WWND 2011 Michael Weber for the HADES collaboration Inclusive e + e - pair production in p+p and p+Nb collisions at E = 3.5 GeV Introduction HADES experiment Effects in cold nuclear matter Relative to  0 Cross sections Comparison with transport models Conclusions Technische Universität München

WWND 2011 Phase diagram Learn about properties of QCD matter at different (T,  ) EM structure of matter hadrons in matter

WWND 2011 Phase diagram Learn about properties of QCD matter at different (T,  ) EM structure of matter hadrons in matter Penetrating probe: Dielectrons Direct coupling of  * to VM (J P =1 - )       e+e-e+e-   e+e-e+e- 

WWND 2011 Dense and hot matter LHC SPS FAIR SIS e+ e-e+ e- e+ e-e+ e- e+ e-e+ e- e+ e-e+ e- e+ e-e+ e- RHIC Accessible in: A  + A reactions Dense matter at SIS: J. Stroth

WWND 2011 Cold nuclear matter Accessible in: p + A /  + A /  + A e+ e-e+ e-

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

WWND 2011 HADESI observabledetector pMDC (Magnet) βTOF(ino) dE/dxMDC TOF(ino) e + /e - RICH HADES [ EPJ A41 243] Production of strangeness: L. Fabbietti

WWND 2011 HADESII HADES [ EPJ A41 243] Beam: p E kin = 3.5 GeV I ~ 6 · /s Target : p 4 cm LH 2 ~1% reaction prob. 3.5 · 10 9 collisions A = 1A = 93 High purity e + e - pair reconstruction Beam: p E kin = 3.5 GeV I ~ 2 · /s Target : 93 Nb 12 x 0.45 mm ~3% reaction prob. 9.2 · 10 9 collisions observabledetector pMDC (Magnet) βTOF(ino) dE/dxMDC TOF(ino) e + /e - RICH

WWND 2011 e + e - pair reconstruction Comb. Background: same event e - e - e + e + correlated BG from  0 /  decays Signal to BG ratio

WWND 2011 e + e - sources in p + p particle production π via resonance decays ( ,N*) η, ω, ρ via phase space Δ through 1 π exchange particle decays form factors mass dep. Width PLUTO, Fröhlich et al, arxiv: cross sections in 4π (mb)  : 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) Δ FF is fixed at the photon point σ ω ~ 16 MeV/c 2 Analysis by Anar Rustamov

WWND 2011 e + e - sources in p + p  - N transition form factor Time like: q 2 > 0 Space like: q 2 < 0 Two–component VDM type model: Wan/Iachello, IJMP A20, 2005 only  relevant for   exclusive analysis Measured in electroproduction NOT Measured   N e-e- e+e+ Analysis by Anar Rustamov

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

WWND 2011 e + e - in p + Nb Scaling to same yield in  0 peak 00   Yield/  0 (p+Nb/p+p) IM: 1.51 HM: 1.37 In  region 1.13 → diff. production and/or absorption mechansims for diff. sources → kinematic observables for diff. mass regions LM IM HM

WWND 2011 pNb vs. pp: p T and y Transverse momentum p T 2 = p x 2 + p y 2 Rapidity y = ½ ln (E+p z )/ (E+p z ) y CM, NN = 1.12 LM y CM, NN

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

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

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

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

WWND 2011 Fast and slow e + e - sources High p: free p+p production Low p:overshoot over p+p different for , , and  High p Low p

WWND 2011 Cross sections p+p: X sect from  pp, elastic p+Nb: X sect from   -,acc comp to HARP Bolshakova et al., EPJC (1997) 1865 Isospin:  pd = 2  pp ( DLS: E kin > 2 GeV ) Wilson et al., PRC 57(1997) 1865 A Scaling: Black disc:  = 2/3 Glauber model:  ~ 0.8 W.Cassing et al., PLB 238 (1990) 25

WWND 2011 R pNb vs. p R pNb = 23.9 ± 1.1R pNb = 27.4 ± 1.4R pNb = 28.8 ± 2.4  pNb = 0.70 ± 0.10  pNb = 0.73 ± 0.13  pNb = 0.74 ± 0.21 High p: free NN prod. + same absorption for all sources LM IM HM  ~0.7

WWND 2011 R pNb vs. p R pNb = 23.7 ± 0.2R pNb = 40.9 ± 1.5R pNb = 48.4 ± 5.3  pNb = 0.70 ± 0.02  pNb = 0.82 ± 0.11  pNb = 0.86 ± 0.36 High p: free NN prod. + same absorption for all sources Low p : enhanced prod. (secondary processes) and/or lower absorption in IM and HM LM IM HM  ~0.7

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

WWND 2011 Transport models - M UrQMD HSD GiBUU Different , ,  contributions !

WWND 2011 Transport models - p T UrQMD HSD GiBUU Different , ,  contributions !  Constrain models ?

WWND 2011 Slow and fast sources (GiBUU) Collisional width in GiBUU for  momentum dependent 20 – 90 MeV ( 2 – 11 x  vac )

WWND 2011 Summary e + e - - pair production p+p and p+Nb at E = 3.5 GeV ( M, p T and y) clear  signal observed (   /M  ~ 2%) significant e + e - - yield with low pair momenta (p < 800 MeV/c) Additional prod. w.r.t p+p at intermediate masses ( hard p T and target rapidity ) A scaling of differential cross sections: (p+Nb/p+p) ~ A  –High p : all masses  ~ 0.7 –Low p :  0 region:  ~ 0.7  region:  ~ 0.82  region:  ~ 0.86 BUT  ~ 0.62  in - medium omega width Comparison with transport models  Better understanding of sources in p + p

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

WWND 2011 The HADES collaboration

WWND 2011 EXTRA SLIDES

WWND 2011 e + e - sources in p + p

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

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

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