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Search for exotic baryon resonances in pp collisions at the CERN SPS

Tatjana Šuša for the NA49 Collaboration Ruđer Bošković Institute, Zagreb,Croatia. Search for exotic baryon resonances in pp collisions at the CERN SPS. ~. (G X 5  Xp 42 MeV ). ~. - -. X 5. Motivation. Chiral soliton model. Anti-decuplet of baryons: J P =1/2 +.

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Search for exotic baryon resonances in pp collisions at the CERN SPS

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  1. Tatjana Šuša for the NA49 Collaboration Ruđer Bošković Institute, Zagreb,Croatia Search for exotic baryon resonances in pp collisions at the CERN SPS

  2. ~ (G X5 Xp 42 MeV) ~ - - X5 Motivation Chiral soliton model Anti-decuplet of baryons: JP=1/2+ D. Diakonov, V. Petrov, M. Polyakov, Z.Phys. A 359 (1997) 305 Q+ observed ! LEPS, DIANA, CLAS, SAPHIR, HERMES, ITEP (ududs) (G≤ 15 MeV) ! Q+(1530) Models: - Correlated quark model - Topological soliton model - Karliner –Lipkin model …. Input N(1710) S(1890) X5(2070) (dsdsu) (ususd) + X5

  3. NA49Experiment VTPC-1 (VTPC-2+MTPC) dp/p2 = 7 x (0.3) 10-4 (GeV/c)-1 dE/dx resolution:3-6 % (sTrig= 28.1 mb)

  4. Data Sample p+p at = 17.2 GeV Before vertex cut After Vertex Cut 19991.2 M 0.66 M 2000 2.7 M 1.5 M 2002 2.9M 1.6 M Total6.8 M 3.76 M Main vertex cuts: 1.) fit converged 2.) x2+y2< 1 cm2 3.) -590.5 cm < z < -572.5 cm i.e. within target

  5. X - y x z MV by bx X5search X-selection: Distance to Bethe- Bloch curve: |dbb| < 3 s |M(pp-) - 1.115683| < 0.015 GeV Z X- Zmain_vtx > 12 cm X- position at main vertex (bx , by): |bx| < 2 cm |by| < 1 cm p(from X- decay) position at main vertex |by|> 0.5 cm L

  6. V0 and X Invariant Mass Spectra X-peak 1640 events L peak |M(Lp)-1.321| < 0.015GeV X+peak L peak 551 events

  7. Primary pion selection (- , + p) p • |dbb| < 1.5 s, dbb .... distance to Bethe-Bloch curve • position at main vertex (bx , by): |bx| < 1.0 cm |by| < 0.5 cm • # of points > 10

  8. X-p Invariant Mass Distribution Structure at 1.86 GeV Candidate for exotic X5 Absolute mass scale correct X(1530)0

  9. X+p Invariant Mass Distribution No clear structures at 1.86 GeV

  10. Additional cuts (X-p- ) events Negative tracks e- p- K P large proton and K+ contamination (X-p+ ) events Positive tracks Purify with momentum and asymmetric dE/dx cut

  11. X- q p- X-X+ p--1.5 s < dbb < 1.5s -1.5 s < dbb < 1.5 s p+-0.5 s< dbb < 1.5s -1.5 s < dbb < 1.5 s p > 3 GeV/c p > 3 GeV/c Additional cuts cos(q) < 0.997(q > 4.5) Backgrond Signal q: lab. angle between X- andp+ insignificant K+ contamination for events with X+

  12. Final Xp Invariant Mass Distributions - +spectra: peak at 1.86 GeV Antibaryon spectra enhancement at same mass C. Alt et al., Phys. Rev. Lett. 92

  13. Mass and width determination M = 1.862 ± 0.002 GeV/c2 M = 1.864± 0.005 GeV/c2 M = 1.862 ± 0.002 GeV/c2 G≤ 18 MeV/c2

  14. Systematics study I Remove resonances: events with By changing: - mass cuts around Xand L - dE/dx cut - number of points cut - bx, by cuts -e.t.c. Peak remains robust !

  15. X-p (VENUS + GEANT + REC.) p: all negative particles p : true negative pions Systematics study II DATA ( X shoulder) No structure at 1.86 GeV

  16. Conclusion • Strong evidence for the existence of a narrow - -resonance at M = 1.862 ± 0.002 GeV/c2 ( ≤ 18 MeV/c2) • At the same mass a peak is observed in the -+spectrum • The corresponding antibaryon spectra shows enhancement atthe same mass • Candidates for the X5baryons !

  17. Outlook Open Questions • Spin ? Parity? Width ? Production mechanism? NA49 • Further improvement of experimental resolution • Data sample increase by up to ~ 30% • Look at other possible decay channels X5 X(1530) p, X5 LK • Look for other pentaquark states NA49’ Project(Expression of Interest CERN-SPSC-2003-038) In particular: • (3-6) • 107 p+p events (new DAQ system) • Neutral pions

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