Dilepton production in 1-2 AGeV energy range: Recent results from the HADES experiment at GSI.

1. Dilepton production in 1-2 AGeV energy range: Recent results from the HADES experiment at GSI. Béatrice Ramstein and Thierry Hennino, Tingting Liu, Malgorzata Sudol Institut de Physique Nucléaire d’Orsay, France

2. Outline • Introduction: • motivations of HADES experiments : dilepton production in Heavy_ion and elementary reactions • short status on medium effects • Inclusive e+e- production in heavy-ion reactions C+C, Ar+KCl • pp and dp reactions: inclusive/exclusive dilepton channels, hadronic channels • Perspectives: • Heavy-ion reactions HADES upgrade  FAIR • pion induced reactions • Conclusions

3. QGP SPS HADES Na60 SIS-100 SIS-300 CBM RHIC AGS hadrons SIS-18 KEK,JLAB,TAPS G7 Motivations of the HADES experiment Exploring the phase diagram of hadronic matter….. • E/A=1-2 AGeV • /o ~ 1-3, T < 100 MeV • N/Apart ≈ 10% ….. using dilepton emission: rare but undistorted probe

4. A faithful probe of the hot and dense phase: the dilepton pair decay of vector mesons R(A=100)5.5 fm e+ Dilepton spectroscopy gives access to vector meson spectral function e- and are undistorted by strong interactions V e+e- JP=1-

5. g* g* e+ r, w e+ e- e- N Interest of vector meson study r, w e+ g* e- • V l+l- JP=1- direct coupling to * • Vector Dominance Model (Sakurai 1969): coupling of a real (or virtual) photon to any electromagnetic hadronic current is mediated by a vector meson • Sensitivity of vector meson mass to Chiral Symmetry Restoration on-shell electromagnetic elastic or transition form factor off-shell

6. Chiral symmetry restoration in medium ≠ 0 “melting of quark condensate “ Klimt, Lutz, Weise, Phys.Lett.B249(1990) 386 SPS - ,-,p beams g,p-,p - beams RHIC LHC SIS 18 SIS100 SIS 200 T [MeV] 300 But the quark condensate is not an observable ! models are needed to relate <qq> to hadron masses - In the limit of massless u,d quarks, exact symmetry of the QCD lagrangian, but not of QCD vacuum and excited states (spontaneous breaking of chiral symmetry)

7. 15% K. Saito et.al Phys.Rev.C55(1997)2637 Medium modifications (I): 1st scenario: « dropping mass » • Scaling inspired by chiral effective models: • Brown-Rho PRL66(1991) 2720 • QCD sum rules : • Hatsuda and Lee PRC46 (1992) 34 , Kwon et al. PRC78 (2008) 055203 • Quark meson coupling (QMC): • Saito et al. PRC55 (1997) 2637

8. N(1520)  r r r r D(1232) + ... + N-1 N-1 Medium modifications (II): schematic summary ! see e.g. Rapp and Wambach 0901.3289 for a recent review I 2nd scenario: « in-medium broadening » • Spectral functions of the mesons in the medium: coupling to resonances Hadronic many-body approach Rapp and Wambach EPJA 6 (1999) 415 Rapp, Chanfray and Wambach NPA 617, (1997) 472

9. QGP SPS HADES Na60 SIS-100 RHIC CBM AGS hadrons SIS-18 Results from ultra-relativistic heavy-ion reactions CERES / CERN Pb (158 AGeV) +Au Na60 / CERN In (158 AGeV) + In CBM   KEK,JLAB,TAPS G7 Adamova et al. PLB 666 (2008) 425 Rapp and Wambach calc. with modified  spectral function Arnaldi et al., PRL 100,022302 (2008) Broadening of  spectral function is favoured, no mass shift + Helios/CERN+Na50/CERN Phenix/RHIC (on-going analysis),

10. + A p+Cu E=12 GeV G7/CLAS/Jlab QGP HADES Na60 SPS SIS-100 SIS-300 CBM RHIC AGS hadrons SIS-18  + AE = 1.2 - 2.2 GeV KEK,JLAB,TAPS Re-analysis of CBELSA/TAPS results: ° in A reactions at E=1.2-2.2 GeV Kotulla et al. PRL 100, 192302 (2008) G7 No effect on  meson Results at =0, small T

11. From C. Djalali, Hadron2009

12. Bevalac (1988-1993) HADES DLS Na60 • pp/pd, Ca+Ca, C+C 1.04 AGeV • mass resolution : 10-20% • 30-40% systematical error 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 not explained even by in-medium mass shifts and broadening DLS puzzle DLS Results for /o ~ 1-3, T < 100 MeV (Heavy-ion collisions E/A=1-2 AGeV) QGP SPS CBM RHIC AGS SIS-100 SIS-300 SIS-18 Bevalac hadrons KEK,JLAB,TAPS G7

13. UrQMD calculations Au+Au 1 AGeV n,p r/r0  15 fm/c     N-1 N-1 Specificity of 1-2 A GeV energy range • N/Apart ≈ 10% • long life time of the dense system (up to 15 fm/c)  ~ 1.3 fm/c, ω~ 23 fm/c • dominance of  resonance • resonance matter (33) r/r0 r/r0 1-2 GeV  tot 

14. Role of resonances in dilepton production at 1-2 AGeV Faessler et al. RQMD Both are linked through electromagnetic form factors • ° Dalitz decay:° e+e- (BR ~ 1.2 %) R  N°,  (1232) , N (1440)… •  Dalitz decay: e+e- (BR ~ 0.6 %) N(1535)  p • /ω vector meson production N(1520)  /ω N , ….. /ωe+e- • Dalitz decay of baryonic resonances: (1232)  Ne+e- (BR ~ 4 10-5) not measured ! + NN Bremsstrahlung NN  NN*  e+e-

15. SIS The Collaboration • Catania (INFN - LNS), Italy • Cracow (Univ.), Poland • Darmstadt (GSI), Germany • Dresden (FZD), Germany • Dubna (JINR), Russia • Frankfurt (Univ.), Germany • Giessen (Univ.), Germany • Milano (INFN, Univ.), Italy • München (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.), LIP, Portugal GSI

16. HADES 2nd generation dilepton spectrometer Side View START • Acceptance:Full azimuth, polar angles 18o - 85o • Pair acceptance  0.35 • Particle identification: • RICH,Time Of Flight, Pre-Shower (pad chambers & lead converter) ( also MDC (K)) • Trigger: • 1st Level: charged particle multiplicity (~10 kHz) • 2nd Level: single electron trigger (~2.5 kHz) • Momentum measurement • Magnet: ∫Bdl = 0.1- 0.34 Tm • MDC: 24 Mini Drift Chambers • Leptons: x~ 140  per cell, p/p ~ 1-2 % • M/M ~ 2% at  peak FW  < 7°

17. IPN contribution 4th plane of drift chambers drift cell (14x10 mm2) active area 3.5 m2 IPN team Jean-Louis Boyard (retired 2008) Jean-Claude Jourdain (retired 2001) Thierry Hennino Tingting Liu (PhD) Emilie Morinière (PhD 2008) Béatrice Ramstein Michèle Roy-Stephan (retired 2006) Malgorzata Sudol (part-time post-doc) …+Jacques Van de Wiele (phenomenology) R&D détecteurs, J. Pouthas, P. Rosier 6 drift chambers constructed at IPN

18. HADES subdetectors Magnet Coils MDC I RICH PreShower LH2 target (IPN) TOF

19. Lepton Identification with HADES Drift Chamber: Track reconstruction Pre-Shower condition RICH pattern e- + + + momentum % velocity momentum ∙ charge C+C 2 AGeV

20. 2005 Ar + KCl 1.75 AGeV medium effects on dielectron spectra, strangeness 2006 p + p 1.25 GeV exclusive ppe+e- (°,) pion production  Dalitz decay first measurement 2007 p + p 3.5 GeV ω line shape and cocktail (,,,,, strangeness 2007 2007 d + p 1.25 AGeV pn Bremsstrahlung /  Dalitz decay d + p 1.25 AGeV 2008 p+Nb 3.5 GeV Reference for medium effects, strangeness HADES experimental program 2001/2 understand dilepton spectrum/test transport models in simple case: low to very low medium effects + check of DLS C + C 2 AGeV C + C 1 AGeV 2004 Validation of detector performance (pp elastic) ° and  Dalitz decays (helicity distribution), “free” cocktail 2004 p + p 2.2 GeV exclusive ppe+e- (°,) 2009HADES upgrade

21. results from Heavy Ion reactions ….

22. Dielectron DATA analysis: normalization to 0 (Nπo = ½ (Nπ- + Nπ+)) Comb. Background (CB) subtraction Mee < 150 MeV/c2 Like-sign pairs Mee > 150 MeV/c2 event mixing Signal: ~ 18400 counts Me+e- ≥ 150 MeV/c2: ~ 646 counts Efficiency correction Measured rates span over 5 orders of magnitude Systematic errors: 20% efficiency correction 10% combinatorial background 15% 0 normalization 22% - 27% Total

23. Calculation: E.L.Bratkovskaya et al. Nucl. Phys. A634 (1998) 168 Hadron String Dynamics E. Bratkovskaya and W.Cassing, nucl-th/0712.0635 DLS DLS M [GeV/c²] HADES NN Bremsstrahlung and the DLS puzzle 1997:transport models failed to explain DiLeptonSpectrometer (Berkeley) data 2007:DLS and HADES agree and are compatible with new transport model calculations G.Agakichiev et al. Phys. Lett. B 663,43 (2008) M [GeV/c²] new (contradictory) treatments of Bremsstrahlung: R. Shyam & U.Mosel, PRC67 (2003) 065202 L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338

24. NN Bremsstrahlung and  Dalitz decay N  graphs N N,  nucleon graphs  graphs nucleon graphs Mee(GeV/c2) NN  NN* NNe+e-   Ne+e- (BR from QED = 4.2 10-5 not measured) q2 = M*2 > 0 Quantum interference Time-Like electromagnetic form factors two recent full quantum mechanical One Boson Exchange calculations: 1) R. Shyam & U. Mosel, PRC 79 (2009) 03520 2) L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338  pp dominated by  graphs  nucleon graphs (Bremsstrahlung) larger in pn than in pp collisions  Yields differ in the two models by factors 3-4 HSD model uses enhanced yield for NN bremsstrahlung, according to K&K OBE model

25. BUU Barz et al. UrQMD Bleicher et al. IQMDThomere et al. RQMD: E.Santini et al C+C 2 AGeV  Dalitz pn HADES data: Phys Rev. Lett 98 , 052302 (2007) Maybe, but a lot of contradictory calculations are pretty good too …. DLS puzzle solved by new calculation of NN Bremsstrahlung ? pn Bremsstrahlung and Delta Dalitz decay are important issues for understanding intermediate mass dilepton yield

26. HSD transport code prediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250. pp -> ppω HSD OBE Inclusive /ω cross-sections not known important ingredient of transport model calculations  can be measured in pp reactions

27.  Dalitz decay : pp E=1.25 GeV • NN Bremsstrahlung: dp (pn) E=1.25 AGeV • °,  production pp E= 2.2 GeV • /ω production: pp E=3.5 GeV E<  production threshold • Specific instrumental tools: LH2 target, proton/deuteron beams, Forward Wall HADES Forward Wall 2 m spectator proton deuteron beam 7 m ( 0.5–7) First results from HI reactions …. …..call for study of elementary processes

28. hadronic channels • pp elastic scattering  tracking, efficiencies, resolutions • pppp°, pppp  consistency of hadronic and leptonic reconstructions • pppn+, pppp°, pppp  pp+-0 check of resonance model ( dilepton cocktail) • pp pppp+-0 (on-going analysis)

29. exclusive pion production channels measured by HADES • absolute normalisation • from elastic scattering • Very good agreement with resonance model • Teis et al. ZPA 356(1997) 421 + refinements in both isospin channels • Consistent picture at E=1.25 GeV and 2.2 GeV • important check for dilepton analysis (° and resonances are dilepton sources !) T. Liu PhD Orsay arXiv:0909.3399[nucl-ex]

30. n qn p p D++ pppp  and  production : acceptance corrected results  production: 1 exchange  decay: 1+1.35cos2 Neutron angular distribution in CM + angular dist. in (+ ,p) ref. frame preliminary p+ D++ preliminary qp+ p Total cross sections compared to systematics preliminary preliminary preliminary T. Liu PhD Orsay arXiv:0909.3399[nucl-ex]

31. inclusive e+e- analysis at E=1.25 GeV/ nucleon

32. Results for pp reaction at 1.25 GeV p p p p D+ * e+ e-- GM(q2) GM(0)~3 0.6m2 q2 = M2inv(e+e-) = M*2 > 0 PLUTO ROOT-based event generator I. Fröhlich et al, arxiv:0708.2382 [nucl-ex] I. Fröhlichet al,arxiv:0909.5373 [nucl-ex] • resonance model: Teis et al. Zeit. Phys.A356(1997) 421+ refinements • ° Dalitz decay  ° = 4.5 mb branching ratio ° → e+e-1.2 % •  Dalitz decay : =3/2  °= 6.75 mb • branching ratio  → Ne+e-(QED :4.2 10-5) Time-like N-  transition electromagneticform factors subm. to PRL 0910.5875[nucl-ex] Iachello and Wan • Resonance model results: • ° Dalitz • Dalitz)+ effect of Iachello FF

33. Quasi-free «  pn » dilepton spectra • Comparison to resonance model • ° Dalitz decay  ° = 8.6 mb branching ratio ° →e+e- 1.2 % •  Dalitz decay : =3/2  °= 12.9 mb New features with respect to pp reactions: • participant neutron Fermi momentum ( Paris potential) •  contribution (due to Fermi motion) subm. to PRL 0910.5875[nucl-ex]  Dalitz ° Dalitz  Dalitz (BR ~ 4.2 10-5)+ + effect of Iachello FF Additionnal sources with respect to pp? NN Bremsstrahlung is absent !!  Check with full One Boson Exchange calculations

34. pp/pn reactions at 1.25 GeV: Comparison to simulations with OBE differential cross sections : R. Shyam & U.Mosel, PRC67 (2003) 065202 L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338 Kaptari & Kämpfer Shyam & Mosel pp OK with Shyam & Mosel overestimated by K&K  critical test for models HADES np data: a new puzzle ??

35. pp/pn reactions at 1.25 GeV:transport model calculations pp allows to dicriminate among models HADES np data: a new puzzle ?? HSD: E.Bratkovskaya et al.NPA 807(2008)214 IQMD: J. Aichelin et al.

36. Towards a consistent description of dilepton spectra from different systems C+C 2 A GeV • Refinement of HSD model • the vector-meson production via the LUND string has been reduced in order to mach pp exp. data better • - the isospin dependence for the channels NN-> V+NN and pi+N-> V+N have been improved p+p 1.25 GeV Ar+KCl 1.75 A GeV C+C 1 A GeV quasi-free n+p 1.25 GeV preliminary • need for description of both HI and elementary collisions with same set of parameters • Not yet fully achieved !

37. N+N dilepton spectra as a reference for A+A     N-1 N-1 • ηcontributions subtracted ! • yield normalized to M(π0) preliminary preliminary x3 • « experimental » indication • for emission from the dense phase • resonance propagation

38. exclusive ppppe+e-analysisE=1.25 GeV

39. ppp+pp°ppe+e- Dilepton mass distribution in very good agreement with ° and  Dalitz decay pp p+ppe+e- consistent with inclusive dilepton analysis Mee>140 MeV/c2 simulation pppe+e-X missing mass MX HADES data preliminary Selection of ppppe+e- channel is efficient mp MX(GeV/c2) exclusive analysis : ppppe+e-at 1.25 GeVusing pe+e- events (i) W. Przygoda’s analysis B.Ramstein Hadron 2009 proceedings 200 events for Me+e- > 140 MeV/c2 preliminary 0.2

40. exclusive analysis (ii) ppppe+e-at 1.25 GeV preliminary p p 1 p2 D+ p e+ q2=M2inv(e+e-)=M2* e- (p,e+,e-) invariant mass(GeV/c2) preliminary cos(CMpe+e-) • reconstructed by (p,e+,e-) Mee>140 MeV/c2 No acceptance corrections ! 2 indistinguishible protons (p1,e+,e-) (p2,e+,e-) acceptance corrected simulation of  production + Dalitz decay (cf hadronic channels) dominance of ppp + p pe+e- room for pp Bremsstrahlung contribution?

41. e-  * e+  N fit 1+B cos2 B=1.110.32 preliminary cos exclusive analysis ppppe+e-at 1.25 GeV: helicities helicity angle   Dalitz decay: +  p g* pe+e-: only transverse g* (if Coulomb interaction is neglected) Helicity distributions *  e+e- d/dΩe~ 1+cos2 True helicity from pe+e- events: *in  ref. frame acceptance corrected • First measurement of  Dalitz decay! • Dalitz decay branching ratio in agreement with QED value (4.2 10 -5) within ~20% • Helicity consistent with 1+cos2

42. / meson production Inclusive e+e- in pp reaction at 3.5 GeV • Inclusive cross sections measurements • Meson production mechanisms • Reference to the pA and AA reactions

43. Vector mesons / at SIS p+p at 3.5 GeVp+ 92Nb 3.5 GeV 40Ar+ 38KCl1.75 AGeV preliminary 2008 “on-line spectrum!” w preliminary h D Very preliminary r sM(w)  15 MeV/c2 Inclusive pp X cross-section Input for transport models • First observations of  production at SIS energies • possibility of check of p+A results from KEK & CBTAPS/CLAS 43

44. Cocktail simulation for pp E=3.5 GeV •  production through  resonance =3/2 ° • production from one pion exchange model • , ,  phase space production • particle decays • relativistic BW with mass dependent width • cross sections • πo : 15  4 mb • η : 1.04  0.03 mb • ω : 0.28  0.07 mb • ρ : 0.36  0.08 mb preliminary A. Rustamov et al. Hadron 2009

45. On-shell/off-shell  production: cocktail subtracted data : p+p E=3.5 GeV • After subtraction of °,  and  • remaining contributions: • and baryonic resonance PLUTO: phase space HSD: LUND string Model UrQMD: production through resonance preliminary Off-shell  production through resonances  meson production needs improvement in transport models Analysis: A. Rustamov (GSI)

46. Future plans • HADES upgrade • Resistive Plate Chambers , 120 << 450 1320 channels, t85 ps • New DAQ electronics ~20kHz LVL1 • New MDCI • Ag+Ag at 1.65 AGeV and Au+Au at 1.25AGeV, 2010-2012 • Pion beams 2012 • move to SIS100 (FAIR), E ~ 8 AGeV • experiments at SIS100 (FAIR)cca 2016-2017 CBM 8 – 45 GeV/u HADES 2 – 8 GeV/u

47. Perspectives of pion beam experiments

48. Motivations of N experiments with HADES • Well-known interactions • Exclusive channels - p   n • Special interest of / subthreshold production (sqrt(s)=1.72 GeV) • Strangeness production  (1405), and K • spectacular destructive interference of /ω production linked to coupling to baryon resonances (S11, S31, D13, D33) • electromagnetic structure of resonances • importance for A, AA dilepton spectra M. Lutz , B. Friman, M. Soyeur Nuclear Physics A 713 (2003) 97–118 p- p → e+e-n sqrt(s)=1.65 GeV Pluto r w E =0.66 GeV p=0.80 GeV/c total See also predictions by Titov & Kämpfer EPJA12 (2001) 217

49. Motivations of dilepton spectroscopy in A experiments • New HSD calculations HSD: preliminary • strong medium effects are predicted HSD’09 => similar to W. Cassing, Y.S. Golubeva, A.S. Iljinov, L.A. Kondratyuk, Phys. Lett. B 396 (1997) 26

50. Conclusions: HADES experiments • Inclusive spectra in pp/pn and heavy-ion reactions: • critical tests for transport models • a consistent description of dilepton production in all systems is needed !! ↓ Prerequisite to medium effect investigations • Ar+KCl • future Ag+Ag and Au+Au reactions • HADES@FAIR • Not mentionned: very fruitful strangeness production program: « Deep sub-threshold - production in Ar+KCl reactions at 1.76 AGeV » PRL 103 (2009) 132301 «  decay: a relevant source for K- production at SIS energies » • Specificity of elementary reactions: • Sensitivity to time-like electromagnetic form factors of nucleon and resonances • Selectivity on dilepton sources (  Dalitz decay,  Dalitz decay,  production,…) • Very new ppppe+e- exclusive analysis ! • First measurement of  Dalitz decay • helicity distributions to be investigated in dp and heavy-ion reactions • perspectives of pion beam experiments ( 2011)