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Hadrons in the nuclear medium - recent experimental results

motivation first observation of medium modifications of the  meson: a.) mass shift b.) in-medium width first observation of an -nucleus bound state: summary and outlook. Hadrons in the nuclear medium - recent experimental results. Volker Metag II. Physikalisches Institut

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Hadrons in the nuclear medium - recent experimental results

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  1. motivation • first observation of medium modifications of the  meson: • a.) mass shift • b.) in-medium width • first observation of an -nucleus bound state: • summary and outlook Hadrons in the nuclear medium - recent experimental results Volker Metag II. Physikalisches Institut Universität Giessen Germany 5th. International Conference on Perspectives in Hadron Physics Trieste, Italy, May 22-26, 2006

  2. hadrons = excitations of the QCD vacuum Mass [GeV] G.E.Brown and M. Rho, PRL 66 (1991) 2720 T.Hatsuda and S. Lee, PRC 46 (1992) R34 Motivation • QCD-vacuum: complicated structure • characterized by condensates • in the nuclear medium: • condensates are changed • change of the hadronic excitation energy spectrum • widespread experimental activities to search for in-medium modifications of hadrons

  3. V. Bernard and U.-G. Meißner NPA 489 (1988) 647 NJL-model K. Saito, K. Tushima, and A.W. Thomas PRC 55 (1997) 2637 Quark-meson coupling model (QMC) decrease of  -mass by  15% at normal nuclear matter density -mass roughly constant model predictions for in-medium masses of mesons

  4. - meson -meson F. Klingl et al. NPA 610 (1997) 297 NPA 650 (1999) 299 M. Post et al., nucl-th/0309085 AT [GeV-2] 1.) lowering of in-medium mass 2.) broadening of resonance for rB m [GeV] q [GeV] Model predictions for spectral functions of r and w mesons

  5. S. Zchocke et al. , Phys. Lett. B 562 (2003) 562 M. Lutz et al. , Nucl. Phys. A 706 (2002) 431 r w structure in spectral function due to coupling to baryon resonances variation in -mass due to density dependence of 4-quark condensate Model predictions for spectral functions of r and w mesons

  6. reconstruction of invariant mass from 4-momenta of decay products: p (12 GeV) A ,  +X • KEK-E325: M. Naruki et al., PRL 96 (2006) 092301: no broadening in the medium!! -meson: • NA60: R. Arnaldi et al., PRL 96 (2006) 162302: In + In (158 AGeV) -spectral function shows strong broadening but no shift in mass experimental approach: dilepton spectroscopy: r, w, f e+e- essential advantage: no final state interactions !! • CLAS (Jlab): C. Tur et al., A , ,  +X • HADES (GSI): planned experiment - p   n on bound proton

  7. J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95 gA   + X p g p0g g M. Effenberger et al.  w p0 g g fraction of -decays in the medium (  0.1 0) :  35% -mass in nuclei from photonuclear reactions advantage: • p0g large branching ratio (8 %) • no -contribution (  0 : 7  10-4) disadvantage: • p0-rescattering

  8. Expected  in-medium signal rescattering of pions in nuclei predominantly proceeds through (1232) excitation: scattered pions have Ekin150 MeV no distortion by pion rescattering expected in mass range of interest

  9. 4 p detector system CB/TAPS @ ELSA Crystal Barrel 1290 CsI TAPS 528 BaF2 front view of TAPS side view E= 900-2200 MeV • = 00 to 3600  = 300 to 1680 • = 00 to 3600  = 50 to 300

  10. comparison of meson masses and lineshapes for LH2 and nuclear targets 0   No change of mass and lineshape for longlived mesons (0, , ) decaying outside nuclei

  11. after background subtraction mNb = 763 MeV;   0.110consistent with m =m0 (1 -  /0) for  = 0.13 inclusive 0 signal for LH2 and Nb target D. Trnka et al., PRL 94 (2005)192303 difference in line shape of  signal for proton and nuclear target

  12. vacuum contribution in-medium contribution decomposition of  signal into in-medium and vacuum decay contributions Nb: in-medium: 45% C: in-medium: 40% lineshape of vacuum contribution taken from LH2 experiment shape of in-medium contribution taken from BUU simulation (P. Mühlich and U. Mosel, NPA (2006)), assuming m = m0(1 - 0.16 /0)

  13. normalization to C!! transparency ratio: = 19 MeV = 34 MeV 37 MeV 94 MeV 74 MeV M. Kaskulov and E. Oset priv. communication P. Mühlich and U. Mosel NPA (2006) access to in-medium  width in-medium  width proportional to  absorption:   vabs

  14. access to in-medium  width in-medium  width proportional to  absorption:   vabs normalization to C!! transparency ratio: = 19 MeV = 34 MeV 37 MeV 94 MeV 74 MeV Comparison to data (D.Trnka et al.) (0)  45 – 95 MeV

  15. Nb LH2 momentum dependence of  signal (Nb-target) • mass modification only for p 0.5 GeV/c determination of momentum dependence of  - nucleus potential requires finer momentum bins  improved 2nd. generation experiment

  16. -, -meson-nucleus potential K. Saito, K. Tsushima, A.W. Thomas, hep-ph/0506314 predictions within the quark meson coupling model (QMC) : E(1s) = -39 MeV; = 29 MeV : E(1s) = -100 MeV; = 31 MeV : E(1s) = -56 MeV; = 33 MeV : E(1s) = -118 MeV; = 33 MeV

  17. The population of meson-nucleus bound states in recoil-free kinematics forward going nucleon takes over photon momentum magic incident energies : E  930 MeV : E  2750 MeV (ELSA)

  18. E. Marco and W. Weise, PLB 502 (2001) 59 quasifree quasifree -mesic states T. Nagahiro et al. N. Phys. A 761 (2005) 92 attractive potential repulsive potential no intensity for negativeenergies signature for -mesic states

  19. E = 2750 MeV simulation E = 1.5 –2.5 GeV p 5° 15° candidates background 740 MeV/c  |p| < 500 MeV/c for p < 140 240 MeV/c bound states expected forp < 140 correlation between  momentum and proton angle

  20. small proton angles 70 < p< 140 large proton angles 180 < p< 280 quasi- free C C LH2 LH2 • for small proton angles: difference between C and LH2 data for negative energies • for large proton angles: similar background distributions for C and LH2 data comparison of dataon LH2 and C

  21. Evidence for carbon data after background subtraction quasifree 70 < p< 140  mesic states theoretical prediction E.Marco and W.Weise PLB 502 (2001) 59 firstevidence for the existence of an -nucleus bound state: here:

  22. An in-medium dropping of the  meson mass has been observed • consistent with • first information on in-medium  width: Summary and outlook • major step forward towards understanding the origin • of hadron masses • first evidence for mesic 11B • remaining open questions •  momentum dependence of -nucleus potential? •  structure in  strength function? •  confirm existence of  mesic states in heavier nuclei • higher statistics needed !! • improved experiments planned at MAMI and ELSA

  23. CBELSA/TAPS collaboration

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