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Study of e+e- Pair Production in p+p and p+Nb Collisions at 3.5 GeV

This study focuses on inclusive e+e- pair production in p+p and p+Nb collisions at 3.5 GeV. It analyzes effects in cold nuclear matter, relative to p0, cross sections, and comparisons with transport models, drawing conclusions on QCD matter properties, EM structure, and hadrons in matter.

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Study of e+e- Pair Production in p+p and p+Nb Collisions at 3.5 GeV

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  1. Michael Weber for the HADES collaboration Inclusive e+e- pair production in p+p and p+Nb collisions at E = 3.5 GeV Technische Universität München • Introduction • HADES experiment • Effects in cold nuclear matter • Relative to p0 • Cross sections • Comparison with transport models • Conclusions WWND 2011

  2. Phase diagram • Learn about properties of QCD matter at different (T,r) • EM structure of matter • hadrons in matter http://www.ice.csic.es/en/graphics/phase.jpg WWND 2011

  3. o,  w,r o, e+ e- g* e+ e- w,r,f g* Phase diagram • Learn about properties of QCD matter at different (T,r) • EM structure of matter • hadrons in matter • Penetrating probe: • Dielectrons • Direct coupling of g* to VM (JP=1-) w,r,f http://www.ice.csic.es/en/graphics/phase.jpg WWND 2011

  4. e+ e- e+ e- e+ e- e+ e- Dense and hot matter • Accessible in: A+ A reactions LHC RHIC SPS FAIR SIS e+ e- Dense matter at SIS: J. Stroth http://www.ice.csic.es/en/graphics/phase.jpg WWND 2011

  5. e+ e- Cold nuclear matter • Accessible in: p + A / g + A / p + A WWND 2011

  6. Cold nuclear matter r meson • Accessible in: p + A / g + A / p + A • Medium effects: mass shift/ (collisional) broadening/ reabsorption/ regeneration • Measure: Spectral shape / Cross sections/ Attenuation • BUT: NON conlusive experimental results P.Mühlich et al., NPA 780 (2006), 187 WWND 2011

  7. HADESI HADES [ EPJ A41 243] Production of strangeness: L. Fabbietti 21.08.2014 WWND 2011

  8. HADESII HADES [ EPJ A41 243] High purity e+e- pair reconstruction A = 1 A = 93 • Beam: p • Ekin= 3.5 GeV • I ~ 6 · 106 1/s • Target : p • 4 cm LH2 • ~1% reaction prob. • 3.5 · 109 collisions • Beam: p • Ekin= 3.5 GeV • I ~ 2 · 106 1/s • Target :93Nb • 12 x 0.45 mm • ~3% reaction prob. • 9.2 · 109 collisions 21.08.2014 WWND 2011

  9. e+e- pair reconstruction • Signal to BG ratio • Comb. Background: • same event e-e- e+e+ • correlated BG from p0 / h decays WWND 2011

  10. e+e- sources in p + p • particle production • π via resonance decays (D,N*) • η, ω, ρ via phase space • Δ through 1 π exchange • particle decays • form factors • mass dep. Width PLUTO, Fröhlich et al, arxiv:0708.2382 • cross sections in 4π (mb) • π: 16 ± 2.6 (from data) • Δ: 7.5 PYTHIA • η: 0.93 ± 0.2 (fit to data) • ω: 0.25 ± 0.05 (fit to data) • ρ: 0.38 ± 0.07 (fit to data) σω ~ 16 MeV/c2 Δ FF is fixed at the photon point WWND 2011 Analysis by Anar Rustamov

  11. e+e- sources in p + p D - N transition form factor Space like: q2 < 0 Measured in electroproduction N D g* e- e+ Two–component VDM type model: Wan/Iachello, IJMP A20, 2005 only r relevant for D exclusive analysis NOT Measured Time like: q2 > 0 WWND 2011 Analysis by Anar Rustamov

  12. h e+e- No direct decay (same for p + Nb):  reduce BR (2.7 x 10-5) at least by factor of 3 WWND 2011 Analysis by Anar Rustamov

  13. e+e- in p + Nb Scaling to same yield in p0 peak Yield/p0 (p+Nb/p+p) • IM: 1.51 • HM: 1.37 • In w region 1.13 → diff. production and/or absorption mechansims for diff. sources → kinematic observables for diff. mass regions LM IM HM p0 h, D r,w,f WWND 2011

  14. pNb vs. pp: pT and y yCM, NN LM LM Transverse momentum pT2 = px2 + py2 Rapidity y = ½ ln (E+pz)/ (E+pz ) yCM, NN = 1.12 WWND 2011

  15. pNb vs. pp: pT and y For both mass bins: • Higher yield • At high pt • Shift to target rap.  add. slow source IM IM HM HM WWND 2011

  16. Fast and slow e+e- sources • Decays inside nucleus • In – Medium effects p cut HSD Simulation for HADES proposal WWND 2011

  17. Fast and slow e+e- sources HM p cut p+Nb Ekin= 3.5 GeV HADES: Significant e+e- yield with low pair momenta ( ~ 35 % in HM) WWND 2011

  18. Fast and slow sources in other Exp. KEK: p + A @ 12 GeV JLAB: g + A @ 0.6 -3.8 GeV S.Leupold, V.Metag and U.Mosel, nucl-th 0907.2388 R.Muto et al., PRL 98 (2007) 042501 WWND 2011

  19. Fast and slow e+e- sources Low p High p • High p: free p+p production • Low p: overshoot over p+p different for r, w, and f WWND 2011

  20. Cross sections • p+p: • X sect from spp, elastic • p+Nb: • X sect from sp-,acc comp to HARP • Bolshakova et al., EPJC (1997) 1865 • Isospin: • spd = 2 spp ( DLS: Ekin > 2 GeV ) • Wilson et al., PRC 57(1997) 1865 • A Scaling: • Black disc: a = 2/3 Glauber model: a ~ 0.8 • W.Cassing et al., PLB 238 (1990) 25 WWND 2011

  21. RpNb vs. p LM HM IM a~0.7 • High p: free NN prod. + same absorption for all sources WWND 2011

  22. RpNb vs. p LM HM IM a~0.7 • High p: free NN prod. + same absorption for all sources • Low p : enhanced prod. (secondary processes) and/or lower absorption in IM and HM WWND 2011

  23. A scaling for vector mesons Fit with Gaussians: • High p: apNb,w = 0.69 ± 0.27 apNb,f = 1.04 ± 2.50 • Low p: apNb,w = 0.62 ± 0.36 Data : p+A @ 12. GeV KEK E325  = 0.710 ± 0.021(stat) ± 0.037(syst)  = 0.937 ± 0.049(stat) ± 0.018(syst) • w: momentum dependent absorption ? • f: production in whole nucleus volume (statistics!) ? T. Tabaru et al.. Phys.Rev. C 74(2006) WWND 2011

  24. Transport models - M Different D, h, r contributions ! GiBUU UrQMD HSD WWND 2011

  25. Transport models - pT Different D, h, r contributions !  Constrain models ? GiBUU UrQMD HSD WWND 2011

  26. Slow and fast sources (GiBUU) Collisional width in GiBUU for w • momentum dependent • 20 – 90 MeV ( 2 – 11 x Gvac) WWND 2011

  27. Summary • e+e- - pair production p+p and p+Nb at E = 3.5 GeV ( M, pT and y) • clear w signal observed (sw/Mw ~ 2%) • significant e+e- - yield with low pair momenta (p < 800 MeV/c) • Additional prod. w.r.t p+p at intermediate masses ( hard pT and target rapidity ) • A scaling of differential cross sections: (p+Nb/p+p) ~ Aa • High p : all masses a ~ 0.7 • Low p : p0 region: a ~ 0.7 h, Dregion: a ~ 0.82 r,w,f region: a ~ 0.86 BUT w a ~ 0.62  in - medium omega width • Comparison with transport models  Better understanding of sources in p + p WWND 2011

  28. Outlook • e+e- - pair production in HI reactions • Regeneration of VM • Higher densities • HADES upgrade • in p- induced reactions • Large X – sections • VM „at rest“ • Beam particle tracking WWND 2011

  29. The HADES collaboration Thank you WWND 2011

  30. EXTRA SLIDES WWND 2011

  31. e+e- sources in p + p WWND 2011

  32. Collisional width of w in GiBUU free width WWND 2011 Priv. Comm. Janus Weil

  33. p + p normalization Elastic scattering Kinematic constraints: Kammerud et al. Phys. Rev. D 4 (1971), 5 WWND 2011 Analysis by Anar Rustamov

  34. p + Nb normalization p- yield in HADES acceptance: Systematic uncertainty Fit to HARP data: pT > 300 MeV/c WWND 2011

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