1. Outline Introduction Simulations & Stategy p+p and p+n at 1.25 GeV
René Magritte: Le Château des Pyrenées, 1959

2. e+e- invariant mass spectrum
History: DLS-Puzzle
e+e- invariant mass spectrum
Excess hard to understand (even present in light C+C system)
So-called „DLS-puzzle“
HADES
signal pairs
Data: R.J. Porter et al.: PRL 79(97)1229
Model: E.L. Bratkovskaya et al.: NP A634(98)168, BUU, vacuum spectral function

3. HADES Layout Geometry Full azimuthal coverage, polar angles 18o - 85o
Particle identification
RICH: CsI solid photo cathode, C4F10 radiator
TOF
TOFino, (RPC in future)
Pre-Shower
Momentum measurement
Superconducting toroid
MDCs: 24 multi-wire drift chambersOnline event selection (level-2 trigger)
1 m

4. Normalization via charged pions [2]
1AGeV [1]
Normalization via charged pions [2]
Simulation Pluto
Thermal emission of particles
No „bremsstrahlung“
Model A: long-lived sources: p0, h, w
Model B: with short-lived resonances r,D
p0 region well under control
Enhancement in h region
Mee [GeV/c²]
[1] G. Agakichiev et al, PLB 663 (2008) 43 [2] G. Agakichiev et al, EPJA 40 (2009) 45

5. HADES vs DLS DLS data confirmed by HADES Conclusion: No „DLS“ puzzle
…but maybe still a „theory“ puzzle)
What else could explain the enhancement?

6. The „New“ Bremsstrahlung
NN ("quasielastic")-non resonant
E.L. Bratkovskaya and W. Cassing arXiv: v1
K&K OBE calculation:
pn bremsstrahlung 4 larger
collection of results from E.L Bratkovskaya & W. Cassing: Nucl.Phys A 807, 214 (2008).
bremsstrahlung OBE calculations:
Kaptari & Kämpfer, NPA 764 (2006) 338:

7. Example: HSD [1] for 1 AGeV
Does larger pn Bremsstrahlung [2] explain excess?
Answer: Yes!
[1] HSD: E.L. Bratkovskaya and W. Cassing arXiv: v1 and private communication [2] L.P. Kaptari and B. Kämpfer, Nucl.Phys. A 764 (2006) 338 [3] DLS Data: R.J. Porter et al. Phys.Rev.Lett. 79 (1997) 1229
M [GeV/c²]

8. pn + D : Coherence Calculation [1] of NN bremsstrahlung + D
(simplified picture!)
Calculation [1] of NN bremsstrahlung + D
Factor 4 larger then previous calculations
Impact of D to HI dileptons?
[1] Kaptari, Kämpfer, NPA 764 (2006) 338
D term only
elastic term only

9. The „Light“ from Dense Matter
, = nucleons
Heavy ion reactions
SIS18/GSI: 1-2 AGeV
Dense matter
Large contribution of resonances
Dalitz decays:
p0/h→ge+e-
w→p0e+e-
D→Ne+e-
N*→Ne+e-
, = resonances e- e+ r g*
10-12 fm/c

10. OBE vs. D Dalitz
„Classical“ D Dalitz decay via resonance model, e.g. transport
[1] Shyam/Mosel, PRC 79 (2009) , „Di-lepton production in N-N collisions revisited“
arxiv:

11. Form Factors Proton form factor Important for HI data Example: +
2 component model [1]
Extended to inelastic channel [2]
1- = VMD part + e- r D p
[2] Iachello/Jackson/Lande, PL 43B (1973) 191 [3] Wan & Iachello, IJMP A20 (2005) 1846, and priv. comm.

12. N-D Transition Form Factor
Extrapolation to time-like region [1]:
Important for di-electrons in medium
NN → NNe+e- must be understood +
[1] Wan & Iachello, IJMP A20 (2005) 1846, and priv. comm.
VMD part e- r e+ D p r g* e-
[2] Schäfer etal, NPA 575 (1994) 429
= D resonance

13. Angular Distributions of Virtual Photons
production plane of source X X e+ g* fe ee
virtual photon
plane
qg*
fg* e-
dilepton decay qe
„Bonus“:
Production angles
Di-Electron decay angles
Usually integrated out
Could this add constraints?
(e.g. resonances)?

14. Open Points D Dalitz decay (D → N e+e-) unmeasured+ p e-
q2 = M2inv(e+e-) = M*2 > 0 *
D Dalitz decay (D → N e+e-) unmeasured
Decay rate (mass-dependent branching ratio)
Form factor
The „bremsstrahlung“
+ → pe+e-
Not measured
Dalitz decay

15. NN HADES experiments p+p: d+p: Beam energy Ebeam = 1.25 GeV LH2 targetFW
p_spec pc
p_c
Side View
Beam energy Ebeam = 1.25 GeV
LH2 target
RICH, MDC+magnet, TOF/SHOWER
p+p:
one week of running in April 2006
~2.6*109 LVL1 events collected (MUL=>3 trigger)
d+p:
two weeks of running in April 2007
~4.8·109 LVL1 events collected (MUL=>2 && FW "p spectator") tag on np -> e+e- X reactions

16. Models & simulation methods
Results
HADES data preliminary
HADES data preliminary
pp data : Mee < 150 MeV/c2: counts, Mee ≥ 150 MeV/c2: counts np data : Mee < 150 MeV/c2: counts, Mee ≥ 150 MeV/c2: counts
Models & simulation methods
Large excess in n+p reactions compared to p+p reactions
Very different mass shape

17. What is Pluto? (The ancient god of the underworld)
ROOT-based event generator
fast simulations (typically evts/min)
Designed (originally) for SIS,
but used also by CBM, PANDA, COSY
Main focus:
Electromagnetic decays,
e.g. h → ge+e-
Sampling of resonances (like D) with mass-dependent width
Open for extensions / „plug-ins“ [1]
[1] IF et al, arxiv: ; IF et al, arxiv:

18. What is Pluto? (The Stategy)
No transport, not a model
A „converter“
functions  particle tracks
Numerical realization
Two modi:
Direct virtual photons, pp→ppg*
Dalitz decays (multi-step pp→pD→ppg*)
Acceptance filter
No preferred model/modus
Information at one single place

19. Numerical Realization: Monte-Carlo Integration
Di-Electron generator E.g, pp→ppg*
1/Nev *
Generator
(flat di-electon distr.)
Physics Model
(Virtual Bremsstrahlung)
=Mean
(of the model) Dm
Taking dynamic range of generator into account: normalized
NO Scaling to b.r.!!!

20. D Dalitz Decay Decay rate from [1] Available form factor(s):
QED transition amplitudes (M1)
F. Iachello‘s 2-component model
Integration
[1] Krivoruchenko et al, PRD 65 (2002) , J. van der Wiele, priv. comm.
Mass-dependent B.R.

21. Invariant Mass Spectra from D Dalitz
Di-Electron mass distribution at different D masses
Crucial!
mD=
VMD
1.8, , GeV
D mass and
branching ratio
QED
VMD: Wan & Iachello, IJMP A20 (2005) 1846, and priv. comm.

22. Broad Resonances 2-step process pp→pD→ppg* Branching ratio
Together with M.C. integration:
Mass-dependent b.r. drops out “for free”
Avoid nasty samplings and re-normalization
Changes event-by-event („breathing“)
Use full spectral shape in 1st step

23. Results for pp at 1.25GeV D cross section from resonance model [1]
VMD and QED
OBE model from Shyam/Mosel [2]
Kaptari/Kämpfer D term
Shyam/Mosel D term
Pluto D (VMD)
Pluto D (QED)
[1] Teis et al. ZPA 356 (1997) 421
[2] Shyam/Mosel, PRC 79 (2009) and priv. comm.

24. Latest pp Results!
Solid lines: R. Shyam, priv. comm. (dat files provided last week)
Dashed lines: Kaptari/Kämpfer
Dotted line (D only): Pluto with VMD
p + p, 1.25 GeV
Sum
D Term
Elastic term
N1520
No FSI

25. Data normalized via pp elastic scattering
Models & Data
Model from Kaptari/Kämpfer Model from Shyam/Mosel
HADES data
preliminary
HADES data
preliminary  
Data normalized via pp elastic scattering

26. Results for pn at 1.25GeV Kaptari/Kämpfer D term Shyam/Mosel D term
Pluto D (VMD)
Pluto D (QED)

27. Fermi Momentum Impulse approximation Breathing of p.s.:
Additional „jittering“ of Dmee
Has been taken into account
dp→ND (p)
dp→pnp0p0(p)
dp→pnh(p) [1]
dp→dh(p) [2]
[1] P. Moskal et al, PRC 79 (2009)
[2] H. Calen et al, PRL 79 (1997) 2642

28. dp Results Solid lines: R. Shyam, priv. comm.
Dashed lines: Kaptari/Kämpfer
Dotted line (D only): Pluto with VMD
d + p, 1.25 AGeV
Sum
D Term
Elastic term
N1520

29. Models & Data for dp
Model from Kaptari/Kämpfer Model from Shyam/Mosel

30. Conclusion HADES has confirmed DLS C+C data
Bremsstrahlung has been proposed to be the solution
pp data
No large „bremsstrahlung“ contribution
„Pluto“ D (with VMD) and S&M D are OK
Hint of a VMD-like transition?
pn data not consistent with any calculation
We are open for discussion & new ideas

31. The Collaboration Ileana Iori (†), Helmut Bokemeyer (†)
Bratislava (SAS, PI), Slovakia
Catania (INFN - LNS), Italy
Cracow (Univ.), Poland
Darmstadt (GSI), Germany
Dresden (FZR), Germany
Dubna (JINR), Russia
Frankfurt (Univ.), Germany
Giessen (Univ.), Germany
Milano (INFN, Univ.), Italy
Munich (TUM), Germany
Moscow (ITEP,MEPhI,RAS), Russia
Nicosia (Univ.), Cyprus
Orsay (IPN), France
Rez (CAS, NPI), Czech Rep.
Sant. de Compostela (Univ.), Spain
Valencia (Univ.), Spain
Coimbra (Univ.), Portugal
in particular
Ileana Iori (†),
Helmut Bokemeyer (†)