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

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 2841+- 82 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

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

Normalization via charged pions [2] C+C @ 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

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

The „New“ Bremsstrahlung NN ("quasielastic")-non resonant E.L. Bratkovskaya and W. Cassing arXiv:0712.0635v1 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:

Example: HSD [1] for 1 AGeV Does larger pn Bremsstrahlung [2] explain excess? Answer: Yes! [1] HSD: E.L. Bratkovskaya and W. Cassing arXiv:0712.0635v1 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]

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

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

OBE vs. D Dalitz „Classical“ D Dalitz decay via resonance model, e.g. transport [1] Shyam/Mosel, PRC 79 (2009) 035203, „Di-lepton production in N-N collisions revisited“ arxiv:0811.0739

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.

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

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)?

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

NN HADES experiments p+p: d+p: Beam energy Ebeam = 1.25 GeV LH2 target FW 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

Models & simulation methods Results HADES data preliminary HADES data preliminary pp data : Mee < 150 MeV/c2: 30032 counts, Mee ≥ 150 MeV/c2: 358 counts np data : Mee < 150 MeV/c2: 41342 counts, Mee ≥ 150 MeV/c2: 1777 counts Models & simulation methods Large excess in n+p reactions compared to p+p reactions Very different mass shape

What is Pluto? (The ancient god of the underworld) ROOT-based event generator fast simulations (typically 100000 evts/min) Designed (originally) for HADES @ 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:0708.2382; IF et al, arxiv:0905.2568

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

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.!!!

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) 017502, J. van der Wiele, priv. comm. Mass-dependent B.R.

Invariant Mass Spectra from D Dalitz Di-Electron mass distribution at different D masses Crucial! mD= VMD 1.8, 1.5, 1.232 GeV D mass and branching ratio QED VMD: Wan & Iachello, IJMP A20 (2005) 1846, and priv. comm.

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

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) 035203 and priv. comm.

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

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

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

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) 015208 [2] H. Calen et al, PRL 79 (1997) 2642

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

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

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

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 (†)