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Quest for in-medium modifications in p+A, +A, +A collisions

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. R(A=100)  5 .5 fm. e+. e-. Main goal:.

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Quest for in-medium modifications in p+A, +A, +A collisions

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  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. R(A=100)5.5 fm e+ e- Main goal: • Studies of vector meson (//) spectral functions in nuclear medium via e+, e- pair spectroscopy at N  0 and T=0 • Dielectron two-body decays of light Vector Mesons r, w, f • High resolution spectroscopy of e+e-pairs, no final state interaction ! Decay length LD= c  R (decay in medium) N(L)=N(0)exp(-L/LD)

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

  4. W. Peters et.al. NPA 632(1998)109:  meson spectral function A(q,m) r = r0   N(1520)   r r r r D(1232) + ... + N-1 N-1 Hadronic scenario: meson spectral function Vacuum: In medium: • m,  depends on meson momentum (q)  A(m,p) spectral function • Connection to chiral symmetry?

  5. 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 M. Effenberger at al PHYS. REV. C 60 044614  decay in Pb nucleus () =vac () + coll()

  6. Experimental considerations: • cross sections • "inside" vs "outside" decays

  7.  cross sections for pp and p SAPHIR (Bonn) CBELSA (Bonn) DAPHNE, TAPS (Mainz) GRAAL (Grenoble) Lund String Model p pA A2/3 KEK (4.6 mb) exclusive •   +   o •   +  - •   +  o •   o  o •  ▲  • K+   K+o • Ko  ’   3.5 12 GeV GeV

  8. "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 on 93Nb with p/ production decay Decay length LD= c  R (decay in medium) N(L)=N(0)exp(-L/LD) 3.5 GeV 1.17 GeV

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

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

  11. ,g w p0 ,p0 N  e+ e- e+ e- Dielectron sources: „free” hadron: dielectron cocktail • Two body meson decays (peaks):  V→ e+e- • 3-body Dalitz-decays (continuum): V→ e+e- X Me+e [GeV/c2}- • CB – e+ e-combinatorial background • Signal/CB usually <1 !

  12. Meson Dalitz (3 body:e+e-X ) decays Vector Meson Dominance (VMD) or Vector Dominance Model (VDM) Dielectron spectrum BR(0e+e-) = 1.2*10-2 BR(e+e-) = 6*10-3 |F(q2)| - electromagnetic form factor : time-like q2>0 q2<0 : space like – probed via electron scattering experiments

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

  14. 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 (GE(q2), GM(q2), Gc(q2)) not known  electromagnetic structure of nucleon Various treatments: C. Fuchs: Phys. Rev. C67 025202(2003) M. Krivoruchenko et al. Ann.Phys296 (2002)299 1. Extended Vector Meson Domince Contribution from several vector mesons – interferences ! Ne+e-=5.02 KeV (BR=4*10-5) 2. Baryons are treated as point like particles (QED) 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

  15. rI =1 2p+4p+... r+w+f KK Inverse process: e+e- * hadrons qq Two body Vector Meson decays JP = 1- Vector Mesons carry same quantum numbers as photon ...quark em.current! I =0,1 + 2 + 3+... At meson pole !

  16. e- e+ Combinatorial Background  Combinatorial Background 0 e+ • 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!! e-  0 e-  • Combinatorial background (CB) e+ and e- comming from different vertices • Ne+e- unlike-sign pairs • Ne-Ne- and Ne+Ne+ like-sign pairs • absolute normalization CB= • Signal • S+-= Ne+e- - CB+- dedicated talk in student session !

  17. E325 Experiment Measures Invariant Mass ofe+e-, K+K- at KEK (1996-2002) in 12GeV p + A // + X reactions, s=5.1 GeV Mass Resolution for e+e- :8.0MeV/c2 for 10.7MeV/c2 for  Beam Primary proton beam (~7*108/s) Target Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress g conversion

  18. Target Configuration Very thin target with clean and high intensity beam Cu C Beam 23mm Vertex Distribution

  19. Experiment KEK-PS E325

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

  21. e+  e- f e+ E325 Model calculations of meson line shape • / mesons are generated uniformly at surface of target nucleus ( ~ A2/3) • momentum distribution: as measured in xp • pole mass downward shift : m*/m = 1 – k1 r/r0 k1 parameter • decay width increase : G*/G = 1 + k2r/r0  k2 parameter • density distribution • Woods-Saxon : • R: C:2.3fm/Cu:4.1fm e- • Detector response : • Blue histogram : Detector Simulation • Red line : Breit-Wigner (gaussian convoluted) fitting result • agrees with measured: Ks+ -

  22. 2/dof =1.8 2/dof =2.3 /=0.7±0.2 /=0.9±0.2 • CB subtracted • Data described (both nuclei) assumming /mass modification m*=m0(1-.092/0) • fit supports  * consistent with the vacuum for / • 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 p+A @ 12 GeV KEK-PS E325 M. Naruki Phys.Rev.Lett 96 (2006) 092301

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

  24. excluded from the fitting Mass modification of  for Cu targets • Integrate the amount of the excess in the above region(0.95~1.01 GeV/c2)  Nexcess • Fit again excluding the region where the excess was seen; 0.95~1.01GeV/c2 Yokkaichi Meson2006 • Best fit (both nuclei) achieved for k1=0.035 and k2=2.6 (*  ) . Mass shift of 

  25. Photon experiment G7 @ JLaB

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

  27. Toroid with 6 coils – 6 sectors

  28. Electron identification in CLAS C Fe C Pb C Ti C ~1.1% D2 dedicated talk in student session !

  29. no acceptance ! Detector acceptance • e-,e+ ID only for 8-450 : EmCal. coverage • electrons in same sector forbidden! Pair acceptance

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

  31. Signal pairs (CB subtracted)

  32. Final result for the  meson spectral function cocktail of free // meson decays subtracted  spectral function k1=0.02±0.02 consistent with 0! • contradiction to KEK PS E325

  33. 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 !

  34. Crystal Barrel & TAPS (CBTAPS) @ ELSA

  35. Detector (photon calorimeter)

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

  37. Reference experiment:  + N

  38. p dependence G7 • g7 is not sensitive to CBTAPS effect ! CB/TAPS @ ELSA D. Trinka Phys. Rev, Lett (2005) 192303 m =722 at averaged =0.60 • m =m0 (1 - k /0); k = 0.14 • mass shift of 

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

  40. HADES experiment @ GSI • 2007: FW hodoscope added • < 70 • proton/pion/HI beams • e+e-/p//K id • mass resolution • M/M~2% at / more details in second lecture !

  41. 3.5 GeV 1.17 GeV /momentum distributions 1.2 • HADES is sensitive to both  / : M ()~1.5% at low p (<1.2) • +A: smaller <p/ > 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

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

  43. p+ Pb @ 3.5 GeV p+Nb @ 3.5 GeV S/B>1 for M>0.5 ! Background for p+A -  conversion beam • segmented target : 106 p/sec 4x Nb segments : 4 x 0.5% I0 1x Be segment (2%) Larger CB!

  44. 10 S/B 1 Pairs from pp @ 3.5 GeV "HADES online pair spectrum from April 2007" • ~ 5.5*109 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/c2 ! 0.5 1.0 Mee to be continued with pA and A with HADES..

  45. Summary • Meson line shape modifications seen in p+A/+A reactions: • E325/KEK : downward  mass shift (~ 9%) • E325/KEK: downward  mass shift (~3%) and broadening • 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 / 

  46. 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 N N*() e+ e- to be continued with pA and A with HADES..

  47. m, pt, y distributions • HADES acceptance and reconstruction efficiency filter: 3.5 GeV 1.17 GeV • HADES acceptance is flat for M> 0.5 GeV/c2

  48. HADES acceptance e+,e- pairs with e+e-> 90

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