1. Summary from lecture I Meson line shape modifications seen in p+A/  +A reactions: 1.E325/KEK : downward  mass shift (~ 9%) 2.E325/KEK: downward  mass shift (~3%) and broadening 3.CBTAPS : downward  mass shift (~14%) and 10-fold! broadening. Strong momentum dependece  m(p). No sensitivity to .. but  G7/CLAS: no mass shift of  (~45% broadening)  contradiction to E325/KEK but CB problem in E325?  G7/CLAS: no effect on  mass shift ... but CLAS acceptance is not sensitive to CBTAPS  no contradiction to CPTABS  New HADES experiment @ GSI : will be sensitive to both  / 

2. Dielectrons from HI: SIS (HADES) and Bevelac (DLS) P. Salabura Jagiellonian University/GSI

3. SIS (BEVALAC) energy regime: 1-2 AGeV Final state (Freeze-out) in heavy ion collisions –approximately 10-15 % pions, baryon resonances (  (1232)) dominated –up to 200 charged particles (Au+Au) Enhancement of baryon density in "fireball" –Comparable to  \  life times :  V =1.3\23 fm/c 15 fm/c   Dense matter Freeze-outFirst chance collisions Au + Au @ 1 AGeV UrQMD: J. Phys. G: Nucl. Part. Phys. 25 (1999) 1859– T  60=80MeV

4. Meson production at SIS/Bevelac energies Production of vector mesons close to the threshold :  S NN <  S THR= 2M N + m  (E kin thresh = 1.92 GeV) –co-operative process :NN  N ,  N  NN  /  or   N ,  N  N * (  )  N  /  –production confined to high density phase –Low production rates: One vector meson decaying into lepton pair per 10 Million reactions ! Investigation of NN and  N collisions is prerequisite for HI ! m T =( m 2 + p 2 T ) 1/2 W. Cassing, E.L. Bratkovskaya / Phys.Rep. 308 (1999) 65Ð233  Yield (arb. units)

5.  e+e - : example thermal source A(m)- Breit-Wigner resonace formula  (m) Mass dependent decay width for e+e- reads fB(M,T) Boltzmann thermal factor fB(M,T) ~ p E exp(-E/T)  →e + e - from C+C @ 2AGeV no cut-off at M=2m  + thermal source

6. Line shape modification of  : intermediate resonaces D. Schumacher, S. Vogel et.al (UrQMD)  produced through Baryonic resonance N * (1520), N * (1720) and  (1700),  (1905) involved e+ e- N N*()N*() 

7. DLS puzzle: Data: R.J. Porter et al.: PRL 79(97)1229 Model: E.L. Bratkovskaya et al.: NP A634(98)168, BUU, vacuum spectral function Strong dilepton enhancement over hadronic cocktails comparable to top SPS energies (CERES)! DLS puzzle: Not explained even by in-medium mass shifts and  broadening Shape of enhancement consistent with  e+e-  but cross section to low (TAPS)

8. TAPS TAPS – Two Arms Photon Spectrometer : m  =(2E  1 E  2 (1-cos(   1  2 )) 1/2 Electromagnetic calorimeter : m  =(2E  1 E  2 (1-cos(   1  2 )) 1/2 C+C @ 1 AGeV small acceptance (mid rapidity) Acc~10 -3

9.  and  from TAPS Cross sections measured and extrapolated to full solid assuming isotropic thermal source at rest in NN CM frame Converted into pair yields:

10. The DiLepton Spectrometer at LBL 1988-1993 at Bevalac 2 Arm-Spectrometer –Minimum opening angle: 40° –Each arm: 40° in Φ, 7.5° in Θ –Trigger on electron-pairs –Mass resolution –30-40% systematical error pp/pd, Ca+Ca 1993 C+C 1.04 AGeV, –Mid-Rapidity: 0.69 –Acceptance –Statistics e+e- e+e- Pair Yields CombNet pairsSyst. Error 476019192841 ± 82±30%

11. HADES detector Side View START FW Acceptance: Full azimuth, polar angles 18 o - 85 o Pair acceptance  0.35 Particle identification: RICH: CsI solid photo cathode, C 4 F 10 radiator, TOF: 384 scintillator rods TOFino: 24 scintillator paddles  MUL limitation, high granularity RPC from 2008 Pre-Shower: 18 pad chambers & lead converters) 2' Level single leg electron trigger (M e >=1)  e+e- >=92%, evt. reduction : 20(pp) -3(ArKCl) Momentum measurement Magnet: B  = 0.36 Tm + MDC: 24 midi drift chambers, single-cell resolution  140 mm, 2005-6: set-up completed (MDC IV)

12. Phase space coverage: HADES vs. DLS mid-rapidity Thermal π 0 and η events: HADES and DLS acceptance HADES acceptance larger but for low masses (M<0.14 GeV/c 2 ) part of phase space covered by DLS not covered by HADES ! Red dashed lines: constant pair momenta in steps of 100 MeV/c

13. Hadron Id momentum vs velocity (β) measurement: momentum vs velocity (β) measurement: 18 0 <  <45 0 TOFINO 45 0 <  <85 0 TOF  ~450 ps  ~120 ps

14. Electron identification-RICH hadron blind  had  t <  lep C 4 F 10 :  t = 18.3 p   GeV/c e+e+ e-e- 00   ~ 15.2 0 Dalitz decay 20% electrons: pi0 Dalitz  e+e+ e-e-  ~ 2.2 0 Conversion 70% : electrons conv. One ring: Two rings (if    Ị needs high res. MDC!

15. Electron Id Electron Id Spatial correlations –RICH rings ↔ MDC tracks –MDC tracks ↔ TOF and PreShower hits PID : e +, e - –β vs momentum correlation –PreShower condition DATA C + C @ 2AGeV hadron admixture < 3% at 1000 MeV/c e-e- e+e+ velocity vs. momentum

16. Pair Analysis Signal/ Background rejection C1 C1 – the only pair cut C2 C2 – selection of "clean" tracks C3- conversion rejection TOF/Shower <9 o RICH MDC I-II C1 C2 C3 C1 C2 C3 <9 0 Close pair Shared detector hit close conversion candidate Relative suppression C1 C2 C3 C + C @ 2 AGeV Signal= N e+e- - CB CB – Combinatorial Background

17. C + C @ 2 AGeV Spectra before efficiency correction normalizated to ½(  + +  - ) yield 23000 ~ 23000 signal pairs for full M ee range 3000 ~ 3000 signal pairs for full M ee > 150 MeV/c 2  (M ee ) ~9 % @ M ee ~0.8 GeV/c 2 2002 set-up with 2 inner MDC only

18. CB Reconstruction Combinatorial background: M < 150 MeV/c 2 - sLS sLs = 2 (checked with MC for HADES ) M > 150 MeV/c 2 - mixed Opposite Sign (mOS) M ee > 150MeV/c 2 Normalization done between 150-550 MeV/c 2 M ee sLS and mOS background show same behavior for M ee > 150 MeV/c 2 For M ee < 150 MeV/c 2 deviations due to correlated background  eeX CC 2AGeV

19. C C pion production (normalization) m t spectra C +C @ 2AGeV anisotropies 2 AGeV 1 AGeV HADES N /A part = 0.137 ± 0.015 A part =8.4 =3.2 fm N /A part = 0.069 ± 0.008 A part =8.6 =3.0 fm Yields extrapolated (LVL1) to 4  : SYS error =11%

20. Comparison to Physics GeneratorsExperiment RawData Raw Data HADES Analysis Physics PairSpectra Pair Spectra Event Generator Efficiency Correction Correction dN/dM AcceptanceFilter Acceptance Filter time consuming but done to cross-check eff.corrections and acc. filters time consuming but done to cross-check eff.corrections and acc. filters final comparison: only in HADES acceptance final comparison: only in HADES acceptance comparison incl. efficiency factors Acc ± (p, , Φ)

21. Acceptance and Efficiency matrices pair production and decay is described by 6 dof (3 production and 3 decay) p (0 – 2 GeV/c), Φ (0 o – 60 o ), Θ (0 o – 90 o ) and similarly CC @ 2 AGeV

22. C+C @ 2 AGeV Coctail A (long lived mesonic components)  0 thermal source, anisotropic angular distribution according to measured  +-  isotropic  m T scaling 18 % 21 % systematic errors: 15 % - efficiency correction 10 % - combinatorial background 11 % -  0 normalization Cocktail A:  0 + η + ω Cocktail B: Cocktail A + Δ(  Ne+e-) + ρ short lived component A. Agakichiev Phys.Rev. Lett 98(2007) 052302

23. C+C @ 1 AGeV Cocktail A:  0 + η + ω = “long-lived” components only  Large excess yield Good agreement in π 0 region Underestimates the data for M ee > 0.15 GeV/c 2 Cocktail B: Cocktail A + Δ + ρ  Contribution from short lived resonances (ρ, Δ, N*) Reduced discrepancy for M ee >0.15 GeV/c 2 Cocktail B underestimates data PRELIMINARY

24. C+C @ 2AGeV p T, Y distributions ω η  M ee < 150 MeV/c 2 : Data well described,  150 < M ee < 550 MeV/c 2 : Underestimation over whole p  range (factor 2).  M ee > 550 MeV/c 2 : Underestimation for high p 

25. P t Spectra @ 1 AGeV Good agreement in π 0 region Underestimation for M ee > 0.15 GeV/c 2 Excess over Cocktail A (  0 + η + ω )  enhancement at low P t PRELIMINARY No Off-shell, multi-step processes  Transport models PRELIMINARY

26. s 2006) Comparison to transport models for CC @ 2AGeV (statu s 2006) RQMD Tübingen C.Fuchs, D. Cozma UrQMD Frankfurt M. Bleicher, D. Schumacher HSD Gießen (v2.5) E. Bratkovskaya, W. Cassing vacuum results Large variation of yield in models due to uncertainties in Baryon decays  must be fixed by data

27. Yield above  for C+C @ 2 and 1 AGeV F(2.0) = 2.07 ± 0.21(stat) ± 0.38(sys) PRELIMINARY C + C 2 AGeV Phys. Rev. Lett. 98, 052302 (2007) F(1.0) = 7.06 ± 0.6(stat) ± 1.58(sys) Yield above  :

28. Excitation function of Pair Excess (Y exc ) DLS F(1.04) = 6.5 ± 0.5(stat) ± 2.1(sys) η DLS Y exc excess scales like pion production ! Y exc (2.0)/Y exc (1.04) = 2.5 ±.5(stat) ± 1.5(sys) TAPS

29. HADES vs DLS : direct comparison for CC at 1 AGeV Fit: M ee < 0.15 GeV/c² 0.15 GeV/c² ≤ M ee ≤ 0.55 GeV/c² P t [GeV/c] PRELIMINARY

30. HADES Data incl. extrapolation in DLS acceptance  HADES and DLS Data agree PRELIMINARY DLS Data: R.J. Porter et al.: PRL 79(97)1229 Direct Comparison: HADES vs DLS PRELIMINARY J. Carroll – presentation International Workshop on Soft Dilepton Production August 20-22,1997, LBNL

31. How can we be sure that our efficiencies are under control ?  Experimental goal:  Verify dielectron efficiency:  dielectron and hadron channels of  0,   hadron channel and branching ratio well known (DISTO, …)  Conditions:  beam intensity: 10 7 p/second  target: LH2 (5 cm length) total signal background M  548 MeV/c 2 M 549 MeV/c 2  /M 2.5 % (3.6) pp  pp  pp  +  -  ° total   ° bg M  548 MeV/c 2 M 546 MeV/c 2  /M 3.3 % M  ° 135 MeV/c 2 M 140 MeV/c 2  /M 18 % pp  pp  ppe + e -   yield 2322  118  yield 24845  348  ° yield 7580  454 EXP: 15.3 ± 1.8 (stat) SIM: 15.6 ± 0.9 (stat)  Resulted yield ratio: => Dielectron efficiency under control SYS error 20%

32. HADES and DLS agree with each other: Do we have now HADES/DLS puzzle for the next 10 years? Hopefully not. Do we understand elementary sources contributing to dielectron cocktail at SIS/Bevelac energies? Radiation from baryonic matter ? NN  NNe+e- bremsstrahlung NN  NN * (  )  NNe+e- bramsstrahlung with resonance excitation i.e  (1232)  Ne+e- Delta Dalitz decay

33. dp and pp @ 1.25 GeV Strategy: study of e+e- sources in pp and pn at s<s thres (E kin =1.25 GeV) for  production Only  0,  and bremsstrahlung contribute: Measure pp  + p  ppe+e- (fix  + ) - assuming  brems (pp) is small Measure pn  e+e-X with dp reactions – determine  brems (pn)

34.  (  ) production in pp, pn @ 1.25 GeV pp  p  + 6.0 mb pn  n  + + pn  p  0 =12. mb (assuming isospin symmetry)  ++ = 3  + S. Teis et al., Z. Phys. A 356 (1997) 421-435  dominant at 1.25 GeV 2/3  +  p  0 2/3  0  n  0 1/3  0  p  - Resonance model and isospin symmetry: 2/3  +,0 = p(n)  0

35. NN - bremstrahlung 1 2 1‘ 2‘ Strong + electromagnetic process e+ e- + = 1 2 1‘ 2‘ baryon resonances + One Boson Exchange Model Kaptari, Kampfer Nucl. Phys. A 764 (2006) 338 Shyam & Mosel (2003)

36. NN- bremsstrahlung-dielectron yield x2 x4 total L.P. Kaptari, B. Kämpfer Nucl. Phys. A 764 (2006) 338 Large isospin (  pn =  pp ) effects

37. Problems in description Bremsstrahlung?  contribution ? No  0 visible in data ! cross-check with "known" physics" missing pp/dp from DLS Data: DLS: W.Wilson Phys.Rev.C(1998) C. Fuchs et al. Phys. Rev. C68 014904(2003) E. Bratkovskaya et al. nucl-th\0008037 (2000) eVDM 10 -1 10 0 10 -1 10 0

38. pp/dp @ 1.25 GeV HADES ~50% analyzed 17008 signal pairs S/B101 0.5 1.0 0.5 1.0 M ee M ee preliminary (run 2006)

39. pp @ 1.25: comparison to simple model cocktail filtered with HADES acceptance and rec. efficiency normalized to  0 yield only 2 sources:  0  e+e-   +  pe+e- decay (BR=4.4*10 -5 )  + yield fixed to  0 with isospin relation: N(  + )=3/2 N(  0 ) factor ~2 missing yield for M>0.15 GeV/c 2 preliminary not eff. corrected

40. pn  e+e- X with d+p @ 1.25 AGeV  forward p spectator tagging in Forward Wall  selection of pn reactions (10% contribution from pp, only) p_spec pcpc d p_c p_spec p_c d+p  p_c n  0 p_spec p_spec p_c d+p  p_c n  p_spec ns p_spect p_c

41. pn @ 1.25 AGeV (MUL=>2 && FW "p spectator") tag on np -> e+e- X reactions ~50% analyzed "online" 37800 signal pairs  +- > 9 0 "On-line spectrum" not efficiency corrected S/B101 0.5 1.0 0.5 1.0 M ee M ee

42. cocktail filtered with HADES acceptance and rec. efficiency cocktail with  and  only: (same as for pp case) factor 4-5 missing yield for M>150 MeV/c 2 (~3 larger than for pp)! larger at higher masses clear evidence for additional sources (bremsstrahlung !? pn>pp) pn @ 1.25 AGeV cocktail not efficiency corrected normalized to same  0 yield p + n @ 1.25 AGeV  e+e- >9 0 "online spectrum"

43. p+n vs p+p vs C+C : scaling to  VERY PRELIMINARY ! p+p vs p+n p+n above p+p by a factor of 3 stronger effect at higher masses  Contribution from bremsstrahlung?  pn Bremsstrahlung > pp Bremsstrahlung factor ~3-4 p+n vs C+C fair agreement over whole range  NN – bremsstrahlung !?  Puzzle explained by bremsstrahlung?

44. HADES – HSD vacuum (version'07) Inv. mass Good agreement Undershoot at ~ 0.4 GeV/c 2 ? P t Distributions M ee < 0.15 GeV/c² -  0 dominated M ee > 0.15 GeV/c²: –Bremsstrahlung important at low p t

45. Summary: Low mass enhancement ("DLS puzzle") : a) HADES+ DLS fully consistent: b) dielectron enhancement scales with beam energy as pion production b) preliminary combined pp and pn data @1.25 GeV show enhancement (pn>pp) above "standard cocktail" :  0  e + e - ,  Ne + e-  Ne + e - BR? NN bremstrahlung ?  e+e- (for pn – Fermi momentum) All components can be fixed from new HADES data Improved cocktail should be then compared to HI data... and finaly fix elementary dielectron cocktail Vector meson production a)  /  region measured in pp @ 3.5 GeV with HADES b) in –medium modifications ? Ar+KCl measured in 2005 and analysis is almost complete c) p+Nb(Be) to hunt for  /  modification in nucleus – run scheduled for beginning of next year

46. Quo Vadis HADES 2006200720082009201020112012 p,d+p 1,25/3,5 AGeV EXP S201 p+p 1,25 GeV EXP S201 p+A upgrade Ni+Ni  +N,A Au+Au SIS Currently discussed in a committee SIS 8 AGeV

47. back-up slides

48.  s NN in dp @ 1.25 GeV and CC @ 1.25 GeV d+p@ 1.25 GeV S 1/2 NN [GeV]  prod 00  For M e+e- >0.5 (  ) region similar  s nn distributions! C+C @ 1 AGeV