Motivations - HADES C+C 1 AGeV, 2 AGeV - measured excess: beam energy dependence p+p, d+p 1.25 (A)GeV - Studies of NN-Bremsstrahlung and  Dalitz decay - N+N (p+p & d+p) to C+C comparison Dielectron production at 1-2 AGeV with HADES Motivations - HADES C+C 1 AGeV, 2 AGeV - measured excess: beam energy dependence p+p, d+p 1.25 (A)GeV - Studies of NN-Bremsstrahlung and  Dalitz decay - N+N (p+p & d+p) to C+C comparison Witold Przygoda, Jagiellonian University, Cracow, for the HADES Collaboration International Conference on Particles And Nuclei PANIC 2008, Eilat Israel Dan Eilat Hotel Big Blue Hall A 10 Nov 2008

2 The case of moderate beam energies t [fm/c] ≈ 10 fm/c comparatively long life-time...at moderate densities c  fm/c Baryonic matter (pions less than 10%)  t(   > 2) ≈ 10 fm/c In-medium invariant mass reconstruction Combinatorial background reduction Strategy: systematic e + e – spectroscopy in proton, pion and heavy ion induced reactions Goal: learn about radiation from baryonic rich matter Meson production at or below threshold : co-operative processes (i.e. multi step processes) off-shell effects elementary sources not known well (i.e. baryon resonance decays)

3 High Acceptance Di-Electron Spectrometer  Installed at the SIS18 GSI Darmstadt (Germany) Beams: pions, protons, nuclei  Spectrometer with high invariant mass resolution (2% at  /  mass)  Utilizes dedicated second level trigger processors to select rare events before mass storing Geometry Full azimuth, polar angles 18  - 85  ( y = 0 – 2 ) Pair acceptance  0.35 ~ channels, segmented solid or LH 2 targets Side View START 1 m

4 Experimental (production) runs p+p 1.25 GeV (2006) 2.2 GeV (2002) 3.5 GeV (2007) d+p C+C 2.0 AGeV (2002) 1.0 AGeV (2004) 1.25 GeV (2007) Ar+KCl 1.75 GeV (2005) p+Nb 3.5 GeV (2008) p+p „Anomalous” excess of dilepton pairs in 0.15 < M ee < 0.5 GeV/c 2 NN-Bremsstrahlung and  Dalitz decays Vector meson  /  /  production p+p Calibration & verification measurement of rare electromagnetic decays  +p/A Ni+Ni/Au+Au ( ) FUTURE

5 log. z axis ! e-e- e+e+ hadron : lepton suppression ~10 -4 Analysis strategies Single electron analysis 2-dim cuts on RICH rings, Shower, p vs , hit matching track fitting quality e+e- pair analysis close pair cuts opening angle  > 9° e-e-e-e- e+e+e+e+  M inv = p1p1 p2p2 Target RICH HADRONS Momentum vs velocity (β) dE/dx in MDC & TOF LEPTONS Corrections for detector and reconstruction efficiencies  acceptance & reconstruction efficiency filters available

6 PLUTO event generator (HADES Collaboration) COCKTAIL A (thermal freezout)  0 and  well known  from m T scaling 1 and 2 AGeV - mass spectrum eff. corrected Phys.Rev.Lett. 98 (2007)  (M ee )~9% at M ee ~0.8 GeV/c 2 Phys. Lett. B 663 (2008) 43 in HADES acceptance syst. error (21%) syst. error (27%) COCKTAIL B : COCKTAIL A + short lived  0 and  NORMALIZATION I. Fröhlich et al. (2007) arXiv: [nucl-ex]

7 C+C – pair excess (Y exc ) : scaling with E beam Excess function F(energy) -  region TAPS 0.3(  sys) F(2.0) = 1.9 ± 0.2(stat) ± 0.3(sys) ± 0.3(  sys) 2.0(  sys) F(1.0) = 6.8 ± 0.6(stat) ± 1.3(sys) ± 2.0(  sys)  HADES and DLS resuls agree  Y exc scales with E beam like pion production R. Averbeck et al. Z. Phys. A 359 (1997) 65 R. Holzmann et al. Phys. Rev. C 56 (1997) R2920 TAPS Collaboration

8 N-N Brehmsstrahlung +  Dalitz decay Collection of results on NN-Bremsstrahlung from: New OBE calculations NN Bremsstrahlung is larger than predicted by SPA pp Bremstrahlung is smaller than pn at ~1 AGeV Kaptari & Kämpfer Nucl. Phys. A 764 (2006) 338 (thick line: propagator of nucleon / baryon resonance) E.L. Bratkovskaya et al. Nucl. Phys. A 807 (2008) 214

9 Recent (>2006) transport calculations M. Thomère et al. Phys. Rev. C 75 (2007) E.L. Bratkovskaya et al. Nucl. Phys. A 807 (2008) 214 need more constrains from measured NN collisions IQMD HSD C+C 2 AGeV NN-Bremsstrahlung adjusted to OBE-calculations ”old” SPA NN-Bremsstrahlung …do they unambiguously explain dilepton excess? BRoBUU H.W. Barz, B. Kämpfer, Gy. Wolf, M. Zetenyi (2006) nucl-th/ v3 see also

GeV -  Dalitz decay  Dalitz decay e+e+ ++ e-e- p Time-like  - N transition  branching ratio?  electromagnetic form factors? q 2 = M 2 inv (e + e - ) = M  * 2 > 0 ** Krivoruchenko et al. Phys. Rev. D 65 (2001) grey band Iachello VMD QED a) parameterization a-la Kaptari & Kämpfer + NN Final State Interactions + isotropic  * production preliminary Q. Wan & F. Iachello Int. J. Mod. Phys. A 20 (2005) 1846 b)  Dalitz decay : a-la Krivoruchenko (QED) + VMD form-factor PLUTO calculations

AGeV – quasifree p-n collisions selection of np reactions by measurement of spectator proton in Forward Wall spectator " tag " Mult.  1 FW (0.5  –7  ) contribution of p+p in p+n after FW selection less than 12% (M M  0 ) a) sum of np-Bremsstrahlung a-la Kaptari & Kämpfer,  and  Dalitz decays b) Fermi momentum of nucleon in deuterium from N-N Bonn potential deuteron spectator proton proton reaction proton e + e – X FSI preliminary PLUTO calculations

12 1 AGeV – pp & 1.25 Dielectron spectrum from C+C: superposition of NN collisions! No indications for in-medium effects (at least for C+C) spectra normalized to  0 measured in C+C and NN 1 AGeV /A part = 0.06 ± (A)GeV

13 Summary 12 C and 2 AGeV analyzed and published –DLS data 1.04 AGeV) confirmed –dilepton pair excess in  region (0.15 GeV/c 2 < M ee < 0.5 GeV/c 2 ) observed –excess seems to scale with energy beam like pion production –new OBE calculations: significantly higher pn-Bremsstrahlung contribution –various transport models describe data although with different scenarios p+p and 1.25 (A)GeV –elementary reactions in order to measure  and NN-Bremsstrahlung –problems with description of pp and pn data –normalized spectra of C+C and pp+pn agree (not due to in-medium effects) –more data to theoretical models in elementary dilepton production channels Vector meson investigation in HADES 3.5 GeV analysis on-going 3.5 GeV measurement just completed 1.75 AGeV analysis almost completed Plans –heavy systems (Ni+Ni, Au+Au) and pion-N/A after HADES upgrade (2009) –further program at FAIR (up to 8 AGeV) anticipated

14 HADES Collaboration