Data analysis of April'2002 SVD-2 run. Pentaquark searches Detection capabilities Charmed mesons

1. Data analysis of April'2002 SVD-2 run. Pentaquark searches • Detection capabilities • Charmed mesons • Pentaquark searches • A.Kubarovsky, V.Popov, V.Volkov • SINP MSU Moscow Talk at VHMP 2004, Dubna, Russia 16-17 April 2005

2. 53,000,000 events collected • ~7% are “beam only”/non-useable • Basic way of pre-analysis: • Finding a primary vertex • Search for a secondary one(s) within different regions and with different detectors/methods to use/CPU time to be consumed  • Average CPU time per event: up to 7 PIII-1000 seconds • Facilities: SINP and SRCC MSU “old” clusters.

3. K-   Region II (5-60 cm)   P-beam K-    p+  Region I(1-35 mm)

4. Primary vertex Z-coordinate in SVD-2 active target Si – 300m; Pb – 220 m; С – 500 m SVD-2 data analysis Pb C Si Si Si Si Si sz = 70÷130µm, sx,y = 7÷10µm

5. K0s and Λ0 effective masses • K0s M = 497.61 ± 0.09 (stat.), σ=4.4 MeV/c2 • Λ0M = 1115.32 ± 0.05 (stat.), σ=1.6 MeV/c2

6. Reconstruction of V0s with the spectrometer (“distant” ones): K0 : σ=5.8 MeV/c2 Λ0 :σ=3.0 MeV/c2

7. The well-known resonances of the same topology as + Arrows correspond exactly to the PDG data

9. Near-threshold D0 production • Search for the D0 K+π- decays • Different reconstruction methods used (not cross-checked yet…) • 24 events over background of 12 V.Volkov

10. A start of pentaquark history • An idea of five-quark states was discussed since late 60’s (Jaffe, Lipkin, Strotteman, …) • First estimates in the Skyrme model (Chemtob, Praszalowicz, Walliser) – 1985... • There were no conclusive results from experimental searches “The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided”.(PDG, 1986)

11. PDG – in “Particle listings under revision”

12. In the ‘2003 fall... • uudds quark content • Predicted by Diakonov,Petrov and Polyakov Z.Phys A 359, 305(1997) • Supposed to be of 1530 MeV/c2 mass, width <15 MeV/c2 and to decay into nK+ or pK0 • Observed by several experiments (LEPS,DIANA,CLAS,…) in both decay modes • The pK0 mode can be searched for by SVD-2:good resolution for K0s andproton identification

13. JLab-d ELSA ITEP SVD-2 JLab-p HERMES ZEUS COSY-TOF CERN/NA49 H1 pp  S+Q+. Experiments zoo Spring8 DIANA

14. PENTA-I : Data sample • ~35,000 events sample for D-meson searches was used • Primary selection mainly based on the presence of well-defined secondary vertices in the close-to-target region • Secondary tracks are measured by vertex detector • Allowed decay length is 1…35 mm (av. 20 mm), means that only ~10% of K0s to be reconstructed

15. Penta-I:Events selection • ≤ 5 charged tracks from primary vertex to reduce combinatorial background • K0s selection: • 2 oppositely charged tracks from secondary vertex • 490 ≤ Mππ≤ 505 MeV/c2 • Mpπ≥ 1120 MeV/c2 (→ not Λ0) • Proton selection: • Positive track from primary vertex • Momentum of 4…21 GeV/c • Absence of a Cerenkov detector hit

16. Penta I : pK0s spectrum analysis • Cut applied: pK0ssystem goes forward in center-of-mass system • Narrow peak at 1526 MeV/c2 is observed • FRITIOF simulated background fails to reproduce the real shape • Peaks in higher mass region can be attribu-ted to Σ*+ bumps

17. Penta I : Cut applied: Pp>PK0 to eliminate bumps (cf. B.Levchenko, arXiv:hep-ph/0401122) • Background can be described in different ways: • FRITIOF simulations • Plain polynom fitting • Random-star (not shown) • All three give the shape compatible to the experimental one

18. Penta I : Pentaquark parameters at SVD-2 • Number of events within a peak: 50 over background of 78, gives a significance of 5.6 σ • Mass: 1526 ± 3(stat.) ± 3(syst.) MeV/c2 • Width Γ < 24 MeV/c2 (below experimental resolution) • Strangeness could not be defined directly, but there are no reported S= -1 states in this mass region We interpret this state as Θ+ pentaquark

19. Penta I : Cross section and A-dependence • We estimate the total cross-section of pA→Θ+ + X as30÷120 μb/nucleon • We fail to find the difference in A-dependence of Θ+events from that of total for inelastic events (~A0.7) The ratio of Θ+ peakevents to the total of K0 events for different target materials

20. Theta masses from different experiments 1520 1540 1560 • Certain mass shift can be observed between the K+n and K0p modes Spring-8 K+n CLAS-d CLAS-p SAPHIR ITEP HERMES K0p COSY ZEUS SVD-2

21. Penta I : second analysis • To be checked… • Possibly could result in a much more narrow peak

22. Penta II : event selection • “Distant” K0 used • Cuts applied: • Number of primary charged tracks: 4 to 6 • Proton candidate momentum: 9 ≤ Pp ≤ 15 GeV/c • No Cerenkov detector hit for a proton • A total of 9,000 events selected

23. Peak is well pronounced (6 σ) No significant change in the effective mass Cross-section and systematic errors are yet to be evaluated Penta II : results

24. Pentaquark II (“distant” K0) • M=1524 ± 2.3 MeV/c2 • σ = 12.0 ± 2.3 MeV/c2 • Nevents = 187 • Significance 5.9 σ No significant mass difference from the previous analysis