1. Quest for in-medium modifications in p+A,  +A,  +A collisions Goal of experiment Dielectron cocktail ( "elementary dielectron sources") Review of existing experimental results: E325@KEK, CLAS@JLAB CBTAPS @ ELSA Plans: HADES @ GSI

2. Why to study in-medium modifications: Origin of masses: 2 mechanisms Standard model: Q u e c ct d e s  b    2/3 -1/3 0 quarks leptons 3 particle families 10 -3 10 -1 1 10 1 10 2 10 3 10 4 10 5 10 -2 m q,l [MeV] e   e   du s c b t leptons quarks  Mass generation via Higss mechanism  „current quark masses”  What is the origin of hadron masses? M proton >> 3m u/d (20 MeV) !  Breakdown of chiral symmetery

3. Chiral symmetry and hadron masses For massless 2-flavour quarks (q=(u,d)) is invariant: Chiral symmetry.... but broken by R L V-Asplitting Ị Ị Hadron mass spectrum reflects chiral symmetry breakdown - parity doublets Ị constituent mass of light quarks is generated by Ị is connected to  properties (Goldstone boson) vac  0+0+ f0f0 QCD gap

4. Quark condensate in dense and hot matter    p - beams SIS 18 SIS 200 T [MeV] 300 LHC RHIC SPS Partial restoration of chiral symmetry Brown-Rho scaling: m  /m = (  / 0 ) 1/3 m  = m 0 (1 -   /  0 ) for T=0 and   0 For low densities: Klimt, Lutz, Weise, Phys.Lett.B249 (1990) 386 Brown,Rho Phys.Lett.66(1991)2720 Dropping mass scenario: m  /  ↓ if  ↑ investigate hadron masses inside nuclear matter Toneev at al.,nucl-th/0503088: 3-fluid hydrodynamics Time evolution of heavy ion collisions

5. Main goal: Studies of vector meson (  /  /  ) spectral functions in nuclear medium via e +, e - pair spectroscopy at  N   0 and T=0 e+ e- R(A=100)  5.5 fm Dielectron two-body decays of light Vector Mesons  High resolution spectroscopy of e + e - pairs, no final state interaction ! MesonMass (MeV/c 2 )  (MeV/c 2 ) c  (fm) Main decaye + e - BR  7681521.3     4.4 x 10 -5  7828.4323.4       7.2 x 10 -5  10194.4344.4K + K - 3.1 x 10 -4 Decay length L D =  c   R (decay in medium) N(L)=N(0)exp(-L/L D )

6. Theoretical predictions (some examples) quark-meson-exchange effective chiral lagrangian 15% K. Saito et.al Phys.Rev.C55(1997)2637 T. Renk et.al Phys. Rev. C 66 (2002) 014902 meson masses scale with density as, order parameter of ChSB Brown-Rho scaling('92) : m V *= m V (1-  /  0 )

7. Hadronic scenario: meson spectral function    Vacuum:   W. Peters et.al. NPA 632(1998)109:  meson spectral function A(q,m) In medium: +   N -1 N(1520) +...  (1232)  N -1  m,  depends on meson momentum (q)  A(m,p) spectral function Connection to chiral symmetry?

8. Mass (width) changes – "trivial" effects Broad resonances (  ) : modification of line shape due mass dependent widths  (m) and due to available phase space : important for HI and NN collisions at low energy (see second lecture) Collision broadening  decay in Pb nucleus  (  ) =  vac (  ) +  coll (  ) M. Effenberger at al PHYS. REV. C 60 044614

9. Experimental considerations: 1. cross sections 2. "inside" vs "outside" decays

10.  cross sections for pp and  p  pA   A 2/3 Lund String Model 3.5 12 GeV KEK (4.6 mb) exclusive SAPHIR (Bonn) CBELSA (Bonn) DAPHNE, TAPS (Mainz) GRAAL (Grenoble)  +   o  +  -  +  o   o  o   ▲    K +   K +  o  K o   ’   pp

11. "Tomography" of vector meson decay Transport calculations for p/  /  + A collisions HSD -E. Bratkovskaya, V. Cassing Phys. Rep. 308(1999) 65) vacuum spectral functions full in-medium meson propagation (scattering in medium) production decay  production on 93 Nb with p/  3.5 GeV 1.17 GeV Decay length L D =  c   R (decay in medium) N(L)=N(0)exp(-L/L D )

12. Beam energy dependence for  /  for  /  E beam ≈1.1-1.5 GeV is optimum (but ~100 times less  for  !) for p/  beams less decays inside protons pions protons for 12 GeV p+A: ~ 6%  decays ~55%  decays For  : factor 2 less in-medium decays as compared to  beams

13. e+,e- spectrum components: "dielectron cocktail"

14. Dielectron sources: „free” hadron: dielectron cocktail Two body meson decays (peaks): 3-body Dalitz- decays (continuum):  M e+e [GeV/c 2 } - CB – e + e - combinatorial background Signal/CB usually <1 ! V → e+e- V → e+e- X      e+ e-    e+ e-

15. Meson Dalitz (3 body:e + e - X ) decays Dielectron spectrum  |F(q 2 )| - electromagnetic form factor : time-like q 2 >0 q 2 <0 : space like – probed via electron scattering experiments Vector Meson Dominance (VMD) or Vector Dominance Model (VDM) BR(  0  e+e-  ) = 1.2*10 -2 BR(  e+e-  ) = 6*10 -3

16. Electromagnetic transition form-factors known from di-muon G-experiment 600 events 33±7 events 60±8 events

17. Dielectron decays : Dalitz decays of Baryons Baryon decays :  (1232), N*(1440),...  Ne + e -  N * (  )->Ne +,e - - not measured in elementary reactions !   (1232): 3 Form. factors (G E (q 2 ), G M (q 2 ), G c (q 2 )) not known  electromagnetic structure of nucleon Extended Vector Meson Domince Contribution from several vector mesons – interferences ! Various treatments: 1. 2. Baryons are treated as point like particles (QED) C. Fuchs: Phys. Rev. C67 025202(2003) M. Krivoruchenko et al. Ann.Phys296 (2002)299 B. Lautrup, J.Smith Phys.Rev D3(1971)1122, C. Ernst et al. PRC 58 (1998) 447, M. Thomere et al PRC 75(2007) 0604902,... gives similar BR as 1   Ne+e- =5.02 KeV (BR=4*10 -5 )

18. Two body Vector Meson decays MesonMass (MeV/c 2 )  (MeV/c 2 ) c  (fm)Main decaye + e - BR  7681521.3  4.4 x 10-5  7828.4323.4   7.2 x 10-5  10194.4344.4K+ K-3.1 x 10-4   I =1 qq 2     4    KK Inverse process: e + e -  *  hadrons I =0,1 + 2  + 3  +... J P = 1 - Vector Mesons carry same quantum numbers as photon...quark em.current! At meson pole !

19.  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

20. Combinatorial Background Combinatorial background (CB)  e + and e - comming from different vertices N e+e- unlike-sign pairs N e- N e- and N e+ N e+ like-sign pairs absolute normalization CB= Signal S +- = N e+e- - CB +-   00 e-e- e+e+ e-e- Combinatorial Background Ị Multi  0 Dalitz decays  small  e+e- External Pair Conversion (EPC) of photons from  0  small  e+e-  e+e- from Dalitz and EPC small but their combination can form large  e+e- and large mass!!  00 e-e- e+e+ dedicated talk in student session !

21. E325 Experiment Measures Invariant Mass of e + e -, K + K - at KEK (1996-2002) in 12GeV p + A   /  /  + X reactions,  s=5.1 GeV Mass Resolution for e + e - : 8.0MeV/c 2 for  10.7MeV/c 2 for  Primary proton beam (~7*10 8 /s) Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress  conversion Beam Target

22. Target Configuration materialbeam intensity(p /spill) Interaction length(%) radiation length(%) C~1x10 9 0.2%0.4% CuX4~1x10 9 0.05%X40.5%X4 23mm C Cu Very thin target with clean and high intensity beam Vertex Distribution Beam

23. Experiment KEK-PS E325

24. Forward LG Calorimeter Rear LG Calorimeter Side LG Calorimeter Front Gas Cherenkov Rear Gas Cherenkov Barrel Drift Chamber Cylindrical DC Vertex Drift chamber 1m Detector Setup 12GeV proton beam B Hodoscope Aerogel Cherenkov Forward TOF Start Timing Counter Trigger: e+ and e- requested in opposite arms

25. Accepted e + e - distributions: mass, p T, y Cu   Note: p =m 0  Y CM

26. E325 Model calculations of meson line shape  /  mesons are generated uniformly at surface of target nucleus (  ~ A 2/3 ) momentum distribution: as measured in xp pole mass downward shift : m*/m = 1 – k 1  /  0  k 1 parameter decay width increase :  */  = 1 + k 2  /  0  k 2 parameter density distribution Woods-Saxon : R: C:2.3fm/Cu:4.1fm e-e- e+e+ e+e+ e-e-   Blue histogram : Detector Simulation Red line : Breit-Wigner (gaussian convoluted) fitting result agrees with measured: Ks   +  - Detector response :

27. CB subtracted  Data described (both nuclei) assumming  /  mass modification m  * =m  0 ( 1 -.092  /  0 ) fit supports  * consistent with the vacuum for  /   /  =0.7±0.2  /  =0.9±0.2 CB shape from mixed event but w.o absolute normalization (fit parameter)! Fit, excluding excess region, gives  /  <0.15 (Cu) and <0.32(C) with CL of 95% ! In contrast to other pp data  /  ~1  2 /dof =2.3  2 /dof =1.8 p+A @ 12 GeV KEK-PS E325 M. Naruki Phys.Rev.Lett 96 (2006) 092301

28.  e + e - modification in KEK-PS E325  only small fraction of  decays is inside medium  selection on     distribution 

29. Fit again excluding the region where the excess was seen; 0.95~1.01GeV/c 2 excluded from the fitting Mass modification of  for Cu targets  Best fit (both nuclei) achieved for k 1 =0.035 and k 2 =2.6 (  *  ). Mass shift of  Yokkaichi Meson2006 Integrate the amount of the excess in the above region(0.95~1.01 GeV/c2)  Nexcess

30. Photon experiment G7 @ JLaB

31. Photon beam: E  =0.6-2.85 GeV E  >1.1 GeV needed for  /  production

32. G7 (CLAS) @ JLaB  + A (C, Fe-Ti, Pb)  e+e- : (since 2002) Beam: 5x10 7  /s from 3 GeV (2/3) and 4 GeV(1/3) electrons

34. Toroid with 6 coils – 6 sectors

35. Electron identification in CLAS C Fe C Pb C Ti C D2D2 ~1.1% dedicated talk in student session !

36. Detector acceptance no acceptance ! e-,e+ ID only for 8-45 0 : EmCal. coverage electrons in same sector forbidden! Pair acceptance

37. Pair spectra Combinatorial background: CB from mixed events with normalization from like-sign pairs :

38. Signal pairs (CB subtracted)

39. Final result for the  meson spectral function cocktail of free  /  /  meson decays subtracted   spectral function k 1 =0.02±0.02 consistent with 0!  contradiction to KEK PS E325

40. Why CLAS result contradicts KEK-E325? KEK-E325 fits CB (no absolute determination!)  G7 spectra with CB fitted (no absolute normalization!)  different results  CB normalization essential !

41. Crystal Barrel & TAPS (CBTAPS) @ ELSA

42. Detector (photon calorimeter)

43. Strategy of the experiment:  0   A   A  (  0  ) A with photons from 3 GeV e - beam

44. Reference experiment:  + N

45. CB/TAPS @ ELSA D. Trinka Phys. Rev, Lett (2005) 192303 m  = m 0 (1 - k  /  0 ); k = 0.14  mass shift of  m  =722 at averaged  =0.6  0 G7 p  dependence  g7 is not sensitive to CBTAPS effect !

46. In-medium  spectral function   in_medium  90 MeV !

47. GSI HADES experiment @ GSI 2007: FW hodoscope added  < 7 0 proton/pion/HI beams e+e-/p/  /K id mass resolution  M/M~2% at  /  more details in second lecture !

48.  /  momentum distributions HADES is sensitive to both  /  :  M (  )~1.5% at low p (  <1.2) 3.5 GeV 1.17 GeV   +A: smaller momentum   +A: low beam intensity, broad focus proton beams:  higher intensity  excellent beam focus (1-2 mm)  reference reaction p +p @ 3.5 GeV done  1.2

49. Expectations for in-medium effect similar effect for pA

50. Background for p+A -  conversion beam 4x Nb segments : 4 x 0.5% I 0 1x Be segment (2%) 10 6 p/sec p+Nb @ 3.5 GeV p+ Pb @ 3.5 GeV S/B>1 for M>0.5 ! segmented target : Larger CB!

51. Pairs from pp @ 3.5 GeV ~ 5.5*10 9 LVL1 events collected (Apr07 -12 days running time) ~70% "online" analyzed (with reasonable calibration and tracking alignment) signal pairs: 54 k (all)   visible i on-line spectrum  ~35 MeV/c 2 ! S/B101 0.5 1.0 0.5 1.0 M ee M ee "HADES online pair spectrum from April 2007" to be continued with pA and  A with HADES..

52. Summary 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  but broadening  possible explanation of contradiction to E325/KEK CB normalization 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 : sensitive to both  / 

53. Line shape modification of  in pp? : intermediate resonaces D. Schumacher, S. Vogel et.al (UrQMD)  produced through Baryonic resonance N * (1520), N * (1720) and  (1700),  (1905) involved to be continued with pA and  A with HADES.. e+ e- N N*()N*()

54. m, p t, y distributions HADES acceptance and reconstruction efficiency filter: HADES acceptance is flat for M> 0.5 GeV/c 2 3.5 GeV 1.17 GeV

55. HADES acceptance e+,e- pairs with  e+e- > 9 0