Tatjana

1
Search for exotic baryon resonances in pp
collisions at the CERN SPS

• Tatjana Šuša
• for the NA49 Collaboration
• Ruder Boškovic Institute, Zagreb,Croatia

2
Motivation
Chiral soliton model
Anti-decuplet of baryons JP1/2
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
NA49 Experiment
VTPC-1 (VTPC-2MTPC)
dp/p2 7 x (0.3) 10-4 (GeV/c)-1
dE/dx resolution 3-6
(sTrig 28.1 mb)
4
Data Sample
pp 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.) x2y2lt 1 cm2
3.) -590.5 cm lt z lt -572.5 cm
i.e. within target
5
X5 search
X- selection

• Distance to Bethe- Bloch curve
• dbb lt 3 s
• M(pp-) - 1.115683 lt 0.015 GeV
• Z X- Zmain_vtx gt 12 cm
• X - position at main vertex (bx , by)
• bx lt 2 cm
• by lt 1 cm
• p (from X - decay) position at main vertex
• by gt 0.5 cm

L
6
V0 and X Invariant Mass Spectra

X- peak
1640 events

L peak
M(Lp)-1.321 lt 0.015 GeV
X peak
L peak
551 events
7
Primary pion selection
(?- ?, ? p)
p

• dbb lt 1.5 s, dbb .... distance to Bethe-Bloch
  
• curve
• position at main vertex (bx , by)
• bx lt 1.0 cm
• by lt 0.5 cm
• of points gt 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
Additional cuts
cos(q) lt 0.997 (q gt 4.5)
Backgrond Signal
q lab. angle between X- and p
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
p- K- p
p 497.6 10 769.0 60 892.0 30 1115.7 10
K 892.0 30 1019.0 10 1520.0 15
p 1115.7 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 !
15
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 at the same mass
• Candidates for the X5 baryons !

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 pp events (new DAQ system)
• Neutral pions