Boris Lemmer and Volker Metag for the A2 collaboration II. Physikalisches Institut

1. In-medium properties of the omega meson from a measurement of the excitation function in photoproduction Boris Lemmer and Volker Metag for the A2 collaboration II. Physikalisches Institut CB@MAMI collaboration meeting, Mainz, March 8-10, 2010

2. V. Bernard and U.-G. Meißner NPA 489 (1988) 647 NJL-model F. Klingl et al. NPA 610 (1997) 297 NPA 650 (1999) 299 P. Mühlich et al., NPA 780 (2006) 187 for rB: splitting of ω meson strength due to coupling to S11 andD13 resonances mass degeneracy of chiral partners expected at high baryon densities 1.) lowering of in-medium mass 2.) broadening of resonance model predictions for in-medium masses of mesons

3. Experimental information on in-medium properties of mesons M. Nanova 1.) analysis of meson line shape on a nuclear target in comparison to a reference measurement on LH2 3 approaches: • limited sensitivity; contributions from high density suppressed ~1/2 2.) in-medium width from measurement of the transparency ratio 3.) sensitivity to different in-medium modification scenarios of the excitation function near the photo-production threshold M. Kotulla et al., PRL 100 (2008) 192302 K. Gallmeister et al., Prog. Part. Nucl. Phys. 61 (2008) 283 Ca

4. K+ and K- mesons in the nuclear medium theoretical predictions: D. B. Kaplan and A. E. Nelson PLB 175 (1986) 57 J. Schaffner-Bielich et al. Nucl. Phys. A 625 (1997) 325 free hadron: hadron in medium of baryon density : concept of an effective mass

5. MK/<Apart> energy above threshold [GeV] Kaons in the nuclear medium K+ and K--production in nucleon-nucleon and nucleus-nucleuscollisions KaoS-collaboration Ni + Ni  K+, K- + X elementary reactions  [b] energy above threshold [GeV] for the same energy above threshold  enhanced K--yield in nucleus-nucleus collisions evidence for K-mass modification in dense matter

6. TAPS: 384 BaF2 modules 10 <  < 200 Crystal Ball: 672 NaI modules 200 <  < 1600 photon beam Ee= 1604 MeV in this experiment: 700 MeV < E < 1400 MeV Crystal Ball /TAPS set up in Mainz data taken in April, June, and September 2008

7. 0; selection of prompt -rays Time cuts: prompt coincidences TAPS  CB; TAPS  tagger; CB  tagger

8. Excitation function for  photoproduction near threshold  + C   + X; 0

9. Excitation function for  photoproduction near threshold  + Nb   + X; 0;

10. Comparison of measured excitation function with GiBUU calculations Boris Lemmer, Stefan Friedrich, Janus Weil preliminary data disfavour „broadening + mass shift“ scenario and favour „Fermi motion + collisional broadening“ scenario

11. in-medium broadening of the  meson to • deduced from measurement of the transparency ratio; • i.e. the  is broadened by a factor 16 relative to its vacuum width Summary and conclusions • information on in-medium properties of the  meson • difficult to extract from an analysis of the  lineshape; • contributions from higher densities suppressed by 1/2 ; • sensitivity shifted to the surface region of the nucleus • excitation function for photo production near threshold sensitive to • different in-medium modifcation scenarios •  photoproduction cross sections on C, Nb deduced from data taken • at MAMIC for E= 900 – 1400 MeV • comparison of data with GiBUU simulations favours a broadening • but not an additional mass shift ot the  meson in a nuclear medium

12. Strength function parametrized by Breit Wigner shape  1/2  1/ 1/ In low density approximation:  for a large in-medium broadening of the meson as observed for the  meson: (M. Kotulla et al., PRL 100 (2008) 192302)  contributions from higher densities suppressed by order of 1/2 • for cases of strong in-medium broadening (, ) it is in principle difficult to observe in-medium shape changes; for details see: J. Lehr and U. Mosel, Phys. Rev.C 64 (2001)042202 S. Leupold, V. Metag, U. Mosel, arXiv:0907.2388 Sensitivity to in-medium signal

13. hadron decay in the medium: • reconstruction of invariant mass from 4-momenta of decay products: • compare with listed in PDG • ensure that decays occur in the medium: •  select shortlived mesons: decay length s= c • for  s  • cut on low meson momenta for  and  mesons 1.3 fm () ; 23 fm () ; 46 fm () • experimentally observed mass distribution = convolution of • spectral function with branching ratio into channel being studied experimental approach:  F. Eichstaedt et al., Prog. Theo. Phys. Suppl. 168 (2007) 495 • avoid distortion of 4-momentum vectors by final state interactions •  dilepton spectroscopy: r, w, f e+e-

14. Information on in-medium properties of mesons 3 approaches: M. Nanova 1.) analysis of meson line shape on a nuclear target in comparison to a reference measurement on LH2 • limited sensitivity; contributions from high density suppressed ~1/2 2.) in-medium width from measurement of the transparency ratio P. Mühlich (PhD-thesis, Giessen 2006) K. Gallmeister et al. Prog. Part. Nucl. Phys. 61 (2008) 283 Ca 3.) sensitivity to different in-medium modification scenarios of the excitation function near the photo-production threshold

15. Strength function parametrized by Breit Wigner shape  1/2  1/ 1/ In low density approximation:  for a large in-medium broadening of the meson as observed for the  meson: (M. Kotulla et al., PRL 100 (2008) 192302)  contributions from higher densities suppressed by order of 1/2 • for cases of strong in-medium broadening (, ) it is in principle difficult to observe in-medium shape changes; for details see: J. Lehr and U. Mosel, Phys. Rev.C 64 (2001)042202 S. Leupold, V. Metag, U. Mosel, arXiv:0907.2388 Sensitivity to in-medium signal

16. Measurement of ω excitation function • additional test for ω broadening and/or mass shift scenario Gi-BUU simulations: P. Mühlich (PhD-thesis, Giessen 2006) K. Gallmeister et al. Prog. Part. Nucl. Phys. 61 (2008) 283 Ca Enhanced  yield for dropping mass scenario below production threshold on free nucleon