1. Motivation Experimental setup Di-lepton results Future plans
Studying short lived sources of dileptons in baryonic matter with HADES
A. Kugler
ÚJF AV ČR Řež
for HADES collaboration
Motivation
Experimental setup
Di-lepton results
Future plans

2. Relativistic Heavy ion collisions
Three stages of collisions:
first chance collisions ~ elementary collisions of nucleons
compression ~ described by transport models
Expansion and freeze-out ~ yields given by thermal models
Compressed matter
Multistep non-equilibrium production of baryon resonances and of mesons →
yield~volume x time ~Aαpart, α>1 l+ l- l+ l- l+ l-
First chance collisions
Bremsstrahlung, hadron resonances …. yield~Apart
Freeze –out
Decays of long-lived particles (p0, h) produced in thermal chemical equilibrium → yield~volume~Apart 2

3. Experimental study of compressed matter at SIS
moderate densities: max/r0  2
temperature: T < 80 MeV
about 30% of matter in baryonic resonances
pion per baryon about 100 times lower in comparison with SPS regime
Evolution of average ρB
t~10fm/c l+ l-
See [R. Rapp and J. Wambach, rXiv:hep-ph/ v1, fig.4.10] and [J. Zhang and C. Gale, Phys. Rev. C50 (1994) 1617]
Dileptons are ideal penetrating probe in contrast with hadrons, however there are many sources of dileptons
Short lived baryon resonances, ρ meson (t~1.3fm/c)
Long lived neutral mesons (p0, h, w) decaying after freeze-out
Dileptons are produced in
Direct decays -> small branching ratios ~10-5
Dalitz decays-> branching ratios ~10-2
How to reveal contribution from compressed matter?
subtract η- Dalitz ~ subtract long lived contribution
normalize to π yield ~ normalization to Apart
compare resulting experimental data from AA to NN 3

4. High Acceptance Di-Electron Spectrometer
Charge hadrons identification
Momentum measurement by superconducting toroidal magnet &tracking in multi-wire drift chambers ΔM/M in w region ~ 2.5%
velocity: tof ≥7ns dt ~ 100ps in TOF
charge: dE/dx in TOF and/or in MDC
Lepton identification:
hadron blind RICH (gthr > 18.3).
Pre-Shower
GSI Darmstadt
SIS accelerator facility.
(HI beams 1-2 A GeV)
Geometry:
full azimuthal angle,
polar angle 18 – 85 deg
FW polar angle 0,5-7 deg
pair acceptance ~ 35%
L~7 m FW

5. Charge Particle Identification (velocity, dE/dx, momentum)
TOF (44° < Q < 88°)
MDC 5

6. Electron/Positron identification
Selection criteria for single
electrons/positrons:
track is associated with
a Cherenkov ring
2) velocity constraints
3) shower condition
e+e- pair analysis :
close pair cuts
(opening angle > 9 deg )
correction on reconstruction
efficiencies
velocity vs charge times momentum
charge times momentum e+ e-

7. Dilepton spectra normalized to the number of neutral pions
Normalization: Np0 = ½ (Np+ + Np-), ± from the same data sample [HADES:EuroPhys.Journal A40, 45 (2009)]
Systematic errors: ~25%, sM(w) = 9%
“hadronic cocktail”: thermal source, only long-lived components included, i.e. p0, h: TAPS data, w: m scaling. R. Averbeck et. al. PRC 67 (2003)
I. Froehlich et al.,arXiv:
Phys. Lett. B 663 (2008) 43
Phys. Lett. B 690 (2010) 118
Excess of pair yield above h-contribution established
Excess in CC scales with Ebeam like π production!
Excess in First chance collision is very slightly below CC
Measured pair excess in C+C scales with beam energy as p0 production
at SIS energies effectively all pions come from resonance decay
Phys.Rev. Lett 98(2007)

8. Dilepton spectra normalized to the number of neutral pionsx3
preliminary
Phys. Lett. B 690 (2010) 118
Normalization: Np0 = ½ (Np+ + Np-), ± from the same data sample [HADES:EuroPhys.Journal A40, 45 (2009)]
Systematic errors: ~25%, sM(w) = 9%
“hadronic cocktail”: thermal source, only long-lived components included, i.e. p0, h: TAPS data, w: m scaling. R. Averbeck et. al. PRC 67 (2003)
I. Froehlich et al.,arXiv:
Excess of pair yield above h-contribution established
Excess in CC scales with Ebeam like π production!
Excess in First chance collision is very slightly below CC
Significant increase of excess in ArKCl
Hinting at Δ resonance decays
Measured pair excess in C+C scales with beam energy as p0 production
at SIS energies effectively all pions come from resonance decay

9. Excess above eta contribution ~ Ebeam ~Apart
over η
π0 and η yields have different scaling with Ebeam
excess scales with Ebeam like π production
π and η yields scales with Apart ~ linear
excess scales with Apart stronger than linear
baryon resonances involved ?
multistep processes involved ?
role of vector mesons ?
π0 and η from TAPS
(min. bias)
We estimate that
pair excess ≈ <Apart>1.4
e+e- pair excess:
HADES
DLS

10. Transport models and HADES HI data Example: HSD transport model for Ar+KCl
Refinement of HSD model
the vector-meson production via the LUND string has been reduced in order to mach pp exp. data better
- the isospin dependence for the channels NN-> V+NN and pi+N-> V+N have been improved
Prediction of HSD model
E.L. Bratkovskaya, W. Cassing,
Nucl.Phys.A 807 (2008)

11. Vector mesons / at SIS
p+(p or 92Nb) at 3.5 GeV
40Ar+ 38KCl AGeV
Spectra normalized to π0 yield
Preliminary
Preliminary
First observations of  production at SIS energies
p+Nb vs. p+p: no change in the ω width, position!
no hints for broadening due to medium!
momentum distribution of ω ?

12. Vector mesons / at SIS
p+(p or 92Nb) at 3.5 GeV
40Ar+ 38KCl AGeV
preliminary
preliminary
~460 counts
σ(ω) = 17.6 MeV/c2
Preliminary
40 counts
MLVL1(ω) = (6.7±2.7) ·10-3
35% of pairs with p < 800 MeV/c in ρ/ω mass range!
First observations of  production at SIS energies
p+Nb vs. p+p: no change in the ω width, position!
no hints for broadening due to medium!
momentum distribution of ω ?

13. Vector mesons / γA → ρ X, CLAS JLab p+(p or 92Nb) at 3.5 GeV
preliminary
~460 counts
σ(ω) = 17.6 MeV/c2
Preliminary
Fig. from S.Leupold et al., nucl-th
35% of pairs with p < 800 MeV/c in ρ/ω mass range!
First observations of  production at SIS energies
p+Nb vs. p+p: no change in the ω width, position!
no hints for broadening due to medium!
momentum distribution of ω ?

14. Tagging quasi-free np reactions with spectator proton with ~pd/2 in FW
FW q < 7o d
psp p pr
Spectator proton momenta at different FW angles
2 < q
2 < q < 4
4 < q < 6
FW operational since April 2007
288 scint. installed ( )
acceptance 0.50 << 70
velocity: dt : 90 – 140 ps , L~7 m
charge ~ dE/dx

15. First chance collisions in HADES
Efficiency corrected data
Quasi-free np in contrast to inclusive dp (DLS) !!!
p0 yield in n+p larger by factor ~2 as known from isospin selection rules!
Not trivial isospin dependence for Mee>0.14GeV/c2 (goes far beyond factor 2)
Very different mass shape above p0 Dalitz decay
Large excess in n+p compared to p+p reactions:
role of Fermi momentum (off-shell propagation of VM)?
NN Bremsstrahlung?
∆ production?
Phys. Lett. B 690 (2010) 118

16. Comparison of pp and np runs with model
Model Calculations:
a) NNNNe+e- OBE calculations Kaptari & Kämpfer (K&K) NPA 764 (2006) 338
b)  , yield constraint by data.  Dalitz decay Krivoruchenko et al. Phys. Rev. D 65 (2001) VMD form- factor (Q. Wan and F. Iachello, Int. J. Mod. Phys. A 20 (2005) 1846)
pn data are not described by model calculations !

17. HADES Experiments November 2002:C+C 2 AGeV [1,4]
February 2004: p+p 2.2 GeV
August 2004: C+C 1 AGeV [2,4]
October 2005: Ar+KCl 1.75 AGeV [3,6,7]
May 2006: p+p 1.25 GeV [5]
April-May 2007: p+p 3.5 GeV, d+p 1.25 GeV [5]
September-October 2008: p+Nb 3.5 GeV
[1] Phys. Rev. Lett. 98, (2007)
[2] Phys.Lett. B 663, 43 (2008)
[3] Filip Krizek, Nuclear Physics, A 830 (2009), pp. 483
[4] Charged pions in C+C paper: EPJ A40 No1, 45 (2009)
[5] Phys. Lett. B 690 (2010) 118
[6]  in Ar+KCl paper: Phys.Rev. C80 (2009)
[7] - in Ar+KCl paper: Phys. Rev. Lett. 103(2009)132301

18. Future plans HADES upgrade
Resistive Plate Chambers , 120 << channels, t85 ps
New DAQ electronics ~20kHz LVL1
Ag+Ag at 1.65 AGeV a Au+Au at 1.25AGeV,
move to SIS100 (FAIR), E ~ 8 AGeV
experiments at SIS100 (FAIR) cca 2015
CBM
8 – 45 GeV/u
HADES
2 – 8 GeV/u

19. FAIR@Darmstadt Applied- and atomic physics - APPA HADES CBM 2011-2016
SIS 100/300
SIS 18
UNILAC
HADES
CBM
Super
FRS
HESR CR
Module 0:Heavy-Ion Synchrotron SIS100
Module 1:HADES/CBM, APPA, and detector calibration.
FLAIR
Module 2:Super-FRS →RIB for NUSTAR.
Module 3:Antiproton facility for PANDA
RESR
NESR
100 m

20. Simulation of Dilepton production at FAIR
CBM: Central
HADES: AGeV

21. Conclusion
Excess of dilepton yield over “freeze-out” contribution observed
scales with Ebeam like π production
scales with size of compressed zone like Apart,  ~1.4
Comparison with transport models done
dilepton and strangeness HADES HI data can be described by refined transport models
model description of HADES p,d-induced data still needs significant effort
HADES experimental data indicate that:
multi-step non-equilibrium processes play substantial role in production of vector mesons and strangeness particles
 No medium effects observed in ω production production

22. The HADES Collaboration
Cyprus:
Department of Physics, University of Cyprus
Czech Republic:
Nuclear Physics Institute, Academy of Sciences of Czech Republic
France:
IPN (UMR 8608), Université Paris Sud
Germany:
GSI, Darmstadt
FZ Dresden-Rossendorf
IKF, Goethe-Universität Frankfurt
II.PI, Justus Liebig Universität Giessen
PD E12, Technische Universität München
Italy:
Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud
Istituto Nazionale di Fisica Nucleare, Sezione di Milano
Poland:
Smoluchowski Institute of Physics, Jagiellonian University of Cracow
Portugal:
LIP-Laboratório de Instrumentação e Física Experimental de Partículas
Russia:
INR, Russian Academy of Science
Joint Institute of Nuclear Research
ITEP
Spain:
Departamento de Física de Partículas, University of Santiago de Compostela
Instituto de Física Corpuscular, Universidad de Valencia-CSIC
17 institutions members

23. Thank You

24. Backup Slides

25. Excess pair yield in C+C: the DLS puzzle
HSD UrQMD
Calculation: E.L.Bratkovskaya et al. Phys. Lett. B445 (1999) 265
Calculation: C. Ernst et al.
Phys. Rev. C58 (1998) 447
DLS Data: R.J. Porter et al.: Phys.Rev.Lett. 79 (1997) 1229
DLS experiment at the Bevalac
1987 – 1993
strong pair excess observed
(excess over cocktail
of known sources)
challenge for transport models
(until recently, no more now?)
DLS Puzzle – Validity of …
experimental data?
theoretical description?

26. HADES vs. DLS: a direct comparison
Project HADES data into DLS acceptance and compare …
J. Carroll at
International Workshop on Soft Dilepton Production August 20-22,1997, LBNL
DLS Data: R.J. Porter et al.: PRL 79 (1997) 1229
 HADES and DLS data agree !

27. Excess above h Assuming excess sources have
similar geometrical acceptance as h:
Multiplicity of h Dalitz decays
contributing to GeV/c2
Extrapolation to 4p

28. Tools for the analysis of pp/pn reactions at E=1.25 GeV
Backbone = PLUTO ROOT-based event generator
I.Fröhlich et al, arxiv: ; I. Fröhlich et al,arxiv:
Inputs:
Full OBE differential cross sections :
R. Shyam & U.Mosel, PRC67 (2003)
L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338
Resonance model:
NNN d/dt according to Dmitriev et al.
(pp→p+) = 6 mb ~ 3/2 (pp→pp° ) , (pn→n+) = 12 mb ~ 3/2 (pn→pn° )
Dalitz decay from QED calculation
a) GM=3
b) Iachello GM (q2) D+
GM(q2 ) p e+
e-- *
*e+e-
d/d~(1+cos2)
Béatrice Ramstein, IPN Orsay, France

29. Tagging quasi-free np reactions in HADES
Spectator proton momenta at different FW angles
Spectra measured at very small FW angles shows same pair excess!
2 < q
2 < q < 4
4 < q < 6

30. Charged hadrons: pion production:
C + C 1AGeV
Yields (LVL1) extrapolated to 4:
1 AGeV
2 AGeV
HADES
N< -+>/Apart = ± 0.009
Apart =8.6 <b>=3.0 fm
N< -+>/Apart = ± 0.023
Apart =8.4 <b>=3.2 fm
Normalization of dilepton yields
Nπo = ½ (N π- + N π+), Syst. error =11%
HADES: EuroPhys.Journal A40, 45 (2009)

31. Model Calculations (long lived sources) compared to data: 1
Model Calculations (long lived sources) compared to data: 1.76 GeV/u Ar+KCl
Mean number of participating nucleons in trigger evts:
<Apart>  38.5 ± 4
( x3 compared to C+C)
Again, strong overshoot above the h Dalitz decay !
First ω peak seen at SIS energies ! ( 40 pairs)
MLVL1(ω) = (6.7±2.7) ·10-3
Preliminary

32. Comparison of HSD model and HADES data
ok for Mee< 0.6 GeV
overshoot ω/ρ peak
Are in-medium effects needed?
Not clear yet…