rate capability.

Demanding:1. Strong magnetic field over large volume but

with field free region around the target

2. Compact RICH with sufficient number of UV photons per ring

3. Fast tracking system with high granularity and minimal material budget

4. Real-time trigger on electron candidates by pattern recognition

1994: Funding approved

2001: Commissioning

6 years of R&D and construction

2002: First production run1 m

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Electron IdentificationElectron Identification

1. Spatial correlation :– Ring center - Track– Track - META hit

2. Velocity condition3. Shower condition

– For p > 500 GeV/c

e-

e+

DATA

C + C @ 2AGeV

hadron admixture < 5% (p>1000 MeV/c)!

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The DLS PuzzleThe DLS Puzzle

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Reconstruction of the signalReconstruction of the signal

• Not corrected for acceptance and efficiency• Normalized to the number of produced p0 (Nπo = ½ (Nπ- + Nπ+))

• Comb. Background (CB)Mee < 150 MeV/c2

Like-sign pairsMee > 150 MeV/c2

event mixing

• Signal: – S+- < 150 MeV/c2:

~17780 counts– S+- ≥ 150 MeV/c2:

~ 646 counts

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C-C @ 1 GeV/u: Dielectron yieldC-C @ 1 GeV/u: Dielectron yield

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Large excess above cocktail!Data: HADES collaboration, Phys. Lett. B 663 (2008) 43-48 PLUTO (Monte Carlo Simulation Tool for Hadron Physics): I. Froehlich et al. , arXiv:0708.2382

PLUTO Cocktail:• Experimental data• mt scaling where not available

Cocktail A: 0 + η + ω “long lived components“

Cocktail B: Cocktail A + Δ + ρ

For a comparison of HADES and DLS results the HADES yield has to be extrapolated to full phase space.

mid-rapidity mid-rapidity

HADES

DLS

HADES and DLS Phase Space CoverageHADES and DLS Phase Space Coverage

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DLS Data: R.J. Porter et al.: Phys. Rev. Lett. 79(97)1229

Direct ComparisonDirect Comparison

HADES and DLS data agree !

J. Carroll – presentation

International Workshop on Soft Dilepton Production August 20-22,1997, LBNL

Remaining uncertainties due to extrapolation into the -Dalitz region (blue and cyan triangles).

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C-C @ 2 GeV/u: Dielectron Yield and CocktailC-C @ 2 GeV/u: Dielectron Yield and Cocktail

Cocktail A: 0 + η + ω “long lived components“

Cocktail B: Cocktail A + Δ + ρ

HADES collaboration, Phys. Rev. Lett. 98, 052302 (2007)

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A closer look to the excess yieldA closer look to the excess yield

At SIS energies effectively all pions come from resonance decay (regeneration).

Excess yield scales like 0 !

Hadron dataR. Averbeck et al., TAPS coll., Z. Phys.

A 359, 65 (1997)R. Holzmann et al., TAPS coll., Phys.

Rev. C 56, R2920 (1997)

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New transport results: HSD New transport results: HSD

E. Bratkovskaya and W. Cassing,, Nucl.Phys.A807:214 250,2008

L.P. Kaptari and B. Kämpfer, Nucl. Phys. A 764 (2006) 338

• New treatment of Bremsstrahlung in HSD! – Off-shell transport– Time integration method

• Based on results of a OBE calculation by Kaptari & Kämpfer– pn Bremsstrahlung larger by a

factor 4 compared to what was used in transport before

• Incoherent treatment– Elastic: soft photon approximation– Inelastic: Explicit Resonance

propagation and decay

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Elementary ReactionsElementary Reactions

2 m

7 m

FW

spectator protondeuteron beam

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PLUTO: resonance model!• Teis et al, ZPA356 (97) 421

• angular distrib. from Dmitriev et al, NPA459 (86) 503

• N 1440 ang. distr. from Aichelin, priv. comm.

• decay angle varied from 1+0.6cos² (Wicklund et al. PRD 35 (87) 2670) to 1-3cos²

Exclusive reconstruction of the Exclusive reconstruction of the ee++ee--

1223

N1440 N1535 1600

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Inclusive Dielectron ProductionInclusive Dielectron Production

Strong isospin dependence!

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Inclusive Dielectron Production & HSDInclusive Dielectron Production & HSD

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preliminarypreliminary

Bremsstrahlung in p-p and n-p collisionsBremsstrahlung in p-p and n-p collisions

N

N

N

N

*

N

N

N

N

VMR

*

N

N

N

N

R

*

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contribution is taken out.

Comparison of C-C and elementary collisionsComparison of C-C and elementary collisions

• C-C yield can be explained by properly scaling p-p and d-p results.• No sensitivity on transverse momentum of spectator proton

No evidence for “medium effects” in C-C collisions20Joachim Stroth

preliminary

Vector MesonsVector Mesons

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Vector meson production in p-p at 3.5 GeVVector meson production in p-p at 3.5 GeV

Preliminary (not efficiency corrected, no normalization)Preliminary (not efficiency corrected, no normalization)

e+e (tot~20 nb! )

~ 35 MeV/c2

• reference for p+A (2008)

• Yield excess below // : :

extended VDM? Ch. Fuchs et al., Phys. Rev. C68 (2003) 014904

Mass resolution in the region < 2,7 %

Cocktail: meson multiplicity from HSD 0// (,) = 0.46/0.031/0.011 (meson/inelastic)22Joachim Stroth

Ar-KCl 1.75 GeV/uAr-KCl 1.75 GeV/u

• Preliminary efficiency correction.• Still uncertainty in absolute

normalization with respect to the cocktail (< factor 2).

• Strong excess in mass region M > 0.5 MeV/c2.

• Cocktail: PLUTO

First signal in HI collisions at SIS-18 energies

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The HADES upgrade projectThe HADES upgrade project

• Cope with multiplicities of Au+Au 1.5 AGeV

• Accept up to 20 KHz trigger rate• Measures:

– Replace TOFINO with high-granularity RPC

– Add forward hodoscope– Upgrade DAQ

(new Trigger and Read-out Board)– Extend RICH radiator– Replace plane I tracking chambers

RPC full size prototype successfully commissioned in November 2007.

Expected resolution below 80 ps.

0 5 10 15 20 25 30

20

40

60

80

100

120

140

160

180

200

Cell number

Tim

e d

iffe

ren

ce

sig

ma

/ s

qrt

(2)

(ps)

Left columnCentral columnRight column

85.1 ps

74.9 ps

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SummarySummary

First round of data taking (almost) finished upgrade in 2009!

C-C @1 and 2 GeV/u• Excess yield above long-lived components established• Can be fully attributed to contributions from NN scattering – no “medium

effects” p-p 1.25, 2.2, 3.5 GeV, d-p 1.25 GeV/u

• Strong isospin effect in e+e- from NN scattering• Elastic or inelastic Bremsstrahlung (,N* Dalitz)?

Ar+KCl 1.76 GeV/u• signal in e+e- decay channel observed• K+, K- ,K0, and reconstructed -> go to SQM 2008

p-Nb 3.5 GeV (running just now)• Is there a mass shift or broadening

Bright future• Au+Au, -induced a SIS-18 and later A+A at SIS-100

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GSIGSI

collaboration

G. Agakishiev8, C. Agodi1, A. Balanda3,e, G. Bellia1,a, D. Belver15, A. Belyaev6, A. Blanco2, M. Böhmer11, J. L. Boyard13, P. Braun-Munzinger4, P. Cabanelas15, E. Castro15, S. Chernenko6, T. Christ11, M. Destefanis8, J. Díaz16, F. Dohrmann5, A. Dybczak3, T. Eberl11, L. Fabbietti11, O. Fateev6, P. Finocchiaro1, P. Fonte2,b, J. Friese11, I. Fröhlich7, T. Galatyuk4, J. A. Garzón15, R. Gernhäuser11, A. Gil16, C. Gilardi8, M. Golubeva10, D. González-Díaz4, E. Grosse5,c, F. Guber10, M. Heilmann7, T. Hennino13, R. Holzmann4, A. Ierusalimov6, I. Iori9,d, A. Ivashkin10, M. Jurkovic11, B. Kämpfer5, K. Kanaki5, T. Karavicheva10, D. Kirschner8, I. Koenig4, W. Koenig4, B. W. Kolb4, R. Kotte5, A. Kozuch3,e, A. Krása14, F. Krizek14, R. Krücken11, W. Kühn8, A. Kugler14, A. Kurepin10, J. Lamas-Valverde15, S. Lang4, J. S. Lange8, K. Lapidus10, L. Lopes2, M. Lorenz7, L. Maier11, A. Mangiarotti2, J. Marín15, J. Markert7, V. Metag8, J. Micel7, B. Michalska3, D. Mishra8, E. Morinière13, J. Mousa12, C. Müntz7, L. Naumann5, R. Novotny8, J. Otwinowski3, Y. C. Pachmayer7, M. Palka4, Y. Parpottas12, V. Pechenov8, O. Pechenova8, T. Pérez Cavalcanti8, J. Pietraszko4, W. Przygoda3,e, B. Ramstein13, A. Reshetin10, M. Roy-Stephan13, A. Rustamov4, A. Sadovsky10, B. Sailer11, P. Salabura3, A. Schmah4, R. Simon4, Yu.G. Sobolev14, S. Spataro8, B. Spruck8, H. Ströbele7, J. Stroth7,4, C. Sturm7, M. Sudol4, A. Tarantola7, K. Teilab7, P. Tlusty14, M. Traxler4, R. Trebacz3, H. Tsertos12, I. Veretenkin10, V. Wagner14, H. Wen8, M. Wisniowski3, T. Wojcik3, J. Wüstenfeld5, S. Yurevich4, Y. Zanevsky6, P. Zhou5, P. Zumbruch4

1 Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud, 95125 Catania, Italy2 LIP-Laboratório de Instrumentação e Física Experimental de Partículas , 3004-516 Coimbra, Portugal3 Smoluchowski Institute of Physics, Jagiellonian University of Cracow, 30-059 Kraków, Poland4 Gesellschaft für Schwerionenforschung mbH, 64291 Darmstadt, Germany5 Institut für Strahlenphysik, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany6 Joint Institute of Nuclear Research, 141980 Dubna, Russia7 Institut für Kernphysik, Johann Wolfgang Goethe-Universität, 60438 Frankfurt, Germany8 II.Physikalisches Institut, Justus Liebig Universität Giessen, 35392 Giessen, Germany9 Istituto Nazionale di Fisica Nucleare, Sezione di Milano, 20133 Milano, Italy10 Institute for Nuclear Research, Russian Academy of Science, 117312 Moscow, Russia11 Physik Department E12, Technische Universität München, 85748 München, Germany12 Department of Physics, University of Cyprus, 1678 Nicosia, Cyprus13 Institut de Physique Nucléaire (UMR 8608), CNRS/IN2P3 - Université Paris Sud, F-91406 Orsay Cedex, France14 Nuclear Physics Institute, Academy of Sciences of Czech Republic, 25068 Rez, Czech Republic15 Departamento de Física de Partículas, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain16 Instituto de Física Corpuscular, Universidad de Valencia-CSIC, 46971 Valencia, Spain

Bonus Slides

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W.K. Wilson (DLS coll.), Acta Phys. Pol. 29 (98) 3291W.K. Wilson (DLS coll.), Acta Phys. Pol. 29 (98) 3291

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E. Bratkovskaya et al. Nucl. Phys. AE. Bratkovskaya et al. Nucl. Phys. A

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Needless to showNeedless to show

preliminary

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Comparison to L. Kaptari, B. KComparison to L. Kaptari, B. Kämpfer calculationsämpfer calculations

[Phys. Rev. C 58, 2667 - 2670 (1998), H. Calén et al., “Measurement of the

quasifree pn→pn reaction”]

Black points – dataRed poins – (data –

Joachim Stroth

Efficiency and acceptance corrected Efficiency and acceptance corrected pp pp data,data,comparison to transport model calculationcomparison to transport model calculation

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IQMD

Δ→e+e-N seems to explain e+e- yield in p+p at 1.25 GeV

Joachim Stroth

Efficiency and acceptance corrected Efficiency and acceptance corrected npnp data, data,comparison to transport model calculation (comparison to transport model calculation (npnp cocktail) cocktail)

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IQMD

preliminary

Data are not explained satisfactorily!

preliminary

Joachim Stroth

Efficiency and acceptance corrected Efficiency and acceptance corrected npnp data, data,comparison to transport model calculation (comparison to transport model calculation (dpdp cocktail) cocktail)

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IQMD

preliminary

dp cocktail do not explain data are not explained satisfactorily!

preliminary

Joachim Stroth

Closer look to HSDCloser look to HSD

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An effect of the internal Fermi motion of the nucleons in the deuteron is hardly visible

Joachim Stroth

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