Hadron Induced two Pion Production in Nuclei

1. Hadron Induced two Pion Production in Nuclei RUDOLF MEIER “Sigma“ medium modifications ? (p,2p) CHAOS Crystal Ball Comparison tophoton induced reaction: open questions (p,2p) TOF WASA

2. “Sigma“ Meson • Particle Data Group 2002 • f0(600) or s • JP = 0+ • m = (400-1200) MeV • = (600-1000) MeV Dominant decay: f0(600) gpp Recent experimental evidence for a s-meson Takamatsu et al. NPA 675, 312c (2000):s seen in p(p-,p0p0)n Aitala et al. PRL 86, 770 (2001):s seen in D decay to 3p Wu hep-ex/0104050 (2001): s seen in J/Y decay 2pw What is it? Pion-Pion S-wave resonance Quark-Antiquark-state Chiral perturbation theory

3. Lutz et al., NPA542, 521(1992) Chiral restoration and particle properties Chiral symmetry: parity doublets of hadrons Not seen in the meson or baryon spectra! g Spontaneous chiral symmetry breaking Restoration of chiral symmetry: Hadrons group to parity doublets! gMedium modifications of masses and widths • Chiral partner of the Pion? • f0(400-1200) or s • JP = 0+ • m = (400-1200) MeV • = (600-1000) MeV Dominant decay: f0(400-1200) gpp • (PDG 2000)

4. p0p0 Identical Bosons! L=0,2,... Crystal Ball, TAPS Prescription for the Experiment • Search for medium modifications of the s (0,0+) meson. • Search for changes of the pp (0,0+) system in matter. • Systems which are (partially) pp (0,0+) : • p0p0 , p+p-(not p+p+) p+p- CHAOS L=0,1,2,... Could couple to r(1,1-)

5. Expect: Changes if s is medium-modified Control channel p+p+ p+p- Pb Pb Ca Ca C C D D Mpp Mpp CHAOS Experiment: Concept • Measure • For different nuclei A: D, 12C, 40Ca, 208Pb Variation of density r • Outgoing Pions should have low energy large mean free path! Control channel: • For different Targets and • channels: compare the pp • invariant mass

6. CHAOS – E653 Collaboration F. Bonutti, P. Camerini, E. Fragiacomo, N. Grion, R. Rui University and INFN Trieste, Trieste, Italy P. Amaudruz, J. T. Brack, L. Felawka, R. Meier, D. Ottewell, K. Raywood, G. R. Smith TRIUMF, Vancouver, BC, Canada M. E. Sevior University of Melbourne, Parkville, Victoria, Australia M. Kermani, G. Hofman, S. McFarland University of British Columbia, Vancouver, BC, Canada E. L. Mathie, R. Tacik University of Regina, Regina, Saskatchewan, Canada E. F. Gibson California State University, Sacramento, California, USA F. Bonutti et al., Phys. Rev. Lett. 77, 603 (1996), ..., Nucl. Phys. A 677, 213 (2000) P. Camerini et al, Phys. Rev. C 64, 067601 (2001)

8. phase space ps, p cut ps, p and Q cut ps, p, Q and F cut Mpp [GeV/c2] Acceptance for the pp Invariant Mass

9. p-p Invariant Mass Distributions on the Deuteron ds/dMppin the CHAOS acceptance (A.U.)

10. Description of the Elementary p2p Reaction Valencia group calculation Vincente-Vacas, Oset, Phys. Rev. C 60, 064621 (1999) Results (in the CHAOS acceptance) ds/dMppin the CHAOS acceptance (A.U.) (p+, p+p-): interference (p+, p+p+): phase space

11. CHAOS (p,pp) data ds/dMpp in the CHAOS acceptance (A.U.) phase space

12. Double ratio

13. pp FSI In vacuum: In medium: additional terms N-hole, D-hole excitation Modification of the pp interaction in medium Vincente-Vacas, Oset, Phys. Rev. C 60, 064621 (1999)

14. Modification of the pp interaction in medium Vincente-Vacas, Oset, Phys. Rev. C 60, 064621 (1999) ds/dMppin the CHAOS acceptance (A.U.) Valencia group calculation Phase space 0.24rn0.7rn

15. Crystal Ball: p- A → p0p0 X Phase space Valencia group calculation

16. TAPS: g A → pp X g A → p0p0X g A → p0p+/- X Valencia group calculation Phase space

17. CHAOS TAPS Photon vs. pion induced reaction • Common features: • Enhancement of low Mpp region • Enhancement comparable for C to Pb • Medium modification of the sigma channel! • Differences: • Enhancement different for elementary reaction to C 4(p,2p)/1.5(g,2p) • Valencia group calculation reproduces (g,2p) enhancement, fails for (p,2p)

18. Photon vs. pion induced reaction • Possible reasons for discrepancies given by Valencia group: • Larger nucleon density probed in (g,2p) • but: artificially higher density in (p,2p) did not help • Accidental cancellation of large parts of the reaction amplitude • in (p,2p): difficulties for reliable prediction. • can be tested in 3rd production mechanism, i.e. (p,2p) • Alternative explanation: • Electromagnetic and hadronic production mechanisms differ • Hadronic mechanism dominated by Roper resonance N*(1440) • Medium modification of Roper decay properties? • Difference between s→2p and other 2p (0+) systems? • can be tested in 3rd production mechanism, i.e. (p,2p)

19. Why pN → NNpp ? • In the low-energy tail of the Roper region: • Roper excitation dominant • Very sensitive to Ns and Dp channels Dp N* N ( pp )I=l=0 Ns • Partly similar, partly different from the situation in ( p, pp ) and ( g, pp ) • Detailed investigations of the elementary reaction underway at COSY-TOF and CELSIUS-WASA Brodowski et al., PRL 88 (2002) • Search for medium modifications in (p,pp) at TOF and WASA!

20. Hadron Induced two Pion Production in Nuclei • Search for medium modifications of the “sigma“ • Pion-induced pion production • TRIUMF-CHAOS • BNL-Crystal Ball • Evidence for “sigma“ medium modifications not described by Valencia group calculations • Similarities, discrepancies to photon-induced reaction • Open questions • Get answers from Proton-induced two pion production in nuclei at COSY-TOF and CELSIUS-WASA!