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

2. 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 MeV) ~

3. NA49 Experiment VTPC-1 (VTPC-2+MTPC) dE/dx resolution: 3-6 %
dp/p2 = 7 x (0.3) (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
M M
M M
M M
Total M M
Main vertex cuts:
1.) fit converged
2.) x2+y2< 1 cm2
3.) cm < z < cm
i.e. within target

5. X5 search X- selection: L Distance to Bethe- Bloch curve:
|dbb| < 3 s
|M(pp-) | < 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

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

7. 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

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. 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

11. 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 s < dbb < 1.5s s < dbb < 1.5 s
p s < dbb < 1.5s s < dbb < 1.5 s
p > 3 GeV/c p > 3 GeV/c
insignificant K+
contamination for
events with X+

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

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

14. Systematics study I Remove resonances:
events with p- K- p p+
497.6 ±10
769.0 ±60
892.0 ±30
±10 K+
±10
±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 !

15. 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

16. Conclusion - - resonance at M = 1.862 ± 0.002 GeV/c2 ( ≤ 18 MeV/c2)
Strong evidence for the existence of a narrow
- - resonance at M = ± 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 !

17. 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 )
In particular:
(3-6) • 107 p+p events (new DAQ system)
Neutral pions