1. Dielectron Spectroscopy
at 1-2 AGeV with HADES
Stefano Spataro
for the collaboration

2. Probes for nuclear matter
Physics Motivations
Heavy-ion collisions at
relativistic energies (12 AGeV) p n
++  K 
T0, B0 r e+ e-
Hadron properties inside the nuclear matter
M, 
Light vector mesons
Probes for nuclear matter
2.9·10-4
K+K- 44
1019 
7.1·10-5
 +- 0 23
783 
4.5·10-5
 
1.3
769 
Branching
ratio e+e-
Main
decay
Mean life
[fm/c]
Mass
[MeV/c2]
Mesons
LEPTONS
NO strong final interaction
Very low probability

3. Physics Motivations CERES/NA45 DLS Models failed to explain data
DLS Ca+Ca 1AGeV
full spectral function
R.Rapp et al., Nucl. Phys.. A617 (1997) 472
DLS C+C 1AGeV
Models failed to explain data
0.15 < M < 0.4 GeV/c2
Explained by medium effects

4. The HADES Spectrometer
Physics GSI
p+p, p+A, +p, +A, 18 AGeV
Geometry
Six sectors on a hexagonal geometry:
2 (azimuthal angle)
18° <  < 85° (polar angle)
Side view
Tracking
Superconducting Magnet (Bmax=0.7T)
24 multi-wire drift chambers
sy  100 mm, Me+e/M1.5% at  mass
Particle identification
RICH: Cherenkov detector
TOF (>45°): tof ~ 150ps
TOFINO (<45°): tof ~ 450ps
Pre-Shower: 18 pad chambers
Online dilepton trigger

5. HADES Experimental Runs
Heavy-Ion collisions
in-medium effects of vector mesons at moderate baryon densities
Commissioning and physics run
Target: 2 x 2.5% interaction length
Low resolution  mass)
650 Mevents
AGeV
November 2002
Published:
PRL 98 (2007)
AGeV
August 2004
Target: 3 x 2% interaction length
2500 Mevents
~ DLS energy
analysis
almost completed
AGeV
September 2005
Target: 4 x 1.5% interaction length
1200 Mevents
ongoing
analysis

6. C+C @ 2 AGeV PRL 98 (2007) 052302 Event generator PLUTO :
Normalization: ½(+ + -) yield
M < 150 MeV/c2: ~ counts
M ≥150 MeV/c2: ~ counts
(Mee) ~ Mee~0.8 GeV/c2
Event generator PLUTO :
thermal source (T=80MeV)
 polar angle distribution from charged  analysis
η (TAPS data)
ρ, ω: m -scaling
Δ scales with 
Cocktail A: ° + η + ω (long lived)
systematic errors:
15 % - efficiency correction
10 % - combinatorial background
11 % - 0 normalization
18%
21%
Cocktail B: A + Δ(Ne+e-) + ρ (short lived)

7. C+C @ 1 AGeV Experimental data Event generator PLUTO :
Normalization: ½(+ + -) yield
M < 150 MeV/c2: ~ counts
M ≥150 MeV/c2: ~ counts
(Mee) ~ Mee~0.8 GeV/c2
Event generator PLUTO :
thermal source (T=55MeV)
 polar angle distribution from charged  analysis
η (TAPS data)
ρ, ω: m -scaling
Δ scales with 
PRELIMINARY
systematic errors:
10 % - efficiency correction
10 % - combinatorial background
11 % - 0 normalization
22–25 %
Cocktail A: ° + η + ω (long lived)
Cocktail B: A + Δ(Ne+e-) + ρ (short lived)

8. + Comparison 1 and 2 AGeV Yield above cocktail A (long lived)
PRELIMINARY
Agreement below 150 MeV/c2
Large relative excess at higher masses
(same shape)
Neutral Meson production (TAPS) +
Pair excess in HADES/DLS
Excess (beyond ) scales like °

9. Ar+KCl @ 1.75 AGeV Lepton analysis: ongoing
PRELIMINARY
Lepton analysis: ongoing
Hadronic analysis: strangeness
30% total statistics
PID: also dE/dx in MDC
Background: mixed event (Ks0,)
PRELIMINARY
PRELIMINARY

10. HADES Experimental Runs
Elementary collisions
exclusive study of dilepton sources
January 2004
Target: 5cm LH2
600 Mevents
analysis
almost completed
 production
pp  pp
Dielectron efficiency
April 2006
3000 Mevents
analysis ongoing
form factor and
BR unmeasured
+ Dalitz decay
pp  p+  ppe+e-
April 2007
analysis just started
Vector mesons direct decay
pp  pp(0) ppe+e-
pn brehmsstrahlung

11. p+p @ 2.2 GeV Verify dielectron efficiency
Target: LH2 (5 cm length)
Beam: 107 p/second
Verify dielectron efficiency
Hadron and dielectron channel for 
EXP: ± 1.8 (stat)
SIM: ± 0.9 (stat)
total
signal
background
M 548 MeV/c2
M MeV/c2
/M %
Hadronic channel  ° bg
M MeV/c2
M MeV/c2
/M %
M° MeV/c2
M MeV/c2
/M %
Electromagnetic channel
 yield
2322  118
24845  348
° yield
7580  454
pppppp+-°
ppppppe+e- 

12. p+p @ 2.2 GeV:  Electromagnetic Form Factor
QED
+-
G.Landsberg, Phys.Rep.128 (1985) 301
 form factor known
check of invariant mass spectrum
preliminary
ppppppe+e- 
 form factor: e+e- (~100 counts)
Phys. Lett. B 59 (1975)
Phys. Lett. B 504 (2001) 275
More statistics expected with 3.5 GeV

13. p+p / n+p @ 1.25 GeV Evidence for additional sources  Form factor ?
PRELIMINARY
n + p, 1.25 GeV
e+e- > 90
ONLINE: ~50% available statistics
Excess above cocktail
~2 (pp), 4-5 (np)
Event Generator PLUTO:
°e+e-, +pe+e-
+ fixed to ° via N(+)=3/2 N(°)
Evidence for additional sources
 Form factor ?

14. preliminary online spectrum
3.5 GeV (april 2007)
preliminary online spectrum
Online spectrum
~70% statistics
 peak visible (~35 MeV/c2)
excess below  pole mass
 line shape
Very important reference
for p+A
(next year)

15. Summary and Outlook Runs p, d, p - induced A + A Status/Comment 2002
C + C 2 AGeV
Published!
2004
C + C 1 AGeV
analysis finished
p + p 2.2 GeV
2005
Ar + KCl 1,75 AGeV
analysis ongoing
2006
p + p 1.25 GeV
2007
p + p 3.5 GeV
d + p 1.25 AGeV
… online results
2008
p + A 3.5 GeV
2008/9
Upgrade RPC, DAQ
2009
Ni + Ni
Planned
2010
 + N, A
2011
Au + Au
> 2011
Hades goes FAIR (8 AGeV)

16. Backup slides

17. The HADES Spectrometer
Averaged over 0 < Y < 2
Opening angle > 9°
PAIR ACCEPTANCE

18. Electron identification
Track matching
Pre-Shower condition
RICH pattern e- + + e- e+
velocity vs. momentum
Data
Monte Carlo e- e+
Momentum * charge [MeV/C]

19. C+C @ 2 AGeV Combinatorial background: Same event like-sign pairs CB =
Mixed event opposite sign pairs

20. C+C @ 2 AGeV: comparison with models
RQMD: M. D. Cozma, C. Fuchs, E. Santini, A. Faessler, Phys. Lett. B 640, 150 (2006)
UrQMD: D. Schumacher, S. Vogel, M. Bleicher, nucl-th/
HSD (v2.5): W. Cassing and E. L. Bratkovskaya, Phys. Rep. 308, 65 (1999).
“free” spectral function
Closer to data than cocktail B
But model calculations
Undershoot between 200<Mee<500 MeV/c2
Overshoot for Mee> 700 MeV/c2

21. Hadron analysis (2 AGeV)
Anisotropy
PRELIMINARY
PRELIMINARY π+ π-
1 AGeV **)
.137 ± .015
.069 ± .008
N<- +>/Apart
2 AGeV *)
Used for the normalization of dilepton spectra, extrapolated to 4
*) 11% syst, error, in agreement with KaoS, TAPS **) preliminary

22. collisional broadening of ω ( Г(ω) = 200 MeV)
Calculation: K. Shekhter, C. Fuchs et al. (Tübingen)
Phys. Rev. C68 (2003)
collisional broadening of ω ( Г(ω) = 200 MeV)
extended VDM + decoherence
BR scaling of VM

23. HADES/DLS comparison PRELIMINARY PRELIMINARY
J. Carroll – International Workshop on Soft Dilepton Production August 20-22,1997, LBNL
DLS Data: R.J. Porter et al.: PRL 79(97)1229

24. P┴ Y Mee < 150 MeV/c2 150 < Mee< 550 MeV/c2
• Data
- Simulation