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

Motivation Chiral soliton model Q+ observed ! (G≤ 15 MeV) ! Q+(1530) 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 …. N(1710) Input S(1890) X5(2070) (dsdsu) (ususd) X5 - - X5 + (G X5 Xp 42 MeV) ~

NA49 Experiment VTPC-1 (VTPC-2+MTPC) dE/dx resolution: 3-6 % dp/p2 = 7 x (0.3) 10-4 (GeV/c)-1 dE/dx resolution: 3-6 % (sTrig= 28.1 mb)

Data Sample p+p at = 17.2 GeV Before vertex cut After Vertex Cut 1999 1.2 M 0.66 M 2000 2.7 M 1.5 M 2002 2.9 M 1.6 M Total 6.8 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

X5 search X- selection: L 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 z x y X - bx by MV

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

Primary pion selection |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 p

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

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

Purify with momentum and asymmetric dE/dx cut 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

Additional cuts cos(q) < 0.997 (q > 4.5) X- X+ Backgrond Signal q: lab. angle between X- and p+ p- X- q 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 insignificant K+ contamination for events with X+

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

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

Systematics study I Remove resonances: events with p- K- p p+ 497.6 ±10 769.0 ±60 892.0 ±30 1115.7 ±10 K+ 1019.0 ±10 1520.0 ±15 By changing: - mass cuts around X and L - dE/dx cut - number of points cut - bx, by cuts -e.t.c. Peak remains robust !

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

Conclusion - - resonance at M = 1.862 ± 0.002 GeV/c2 ( ≤ 18 MeV/c2) 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 at the same mass Candidates for the X5 baryons !

Outlook NA49’ Project (Expression of Interest CERN-SPSC-2003-038) 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